Skip to main content

Full text of "Essentials of irrigation and cultivation of orchards"

See other formats 


Division       of       Agricultur  a  I       Sciences 


UNIVERSITY  OF  CALIFORNIA 


ESSENTIALS  of 
IRRIGATION  and 
CULTIVATION  of 

ORCHARDS 

F.  J.  Veihmeyer    «    A.  H.  Hendrickson 


..,„.■. 


CALIFORNIA  AGRICULTURAL 
lent  Station 
Extension      Service 


CIRCULAR  486 


The  main  purpose  of  irrigation: 


ENOUGH  IS  ENOUGH! 


MORE  IS 
WASTEFUL 


To  keep  the  soil  supplied  with  readily 
available  moisture  AT  ALL  TIMES. 

Nothing  is  gained  by  irrigating  often 
enough  to  keep  the  soil  moisture  at  a 
high  level. 

But  if  the  soil  is  allowed  to  become  dry 
for  a  considerable  time,  this  may  result 
in  harmful  effects  on 
Tree  size 
Yield 
Fruit  size 
Fruit  quality. 


The  main  purpose  of  cultivation: 


To  remove  weed  competition.  Cultiva- 
tion itself  does  not  conserve  moisture. 


This  circular  presents  the  principles  on  which  good  irrigation  and  cultivation  prac- 
tices in  orchards  are  founded.  The  same  principles  also  apply  to  other  crops.  The  in- 
formation of  this  publication  is  based  on  research  conducted  in  California. 

This  circular  replaces  Extension  Circular  50,  originally  issued  1930 


The  Authors: 

F.  J.  Veihmeyer  is  Professor  of  Irrigation,  Emeritus,  and  Irrigation  Engineer,  Emeritus, 
in  the  Experiment  Station,  Davis.  A.  H.  Hendrickson  is  Pomologist,  Emeritus,  in  the 
Experiment  Station,  Davis. 


FEBRUARY,  1960 


Research  has  shown  that: 

often  contrary  to  widespread  belief,  deciduous  fruit  trees 
need  readily  available  moisture  all  year  around;  but  no 
additional  benefits  are  gained  by  adding  more  water  to 
that  already  available. 

Beneficial  results  of  good  irrigation  practices,  such  as 
increased  yields,  are  cumulative  and  require  many  years 
to  appear. 

Harmful  results  of  poor  irrigation  practices,  such  as 
decreased  fruit  size,  may  show  up  immediately  when  trees 
are  not  supplied  with  readily  available  moisture. 

Large  trees  need  no  more  water  than  small  ones,  as 
long  as  the  area  shaded  by  the  leaves  is  the  same. 

Cover  crops  in  the  orchard  do  not  conserve  soil  mois- 
ture. 

Withholding  irrigation  does  not  make  the  trees  send 
down  their  roots  more  deeply  into  the  soil. 


Contents 

IRRIGATION  OF  ORCHARDS 4 

Water  in  soils 4 

Measuring  soil  moisture 6 

Methods  of  irrigation 8 

Use  of  water  by  trees 13 

Tree  responses  to  soil-moisture  conditions 14 

Irrigation  during  the  growing  season 20 

Seasonal   irrigation 23 

Influence  of  irrigation  on  root  distribution 24 

CULTIVATION  OF  ORCHARDS 25 


IRRIGATION  OF  ORCHARDS 


Soil  moisture  in  this  circular  is  dis- 
cussed from  the  standpoint  the  grower  is 
most  interested  in — the  availability  of 
water  to  the  trees.  For  a  clearer  under- 
standing of  the  terms  used  in  this  pub- 
lication the  most  important  ones  are 
defined  and  briefly  discussed  on  pages  4 
to  10  under  the  line. 

WATER   IN   SOILS 

How  water  is  stored  in  soil.  Soil 
is  a  porous  material  composed  of  par- 
ticles of  many  different  sizes  touching 
each  other,  but  leaving  space  in  between. 
This  space  is  called  pore  space  and  is 
the  place  where  water  in  soil  is  stored. 

Soil  is  a  reservoir  for  water.  The 
soil  in  which  tree  roots  are  growing 
is  like  a  reservoir  containing  various 
amounts  of  water  at  different  times  a 
year. 


In  California,  the  soil  containing  these 
roots  is  ordinarily  filled  to  its  field 
capacity  at  the  beginning  of  the  growing 
season.  In  unirrigated  mature  orchards 
where  drainage  is  unrestricted,  the 
readily  available  moisture  in  the  soil 
occupied  by  the  roots  is  usually  ex- 
hausted before  the  end  of  the  growing 
season.  The  trees  then  remain  wilted 
until  fall  rains  renew  the  water  supply. 
In  other  words,  the  trees  use  all  the 
readily  available  water  and  then  exist 
as  best  they  can.  This  situation  can  only 
be  remedied  by  irrigation. 

How  soils  are  wetted.  After  an 
irrigation,  the  soil  throughout  the  por- 
tion wetted  is  at  uniform  moisture  con- 
tent. The  water  moves  downward  mostly 
by  gravity;  capillarity  cannot  be  de- 
pended upon  to  distribute  moisture  uni- 
( Continued  on  page  6) 


GLOSSARY 


O      F 


TERMS 


SATURATION 

Water  fills  pore  space  between   soil   particles 
almost  completely. 


FIELD  CAPACITY 

Water  remaining  after  drainage  exists  in  form 
of  wedges  between  particles. 


Saturation  of  the  soil  is  the  condi- 
tion in  which  the  pore  spaces  are  filled 
almost  completely  with  water.  A  soil 
is  saturated  or  nearly  so  for  a  short 
time  after  water  is  applied  until  drain- 
age takes  place. 

Field  capacity  is  all  the  water  a  soil 
will  hold  after  drainage  has  taken 
place.  During  drainage,  water  moves 
downward  and,  to  a  lesser  extent,  side- 
ways, by  gravity.  A  limited  amount  of 
water  is  also  capable  of  moving  by 
capillarity,  but  you  cannot  depend  on 
it  for  moving  water  in  the  soil. 

At  field  capacity,  each  soil  particle 
is  completely  surrounded  by  water  but 
most  of  it  exists  in  the  form  of  wedges 
between  the  soil  particles  at  their 
points  of  contact.  It  is  from  these 
wedges  that  plants  get  most  of  their 
water. 


[4] 


SANDY  LOAM 


CLAY  LOAM 


18    12     6      0      6    12    18  30   24   18    12     6     0     6     12    18   24    30 

DISTANCE— INCHES  FROM  CENTER  OF  FURROW 

light  irrigation  simply  wets  a  shallower  depth  to  its  field  capacity  than  heavy  irrigation  does. 
Soils  cannot  be  partially  wetted.  They  must  be  completely  wetted  or  not  at  all. 


USED 


I    N 


THIS 


CIRCUL     AR 


When  irrigation  water  is  applied  to 
a  soil,  that  soil  is  moistened  to  its 
field  capacity  to  a  definite  depth,  which 
depends  on  the  dryness  of  the  soil  and 
the  amount  of  water  used. 

The  moistened  portion  of  a  drained 
soil  of  uniform  texture  and  structure 
reaches  its  field  capacity  two  or  three 
days  after  a  rain  or  irrigation.  This 
time  span  is  increased  if  there  are  lay- 
ers of  soil  which  hinder  the  movement 
of  water. 

The  field  capacity  is  the  approxi- 
mate starting  point  from  which  trees 
begin  to  use  water  from  the  soil  in 
their  normal  functions  of  growth  and 
fruiting.  They  may,  however,  use 
some  water  during  irrigation  before 
the  field  capacity  is  reached. 

Field  capacity  is  influenced  by: 

Soil  texture — the  size  of  the  soil  par- 
ticles, indicating  coarseness  or  fineness 


of  the  soil.  In  general,  the  fine-textured 
soils  such  as  clays  and  loams  hold  more 
water  at  field  capacity  than  the  sands. 
This  is  so  because  most  of  the  particles 
in  fine-textured  soils  are  very  small, 
therefore  have  more  particles  in  a 
unit  volume  of  soil,  and  consequently 
have  more  water-holding  wedges.  The 
amount  of  water  may  be  greatly  in- 
fluenced by  the  extremely  small  par- 
ticles (colloids)  in  the  soil.  Adding 
organic  matter  will  only  slightly  in- 
crease the  field  capacity,  and  usually 
only  in  the  surface  layer.  The  amount 
of  water  in  the  soil  at  saturation  will 
not  vary  much  with  texture  of  soils 
from  the  same  rock  material. 

Soil  structure — the  pattern  in  which 
soil  particles  are  arranged.  It  may  in- 
fluence the  water  penetration  of  the 
soil. 

Uniformity  of  soil  texture  and  struc- 


[5] 


formly.  Hence,  a  light  irrigation  simply 
wets  a  shallower  depth  to  its  field  ca- 
pacity than  a  heavy  one  does;  it  does  not 
bring  about  a  moisture  condition  less 
than  the  field  capacity.  Or,  simply  stated, 
soils  cannot  be  partially  wetted;  they 
must  be  completely  wetted  or  not  at  all 
(see  the  drawing  on  page  5). 

The  field  capacity  may  be  exceeded, 
of  course,  in  undrained  soils.  On  the 
other  hand,  portions  of  the  soil  will  re- 
main dry  where  furrows  are  too  far 
apart,  as  lateral  movement  caused  by 
capillarity  is  very  limited.  A  plow  sole 
or  decided  differences  in  soil  texture  or 
structure  will  increase  the  lateral  move- 
ment. 

MEASURING   SOIL  MOISTURE 

When  is  irrigation  necessary?  This 
question  may  be  answered  by  measuring 
the  amount  of  moisture  in  the  soil.  How- 
ever, not  all  the  water  remaining  in  the 
soil  at  a  given  time  can  be  used  by  the 


trees.  (See  discussion  of  readily  avail- 
able moisture  on  page  8  below  the  line) . 

You  may  measure  soil  moisture  by 
several  methods,  but  the  quick  and  easy 
ones   are  not  always  the  most  reliable. 

Soil  sampling.  The  best  method  of 
measuring  soil  moisture  is  by  taking  soil 
samples  from  various  depths  and  loca- 
tions, with  a  soil  tube  or  auger,  drying 
the  samples  in  an  oven,  and  calculating 
the  water  content.  This  method  is  accu- 
rate, gives  a  true  value  for  the  undis- 
turbed core  of  soil,  and  makes  it  possible 
to  secure  an  adequate  number  of  samples. 
It  has  the  disadvantage  of  needing  con- 
siderable equipment  and  requiring  two 
days  or  more  for  the  drying,  weighing, 
and  calculating. 

In  California,  the  growing  season  is 
rainless  or  nearly  so,  and  the  climatic 
conditions  affecting  plants  are  similar 
year  after  year.  After  sampling  your  soil 
for  several  years  you  will  find  that,  if 
the  winter  rainfall  has  been  adequate  and 


Glossary  continued 

ture.  For  instance,  if  a  fine-textured 
soil  (clay)  overlies  a  coarser  soil  (sand 
or  gravel),  the  zone  immediately  above 
the  clay  will  have  a  higher  field  ca- 
pacity than  the  clay  would  have  if  it 
were  uniform  throughout. 

Soil  depth.  A  shallow  soil  holds 
more  water  in  a  unit  depth  at  field 
capacity  than  a  deep  soil  of  the  same 
kind;  but  this  difference  is  not  marked 
in  soils  deeper  than  about  2  feet. 

Presence  of  water  table.  The  mois- 
ture content  just  above  the  water  table 
is  greater  than  that  which  this  soil 
would  have  if  it  were  drained.  The  dis- 
tance above  the  water  table  affected 
this  way  is  greater  in  clays  than  in 
sands.  However,  the  amount  of  water 
held  in  the  soil  occupied  by  the  roots 
is  increased  measurably  only  by  shal- 
low water  tables,  and  not  by  deep  ones. 


Moisture  equivalent  is  a  common 
laboratory  method  of  measuring  soil- 
moisture  conditions.  It  helps  estimate 
the  amount  of  water  the  soil  will  hold 
shortly  after  a  rain  or  irrigation.  It 
would  be  preferable  to  measure  field 
capacity  directly,  but  this  is  not  always 
practical.  The  moisture  equivalent 
measure  agrees  closely  with  the  field 
capacity  in  most  fine-textured  soils,  but 
usually  is  lower  than  that  of  the  sands. 

"Wilting  coefficient"  (the  moisture 
equivalent  divided  by  the  factor  1.84) 
has  been  assumed  as  the  point  above 
which  plants  would  not  wilt.  However, 
this  relationship  does  not  hold  for  all 
soils.  There  are  enough  exceptions 
to  make  its  use  doubtful. 

Before  you  accept  recommendations 
for  irrigation  practices  based  on  mois- 
ture equivalent  alone,  be  aware  of  the 
limitations  of  the  methods  and  of  the 
lact  that  it  cannot  be  used  to  measure 


[6] 


irrigations  during  the  growing  season 
have  penetrated  to  a  uniform  depth  each 
time,  the  dates  when  you  need  further 
irrigations  will  fall  at  about  the  same 
time  each  year.  For  example,  a  mature 
deciduous  fruit  orchard  in  the  Sacra- 
mento Valley  on  fine-textured  soils  with 
a  depth  of  at  least  six  feet  will  not  need 
irrigation  until  the  latter  part  of  June 
in  a  year  of  normal  rainfall.  If  the  soil 
is  wet  to  a  depth  of  six  feet,  a  second 
irrigation  will  not  be  needed  until  after 
mid-August. 

Instruments  with  immediate  read- 
ings. Various  devices  exist  to  measure 
soil  moisture  without  the  drudgery  of 
soil  sampling.  In  general,  these  instru- 
ments have  the  advantage  of  immediate 
readings.  But  they  also  have  serious  dis- 
advantages. Some  are  not  always  reliable 
at  low  moisture  contents,  others  are  not 
accurate  at  high  moisture  contents.  Their 
calibrations  may  not  remain  correct  after 
these  instruments  have  been  in  place  for 


some  time.  Finally,  there  are  usually  not 
enough  of  these  instruments  to  give  an 
adequate  number  of  readings,  and  they 
must  of  necessity  be  placed  in  holes 
where  the  soil  has  been  disturbed,  and 
the  soil  may  not  have  been  repacked  to 
the  same  density  of  the  undisturbed  soil. 
Roots  must  penetrate  into  the  region  of 
disturbed  soil  before  the  reading  of  the 
instrument  becomes  meaningful.  To 
take  a  quantitative  measurement  of  soil 
moisture  content,  these  devices  must 
be  calibrated  by  testing  them  in  soil  of 
known  moisture  content.  This  requires 
that  the  tests  be  made  on  all  the  kinds 
of  soils  in  the  orchard.  Nevertheless, 
these  instruments  will  show  you  that 
changes  in  soil  moisture  are  taking  place, 
and  may  cause  you  to  pay  more  attention 
to  your  soil  moisture  conditions. 

The  Atmometer  is  an  inexpensive  de- 
vice used  to  estimate  the  extraction  of 
water  from  the  soil  (and  thus  the  neces- 
sity for  the  next  irrigation)  by  measuring 


accurately  the  amount  of  water  avail- 
able for  plants. 

Permanent  wilting  percentage 
(PWP)  is  the  moisture  content  of  the 
soil  below  which  the  trees  cannot 
readily  obtain  water;  plants  will  wilt, 
and  will  not  recover  unless  water  is 
applied  to  the  soil.  In  a  given  soil  all 
plants  will  wilt  permanently  at  the 
same  moisture  content. 

Wilting  or  drooping  of  leaves  is  the 
most  common  symptom  that  the  PWP 
has  been  reached.  Some  plants  will  not 
wilt  but  show  other  signs  such  as  de- 
creased plant  or  fruit  growth,  or 
change  of  color  of  leaves. 

Because  it  is  difficult  to  obtain  pre- 
cise results  in  soil  sampling  and  to  de- 
termine exactly  when  a  tree  is  wilted, 
this  circular  uses  the  term  PWP  as  a 
narrow  range  of  soil-moisture  contents 
within  which  wilting  takes  place.  The 
PWP  covers  a  range  of  about  1  per  cent 


C6 

P.W.P. 

Water  between  particles  is  reduced  to  a  point 
that  it  becomes  unavailable  to  plants. 

of  soil  moisture  in  fine-textured  soils 
and  about  i/2  per  cent  in  sands.      \ 

Drooping  of  leaves,  usually  in  the 
late  afternoon,  is  a  sign  that  soil  mois- 
ture has  been  reduced  close  to  the 
PWP.  If  this  wilted  condition  is  still 
noticeable  when  transpiration  begins 
the  following  morning,  for  most  soils 
this  means  that  the  PWP  has  been 
reached  in  that  part  of  the  soil  which 


[7] 


the  water  evaporated  from  the  atmom- 
eters.  The  instrument  consists  of  two 
water-filled  hollow-porous  porcelain 
spheres  with  thin  walls,  one  white,  and 
the  other  black.  The  white  sphere  reflects 
most  of  the  solar  energy  falling  on  it, 
while  the  black  one  absorbs  most  of  it. 
This  makes  the  difference  in  evaporation 
between  the  black  and  white  sphere  a 
good  measure  of  insolation  (sun  energy) . 
The  pair  of  atmometers  act  like  a  pyrheli- 
ometer. 

Experiments  have  shown  that  extrac- 
tion of  moisture  from  the  soil  by  the 
trees  is  correlated  with  the  difference  in 
evaporation  from  the  black  and  white 
atmometers.  Hence,  when  you  know  the 
relationship  between  the  extraction  of 
soil  moisture  and  evaporation  of  water 
from  the  atmometers,  you  can  calculate 
with  a  fair  degree  of  accuracy  when  the 
next  irrigation  is  necessary.  Many  fruit- 
growing counties  of  California  are 
equipped  with  atmometers,  and  records 


of  evaporation  from  them  can  be  ob- 
tained from  the   Farm  Advisors. 

METHODS  OF   IRRIGATION 

Your  method  of  irrigating  orchards 
will  depend  on  the  size  of  stream  you 
have  available,  on  topography,  soil, 
climate,  and  the  general  tendency  to 
follow  the  practice  prevalent  in  your 
locality. 

Furrows  can  be  used  if  the  slope  of 
your  land  and  the  size  of  your  stream 
are  such  that  they  will  not  cause  erosion. 
Space  the  furrows  closely  enough  to- 
gether so  that  the  wetted  areas  meet  and 
the  water  is  kept  in  the  furrows  until 
it  has  penetrated  to  the  desired  depth. 
Regulate  the  length  of  the  furrow  so  that 
the  difference  in  penetration  between  the 
upper  and  lower  ends  is  not  great.  Also 
regulate  the  size  of  the  irrigation  stream 
in  accordance  with  rate  of  water  intake 
by  the  soil. 


Glossary  continued 

is  filled  by  the  major  portion  of  the 
root  system;  the  normal  activities  of 
the  tree  are  then  limited. 

Readily  available  moisture  is  the 
moisture  above  the  PWP.  Soil  mois- 
ture is  also  present  below  the  PWP 
but  is  held  so  tightly  by  soil  particles 
that  plant  roots  cannot  absorb  it  rap- 
idly enough  to  prevent  wilting.  The 
water  in  the  soil  above  the  PWP, 
throughout  the  entire  range  of  mois- 
ture contents  up  to  field  capacity,  can 
be  used  by  plants  with  equal  ease — 
it  is  readily  available  moisture. 

The  simplest  way  to  determine  the 
amount  of  readily  available  moisture 
in  a  soil  at  known  field  capacity  is  by 
growing  a  plant  on  it  and  finding  the 
PWP. 

The   field  capacity  of  clay  soils  is 


greater  than  that  of  sandy  soils,  but 
some  sandy  soils  contain  more  readily 
available  moisture  than  some  clays. 
The  readily  available  water  varies 
from  about  one-quarter  to  three-quar- 
ters of  the  moisture  equivalent.  These 
differences  do  not  depend  on  the  tex- 
tural  grade  (that  is,  whether  it  is  clay, 
loam,  or  sand). 

The  amount  of  readily  available 
water  is  like  the  water  that  can  be 
drawn  from  a  faucet  on  the  side  of  a 
barrel  filled  with  water.  The  amount 
of  water  in  the  barrel  is  the  field  ca- 
pacity. The  water  above  the  faucet 
may  be  drawn  off  and  is  the  readily 
available  moisture.  The  water  below 
the  faucet  is  present  but  cannot  be 
drawn  off.  The  lower  the  faucet,  the 
more  water  can  be  drawn  off.  Some 
soils  are  like  barrels  with  low  faucets; 
others  like  those  with  high  faucets. 


[8] 


Straight  furrows  are  adapted  to  relatively  flat  lands  that  can  be  graded  to  slopes 
of  .15  per  cent  or  less.  Sometimes  straight  furrows  are  used  on  steeper  slopes,  but  there 
the  stream  must  be  controlled  carefully  or  erosion  will  occur.  On  sandy  soils  a  good 
length  of  furrows  is  about  300  feet;  on  medium-textured  soils  about  600  feet;  on  clays 
it  may  be  longer. 


SATURATION 


-FIELD  CAPACITY 
(READILY  AVAILABLE 
MOISTURE) 


P.W.P.— END 
-I- OF    READILY 
AVAILABLE 
WATER 


^  ^  DRAINAGE 

[9] 


CONTOUR  FURROWS 


■Affc 


-m 


•»fS-t 


-r  v.#-  -::;^x¥^  • 


Graded  contour  furrows  are  sometimes  used  on  steep  slopes,  i.e.,  on  land  with  slopes 
up  to  25  per  cent,  the  grades  of  the  furrows  being  .5  to  1.5  per  cent,  depending  on 
soil  type. 


Glossary  concluded 

This  analogy  needs  to  be  modified, 
however.  Some  of  the  water  in  the  soil 
below  the  PWP  can  be  used  by  plants, 
although  the  rate  of  use  is  drastically 
reduced. 

Field  capacity  and  PWP,  for  all 
practical  purposes,  are  constant  for  a 
given  soil.  Additions  of  organic  or  in- 
organic fertilizers,  in  amounts  usually 
applied  in  orchards,  will  not  change 
these  soil  properties  enough  to  be  of 
practical  importance.  For  example:  an 
increase  in  the  field  capacity  by  add- 
ing manure  also  increases  the  PWP; 
hence,  the  readily  available  moisture 
remains  unchanged  for  all  practical 
purposes. 

Cultivation  with  heavy  machinery 
or  traffic  through  the  orchard,  espe- 
cially on  wet  soil,  may  result  in  soil 
compaction.  In  some  soils  a  slight  in- 
crease in  density  of  the  soil  will  pre- 
vent the  roots  from  penetrating  into 
it.  Field  capacity  and  PWP  determina- 
tions under  these  conditions  may  be 


misleading  if  they  are  made  on  frag- 
mented samples. 

Optimum  moisture,  a  "best"  soil 
moisture  content  for  plant  growth,  is 
a  false  idea.  It  usually  is  expressed  by 
a  moisture  percentage  from  the  field 
capacity.  However,  there  is  no  one  per- 
centage of  readily  available  moisture 
at  which  plants  grow  better  than  at 
another.  Even  if  there  were  such  a  the- 
oretical optimum  moisture  content  of 
a  soil,  it  could  not  be  maintained,  and 
therefore  would  be  of  no  practical  im- 
portance. All  attempts  to  maintain  any 
soil-moisture  content  lower  than  field 
capacity  have  failed  because  movement 
of  moisture  in  the  soil  by  capillarity 
is  too  slow  to  bring  about  a  uniform 
distribution  in  the  soil. 

Wet  soil  is  soil  containing  readily 
available  moisture.  It  is  not  necessarily 
a  wet-looking  soil,  nor  a  soil  which, 
when  squeezed,  contains  water. 

Dry  soil,  accordingly,  is  soil  from 
which  plants  cannot  readily  obtain 
water,  regardless  of  whether  the  water 
is  present  or  not. 


[10] 


ZIGZAG  FURROWS 


,  ,ts>i 


m     • 


Zigzag  or  check-back  furrows  may  be  used  to  reduce  the  grade  of  the  furrows  or 
to  insure  wetting  the  soil  in  the  tree  rows. 


Rectangular  checks,  either  for  single  tree  or  multiple-tree  basins,  are  adapted  to 
land  that  can  be  leveled  within  .2  foot  per  100  feet.  You  may  enclose  one  or  more  trees 
in  a  basin.  For  a  large  number  of  trees  use  long  rectangular  checks. 


fc 


:*».: 


■  • 


¥■;.■,.  »?** 


•  «•  v*/.»*V*r 


RECTANGULAR  CHECKS 


'***-* 


».  W*:.*  •'  •■•:i-X. 


tyM 


,?     '-^%;^ 


8 


*V'fr 


5  ***;#^<!  1 


ss 


Contour  checks,  which  are  used  extensively,  reduce  the  need  for  land  grading  and 
probably  lower  labor  costs.  They  require  large  streams  for  irrigation.  The  levees  are 
put  on  contours  with  a  difference  in  elevation  of  about  .2  foot. 


Land  may  be  prepared  by  constructing  contour  terraces — suitable  grades 
for  furrows  may  be  obtained. 


Sprinklers  can  be  used  under  a 
variety  of  conditions,  and  are  adapted  to 
rolling  or  steep  lands.  Their  advantage 
for  irrigating  orchards  is  that  they  need 
no  leveling,  and  that  they  can  make  use  of 
small  streams  of  water.  Usually  sprin- 
kling results  in  an  approximate  uniform 
wetting  of  the  soil. 

The  disadvantages  of  sprinkling  are: 

It  requires  high  capital  investment  and 
maintenance  costs. 

The  amount  of  water  that  usually  can 
be  applied  at  one  setting  of  the  pipelines 
is  limited.  The  depth  of  application  for 
a  given  time,  which  in  orchards  is  gener- 
ally limited  by  two  moves  of  the  pipe- 
lines in  24  hours,  is  controlled  by  the 
rate  at  which  the  soil  will  take  water, 
and  the  discharge  rate  of  the  nozzles. 
Lengthening  of  time  of  setting,  of  course, 
means  increasing  the  amount  of  equip- 
ment to  sprinkle  the  entire  area  in  a 
given  time,  and  adding  to  the  cost  of 
the  pipe  and  sprinklers. 

The  operation  of  sprinklers  is  in- 
flexible after  the  original  design  of  the 
system  is  adopted.  Low  hanging  branches 
of  trees  may  interfere  with  the  spray 
from  the  sprinklers  and  cause  uneven 
distribution  of  water. 


USE   OF  WATER   BY   TREES 

Water  taken  from  the  soil  by  trees  is 
almost  entirely  given  off  as  water  vapor 
through  the  leaves.  This  "transpiration" 
is  like  evaporation  from  a  piece  of  wet 
paper;  it  may  be  controlled  to  some 
extent  by  internal  conditions  within  the 
plant. 

External  factors  influencing  tran- 
spiration, other  than  soil  moisture,  are 
sunlight,  temperature,  humidity,  and 
wind  (drawing  below).  Of  those,  the 
amount  and  quality  of  sunlight  is  prob- 
ably the  most  important.  High  tempera- 
tures are  usually  accompanied  by  low 
humidities.  This  tends  to  increase  tran- 
spiration. Transpiration  may  be  less  on 
a  calm  than  on  a  windy  day,  but  does 
not  increase  in  direct  proportion  to  wind 
velocity.  The  effect  sometimes  noticed 
on  leaves  after  a  period  of  strong  winds 
is  probably  due  to  the  combination  of 
low  relative  humidity,  high  temperatures, 
intense  sunlight,  and  mechanical  injury. 

Size  of  trees  does  not  influence  the 
amount  of  water  used  by  the  orchard. 
If  the  ground  area  shaded  by  the  leaves 
of  the  trees  is  the  same,  the  use  of  water 
is   the  same   regardless  of  whether  the 


V  M 


ml 


SPRINKLERS 


m. 


THESE  FACTORS  INFLUENCE 


W    TRANSPIRATION 

Q-  •_♦  ^*- 


/ 1  \ 

SUN 
HUMIDITY 


Uj>- 


/N 


^ 


i. 


WIND 


I 


TEMPERATURE 


trees  are  large  or  small.  The  shading  or 
ground  coverage  is  determined  by  the 
spacing  of  the  trees  and  their  growth 
habit.  What  counts  is  not  the  number  of 
leaves  on  the  trees  but  rather  the  number 
of  leaves  directly  exposed  to  sunlight. 
Our  experiments  show  that  the  use  of 
water  from  large  walnut  trees  is  almost 
the  same  as  that  from  low-growing  plants 
such  as  alfalfa,  if  the  ground  is  fully 
covered  by  the  foliage  of  the  plants. 

Presence  or  absence  of  fruit  does 
not  materially  influence  the  amount  of 
water  used  by  the  trees. 

Time  of  year.  Transpiration  by  de- 
ciduous trees  is  confined  almost  entirely 
to  that  part  of  the  year  when  leaves  are 
present,  although  some  water  is  used 
during  the  leafless  period.  Our  experi- 
ments with  prunes,  peaches,  and  apricots 
indicate  that  the  use  of  water  depends 
upon  the  ground  coverage  of  the  trees, 
and  not  upon  the  particular  kind.  How- 
ever, this  may  not  apply  to  trees  differing 
from  deciduous  trees  as  widely  as  the 
olive  and  orange,  when  the  total  seasonal 
use  is  considered. 

Evergreen  trees  use  water  throughout 
the  year,  but  the  amounts  used  in  the 
winter  usually  are  much  less  than  in  the 
summer. 

Use  of  water  above  the  PWP.  The 

extraction  of  water  from  the  soil  by  trees 
is  not  affected  by  soil  moisture  until  it 
is  reduced  to  the  PWP  in  that  part  of  the 
soil  that  is  reached  by  the  roots.  The  use 


of  water  is  influenced  by  the  factors  men- 
tioned before,  but  their  effect  is  much 
more  marked  when  the  moisture  is  above 
the  PWP. 

At  the  PWP  or  below,  trees  use 
water  very  slowly  even  if  climatic  condi- 
tions are  favorable  for  transpiration.  The 
rate  of  use  varies  with  the  surrounding 
conditions,  and  may  not  be  the  same 
during  each  hour  of  the  day.  Experiments 
in  deciduous  orchards  under  fairly  uni- 
form climatic  conditions  show,  however, 
that  the  rate  of  moisture  use  is  sub- 
stantially constant  day  by  day  until  the 
soil  moisture  is  reduced  to  about  the 
PWP.  Of  course,  changes  in  evaporation 
conditions  during  the  day  and  from  day 
to  day,  such  as  between  a  foggy  and  a 
bright  day,  will  cause  corresponding 
changes  in  transpiration  rates.  Transpi- 
ration is  highly  correlated  with  solar 
energy. 

TREE   RESPONSES  TO 
SOIL-MOISTURE   CONDITIONS 

Fruit  growth  is  retarded,  and  other 
symptoms  appear,  when  the  soil  contain- 
ing most  of  the  roots  has  been  reduced 
to  the  PWP.  The  degree  of  injury  de- 
pends on  the  length  of  time  the  soil  re- 
mains in  this  condition. 

Irrigation  experiments  in  California 
deciduous  orchards  yielded  many  results 
and  observations  on  the  response  of  fruit 
trees  to  various  soil-moisture  conditions, 
particularly  those  in  which  trees  were 
allowed  to  remain  permanently  wilted 
for  relatively  long  periods. 

Tree  responses  to  irrigation  practices 
fall  into  two  general  classes: 

Short-term  results:  the  response  is 
immediate  and  takes  effect  the  same 
season.  In  general,  immediate  results  are 
usually  harmful,  and  follow  changes  in 
practice  involving  neglect  or  ceasing  to 
irrigate,  especially  during  certain  critical 
periods.  Decreased  size  in  many  fruits, 
delay  in  maturity  of  pears,  and  a  lowered 
percentage  of  well-filled  shells  in  walnuts 
are  some  of  the  results  that  immediately 


[14] 


follow  a  failure  to  keep  the  trees  sup- 
plied with  readily  available  moisture. 

Long-term  results:  the  response  to 
a  given  irrigation  program  appears 
slowly  and  is  sometimes  apparent  only 
after  several  years.  In  general,  the  bene- 
ficial results  are  those  requiring  several 
years  of  good  irrigation  practice.  In- 
creases in  yield,  for  instance,  are  as  a 
rule  the  reward  for  the  long-continued 
practice  of  keeping  the  trees  supplied 
with  readily  available  water  throughout 
the  year. 

Briefly,  the  initiation  of  good  irriga- 
tion practice  does  not  bring  about  the 
beneficial  results  as  if  by  magic,  but  lack 
of  moisture  may  cause  serious  loss  the 
same  season. 

Effect  of  Irrigation  on  Tree  Size 

One  of  the  most  noticeable  responses 
of  peach  and  prune  trees  was  the  slow 
but  comparatively  steady  gain  in  size,  as 
measured  by  the  cross-section  areas  of 
tree  trunks,  of  the  irrigated  over  the  un- 
irrigated  trees. 

At  the  beginning  of  the  experiment,  the 
plots  were  divided  into  treatments  in  such 
a  way  that  the  average  sizes  of  trees  and 
the  average  yields  were  about  equal. 

After  two  years,  the  trees  in  the  irri- 
gated plots  showed  evidence  of  growing 
faster  than  those  in  the  unirrigated  plots. 
As  the  experiment  progressed,  the  un- 
irrigated trees  and  those  in  the  interme- 
diate plots  in  which  the  soil-moisture  was 
reduced  to  the  PWP  for  varying  periods, 
continued  to  grow  more  slowly,  until  the 
irrigated  trees  were  distinctly  the  largest, 
with  the  intermediate  plots  in  second 
place,  and  the  dry  plots  the  smallest.  But 
trees  in  frequently  irrigated  plots  did  not 
grow  faster  than  in  less  irrigated  plots 
where  the  soil  was  not  reduced  to  the 
PWP. 

Walnut  trees  did  not  respond  in  size  to 
differential  treatment  as  quickly  as  either 
the  peach  or  prune  trees.  In  fact,  the  trees 
in  the  dry  treatment,  which  were  the  larg- 
est at  the  beginning  of  the  experiment, 


POOR  IRRIGATION   PRACTICES 
MAY   RESULT   IN 


06 


SMALLER  YIELD 

SMALLER  FRUIT 
POORER  QUALITY 

STUNTED  TREES 


remained  so  for  several  years.  This  may 
be  due  to  the  deep-rooting  habit  and  wide 
spacing  of  walnut  trees. 

Gains  in  size  due  to  irrigation  were 
comparatively  slow  in  prune,  peach,  as 
well  as  walnut  trees. 

Effect  of  Irrigation  on  Yield 

All  plots  of  the  prune  orchards  pro- 
duced about  equally  before  the  various 
irrigation  treatments  were  begun.  A 
change  in  production  started  with  the 
first  year  when  the  plots  were  irrigated 
according  to  project  plan.  The  irrigated 
plots  moved  ahead  of  the  unirrigated 
ones,  and  maintained  their  relative  po- 
sitions ever  since.  This  change  was  due 


The  results  reported  here  are  sum- 
marized from  long-time  experiments 
with  fruit  trees  at  Davis,  and  from  other 
experiments  carried  on  for  compara- 
tively short  periods  in  widely  separated 
fruit-growing   areas   in   California. 

All  experiments  in  the  field  were 
carried  out  in  such  a  way  that  certain 
plots  had  readily  available  moisture  at 
all  times,  while  others  were  subjected 
to  dry  soil  conditions  after  the  moisture 
from  winter  rains  was  exhausted.  In 
some  cases  intermediate  treatments  were 
also  used.  At  Davis  the  irrigation  treat- 
ments were  greater  in  number,  and  each 
treatment  was  replicated  several  times. 


[15] 


to  the  decrease  in  crop  on  the  dry  plots, 
rather  than  a  marked  increase  in  crops 
from  the  irrigated  ones.  The  frequently 
irrigated  plots  did  not  produce  more 
fruit,  on  the  average,  than  the  intermedi- 
ate ones  in  which  just  enough  irrigation 
was  given  to  keep  the  soil  moisture  above 
thePWP. 

The  increasing  differences  in  yields 
between  the  irrigated  and  the  unirrigated 
treatments  are  due  to  the  delayed  effects 
of  irrigation. 

The  yields  of  the  intermediate  plots 
indicate  that  some  irrigation,  while  the 
crop  is  on  the  trees,  is  better  than  none 
at  all,  but  this  treatment  is  inferior  to 
that  where  irrigation  is  frequent  enough 
to  provide  readily  available  moisture  at 
all  times. 

Similar  results  were  obtained  with 
peaches.  The  immediate  effect  of  stopping 
irrigation  during  the  growing  season  was 
a  reduction  in  crop.  This  was  largely  due 
to  a  reduction  in  the  size  of  the  fruit 
which  left  a  large  percentage  unmarket- 
able. The  intermediate  plots  (at  PWP  for 
short  periods)  fell  behind  the  irrigated 
ones  in  production  during  the  third  sum- 
mer of  differential  treatment. 

Relation  of  Tree  Size  to  Yield 

Our  experiments  dealing  with  the  irri- 
gation of  peach  and  prune  trees  con- 
firmed observation  in  many  places  about 
a  general  relationship  between  tree  size 
and  yield.  These  results  indicate  that  fruit 
trees  grown  in  areas  where  irrigation  is 
not  possible  are  generally  smaller  and 
yield  less  than  those  in  areas  where  irri- 
gation water  is  applied  when  needed. 
These  results  also  indicate  the  relatively 
slow  yield  response  of  trees  to  irrigation, 
particularly  in  areas  similar  to  Davis 
where  winter  rainfall  is  ordinarily  suf- 
ficient to  provide  water  for  a  consider- 
able portion  of  the  growing  season. 

Effect  of  Irrigation  on  Fruit  Size 

Maintenance  of  readily  available  soil 
moisture  allows  the  fruit  to  grow  nor- 

[16] 


mally  according  to  the  characteristic  of 
the  particular  kind  of  fruit.  Lack  of  read- 
ily available  soil  moisture  while  the  fruit 
is  growing  causes  an  immediate  check  in 
growth.  Slow  growth  of  fruit  on  peach, 
pear,  and  prune  trees  has  been  repeatedly 
found  at  Davis  when  the  soil  moisture  in 
the  upper  5  or  6  feet  of  soil  is  reduced 
to  the  PWP.  Similar  results  have  been 
observed  in  the  San  Joaquin  Valley  on 
Japanese  plums,  on  soils  holding  a  com- 
paratively small  amount  of  available 
moisture. 

In  some  areas,  where  winter  rainfall 
is  ample  and  the  soil  holds  a  compara- 
tively large  supply  of  moisture,  certain 
early  fruits  may  be  grown  to  maturity 
without  irrigation  and  reduction  in  fruit 
size,  because  the  amount  of  moisture  is 
sufficient  to  supply  the  needs  of  the  tree 
at  least  until  the  crop  is  mature.  Late 
fruits  in  these  areas,  however,  show  the 
characteristic  responses  to  lack  of  mois- 
ture. 

Almond  and  walnut  fruits  grow  rapidly 
during  the  spring  months  and  usually 
attain  full  size  before  the  available  soil 
moisture  is  exhausted.  Reduction  in  sizes 
of  almonds  and  walnuts  is  generally  only 
noticeable  in  unirrigated  areas  following 
winters  of  exceptionally  light  rainfall. 

Size  of  some  kinds  of  fruits  is  also  re- 
lated to  the  numbers  of  fruits  on  the  trees. 
If  the  fruits  are  not  thinned,  the  final 
sizes  may  be  unsatisfactory  in  spite  of 
good  irrigation.  Of  course,  if  the  trees 
do  not  get  sufficient  water  while  the  crop 
is  growing,  the  fruits  will  be  very  small. 

On  soils  holding  comparatively  large 
amounts  of  readily  available  moisture  in 
the  cool  coastal  regions,  fruits  may  grow 
to  normal  size  even  though  unirrigated. 

In  general,  fruits  may  be  expected  to 
attain  normal  size,  if  the  usual  thinning 
practice  is  followed,  and  if  the  soil  mois- 
ture does  not  remain  at  the  PWP  while 
the  fruits  are  growing. 

Fruits  stunted  in  growth  because  of 
he  lack  of  moisture,  begin  to  grow  more 
rapidly    than    previously    if   the    supply 


is  replenished,  but  they  always  remain 
smaller  than  similar  fruits  not  allowed 
to  suffer  for  water. 

Effect  of  Irrigation 
in  Soil  with  Readily 
Available  Moisture 

If  trees  have  readily  available  moisture, 
no  measurable  beneficial  results  are  ob- 
tained by  adding  more  water.  In  this  case 
it  does  not  follow  that,  if  a  little  is  good, 
more  is  better. 

In  our  experiments  neither  the  rate  of 
growth  nor  yields  were  measurably  in- 
creased in  apple,  peach,  plum,  prune  and 
Avalnut  orchards  by  irrigating  while  the 
soil  moisture  was  above  the  PWP.  No 
benefits,  neither  immediate  nor  delayed, 
were  achieved  by  this  treatment. 

This  finding  once  more  confirmed  that 
plants  can  take  water  readily  from  soil, 
no  matter  what  its  moisture  content,  as 
long  as  it  is  not  reduced  to  the  PWP. 
Transpiration  will  not  decrease  with  de- 
creasing soil  moisture,  growth  will  not  be 
retarded,  yields  will  not  be  lessened  until 
PWP  is  reached.  Neither  will  quality  of 
the  irrigated  product  deteriorate  under 
these  conditions. 

Water  Intake  by  Trees  Above  the 
PWP — Scientific  Considerations 

It  is  sometimes  reasoned  that  be- 
cause the  resistance  to  removal  of  water 
from  the  soil  by  the  plants  increases  as 
the  soil-moisture  content  decreases, 
transpiration,  growth,  and  other  plant 
processes  likewise  must  decrease.  We 
believe  this  reasoning  is  incorrect. 

Almost  all  of  the  water  is  taken  by 
the  plant  from  the  soil  and  literally 
pulled  through  the  plant,  due  to  the 
different  condition  of  the  water  within 
the  leaves  from  that  of  the  surround- 
ing air. 

The  energy  required  to  take  water 
from  the  evaporating  cells  in  the  leaves 
is  many  times  that  needed  to  take  water 
from  the  soil  with  its  moisture  at  the 
PWP.   Thus   the  relatively   slight  in- 


crease in  energy  requirements,  as  the 
soil  moisture  decreases,  is  ineffective  in 
reducing  water  uptake  by  the  plant. 
When  the  PWP  is  reached,  however, 
there  is  a  great  increase  in  the  tightness 
with  which  the  water  is  held  by  the 
soil,  and  the  plants  will  wilt.  Failure 
to  supply  adequate  water  to  the  plant 
by  the  soil  may  not  be  due  to  an  exces- 
sive amount  of  energy  required  to  re- 
move it  from  the  soil  at  low  moisture 
contents.  There  is  the  possibility  that 
slow  movement  of  water  into  the  plant 
roots  at  low  soil-moisture  contents,  or 
slow  rate  of  root  growth  into  moist 
soil,  may  be  the  cause  of  water  defi- 
ciencies. Changes  within  the  plant,  if 
brought  about  by  low  soil  moisture, 
may  also  affect  water  uptake  by  the 
plant. 

Whether  these  things  actually  occur 
can  only  be  determined  by  empirical 
trials.  As  stated  before,  our  experi- 
ments with  fruit  trees  have  shown  that 
their  water  needs  can  be  supplied  so 
long  as  the  soil  moisture  is  not  reduced 
to  the  PWP  in  the  soil  occupied  by 
the  majority  of  the  roots.  It  must  be 
assumed,  therefore,  that  such  changes 
do  not  occur  or  that,  if  they  do,  their 
effects  are  not  measurable  until  the 
soil  moisture  is  reduced  to  the  PWP. 

Water  is  lost  from  the  plant  largely 
through  openings  in  the  leaves.  These 
are  called  stomata  and  are  capable  of 
either  closing  or  opening.  Hence,  it  is 
reasoned  that  the  condition  of  the 
stomata  will  affect  transpiration.  Con- 
siderable study  has  been  given  to  them 
to  find  out  whether  the  amount  of 
readily  available  soil  moisture  in- 
fluences their  behavior.  Our  experi- 
ments show  that  stomata  behave  nor- 
mally until  the  soil  moisture  is  reduced 
to  the  PWP. 

Photosynthesis  does  not  decrease  with 
decreasing  soil  moisture.  Also,  microbial 
activity  in  the  soil  is  not  reduced  unless 
the  PWP  is  reached. 

The  records  of  yields   of  fruit  from 


[17] 


trees  in  our  experiments  extending  over 
many  years  also  illustrate  the  fact  that 
water  is  readily  available  between  the 
field  capacity  and  the  PWP.  The  table 
shows  some  records  from  orchard  plots 
on  good  deep  soils  with  sufficient  rain 
to  wet  the  soil  to  a  depth  of  from  6  to  9 
feet  each  season. 

Plots  irrigated  frequently  kept  the  soil 
moisture  at  a  relatively  high  level  while 
the  plots  irrigated  infrequently  just  had 
enough  water  to  keep  the  soil  moisture 
within  the  top  6  feet  of  soil  from  being 
at  the  PWP  for  an  appreciable  length  of 
time.  The  differences  in  yield  were  not 
significant. 

The  High  Cost  of  Needless  Irrigation 

Here  is  an  example  of  the  relative 
costs  of  growing  prunes  under  frequent 
irrigation  (with  the  intent  of  main- 
taining the  soil  moisture  at  a  high 
level)  as  compared  to  infrequent  irri- 
gation which  was  just  enough  to  keep 
the  soil  moisture  from  remaining  at 
the  PWP  for  more  than  2  to  3  weeks 
during  the  growing  season.  For  13 
years  portions  of  a  prune  orchard  were 
irrigated  in  this  manner.  The  yield 
from  the  frequently  irrigated  portions 
was  46.9  tons  of  dried  fruit  per  acre. 
The  total  cost  of  irrigation  was  $818.80 
for  the  13-year  period,  and  the  income 


was  $7,504.  The  infrequently  irrigated 
portions  yielded  46.6  tons  per  acre — 
at  a  cost  of  $579.60  for  irrigation,  and 
an  income  of  $7,456. 

Local  Conditions  and  Irrigation 

Ordinarily,  soil  moisture  is  readily 
available  between  the  limits  of  field 
capacity  and  PWP.  Yet,  local  conditions 
often  will  determine  when  to  irrigate. 

"Soil  moisture,"  in  this  circular,  means 
moisture  in  that  part  of  the  soil  that  is 
in  contact  with  the  absorbing  portions 
of  the  roots.  If  the  roots  do  not  thor- 
oughly penetrate  the  soil,  there  will  be 
some  parts  of  the  soil  that  will  not  supply 
water  to  the  plant.  Under  such  conditions, 
soil  sampling  will  not  give  a  true  picture 
of  the  moisture  conditions.  Our  observa- 
tions of  deciduous  fruit  trees  and  grape 
vines  show  that  these  plants  have  good 
root  systems.  There  are,  however,  in- 
stances where  the  soil  is  too  dense  to 
permit  root  penetration. 

Shallow  soils,  high  water  tables,  alkali 
and  high  salinity  also  may  be  controlling 
factors  in  timing  irrigations. 

Effects  of  Lack  of  Available 
Soil  Moisture 

Lack  of  soil  moisture  during  the  grow- 
ing season  is  almost  always  followed  by 
immediate  and  harmful  effects.  The  most 


Crop 

Number  of 

years  in 
experiments 

Irrigations 

Cumulated  yield 
in  pounds  per  tree 

Walnuts  (hulled) 

23 

Frequent 
Infrequent 

2,469.7 
2,498.8 

Prunes  (fresh  fruit) 

19 

Frequent 
Infrequent 

6,219.6 
6,046.4 

Almonds  (hulled) 

5 

Frequent 
Infrequent 

105.35 
107.35 

Apricots  (fresh  fruit) 

11 

Frequent 
Infrequent 

1,966.9 
1,878.2 

[18] 


common  is  a  reduction  in  final  size  of 
fruits;  this  not  only  reduces  the  total 
yield,  but  materially  reduces  the  amount 
of  the  marketable  crop  with  those  fruits 
that  must  meet  a  certain  legal  minimum 
size  to  be  salable. 

Lack  of  available  moisture  affects 
various  fruits  differently.  Walnuts,  for 
instance,  growing  rapidly  during  the 
early  part  of  the  season,  ordinarily  attain 
full  size  before  the  soil  moisture  from 
winter  rains  is  exhausted.  Lack  of  soil 
moisture  later  in  the  season,  however, 
results  in  a  larger  proportion  of  blanks 
or  partly  filled  shells.  The  weight-volume, 
or  number  of  pounds  of  walnuts  per  cubic 
foot,  of  Concord  Walnuts  at  Davis  has 
been  consistently  lower  in  the  dry  plots 
during  the  past  several  years  than  in  the 
irrigated  ones. 

These  responses  to  lack  of  soil  moisture 
during  the  growing  season  are  fairly 
common.  They  occur  year  after  year  in 
many  orchards,  but  are  not  cumulative 
in  effect  nor  are  they  generally  carried 
over  from  one  year  to  the  next. 

If  you  have  small  fruit  one  year  be- 
cause you  were  unable  to  irrigate  at  the 
proper  time,  you  may  expect  fruit  of 
normal  size  the  following  season  unless 
you  again  let  the  moisture  become  ex- 
hausted. 

Yield  losses  and  fruit-size  reduction 
result  if  an  irrigated  orchard  suddenly 
is  deprived  of  water.  This  is  not  difficult 
to  understand.  The  size  of  the  aerial  por- 
tion of  the  tree  and  the  amount  of  crop 
set  are  largely  determined  by  the  treat- 
ment given  the  trees  in  previous  years. 
The  large  leaf  area,  or  large  ground 
coverage  by  the  trees,  rapidly  exhausts 
the  available  moisture.  Unless  the  mois- 
ture is  replenished  the  tree  wilts,  the 
fruits  slow  down  in  growth,  and  some  of 
the  new  shoots  may  die. 

These  points  were  illustrated  in  experi- 
mental orchards  at  Davis.  An  entire  or- 
chard was  given  uniform  treatment,  in- 
cluding regular  irrigation.  All  trees  set 
a  uniformly  heavy  crop.  After  a  number 


of  years  certain  plots  were  given  a  dif- 
ferent irrigation.  In  some  cases,  the  ir- 
rigation schedule  was  continued  as  before 
and  continued  to  bear  heavy  crops.  In 
other  plots,  irrigation  was  stopped.  The 
trees  in  these  dry  plots  exhausted  the 
supply  of  moisture  before  the  fruits  at- 
tained full  size.  The  result  was  a  reduc- 
tion of  crop  from  the  trees  in  these  dry 
plots,  and  a  reduction  in  the  size  of  fruits. 

In  the  prune  orchards,  for  instance, 
the  average  yields  were  290  pounds  from 
the  irrigated  plots,  and  172  pounds  from 
the  dry  plots.  The  year  before,  the  aver- 
age yields  on  these  same  two  groups  of 
plots  (but  before  differential  treatment) 
had  been  147  and  163  pounds.  Fruit-size 
differences  were  even  more  marked:  the 
irrigated  plots  produced  an  average  of 
21.6  per  cent  of  large  fruit;  the  dry  plots 
only  2.7  per  cent. 

In  the  peach  experiment  the  average 
yields  of  all  the  plots  were  substantially 
the  same  before  differential  treatment. 
The  next  year,  irrigated  plots  averaged  a 
yield  of  232  pounds,  dry  plots  149 
pounds.  In  addition  to  the  yield  loss,  a 
large  proportion  of  the  crop  from  dry 
lots  was  unsalable  because  fruits  did  not 
reach  the  minimum  size  stipulated  by  the 
canneries. 

Effect  of  Irrigation  on  Fruit  Quality 

It  was  believed  that  irrigation  at  cer- 
tain periods  of  the  growing  season  has 
an  immediate  and  injurious  effect  on 
fruit  quality.  Our  experiments  have 
shown  that  this  definitely  is  not  the  case. 
The  highest  quality  is  obtained  when 
trees  are  supplied  with  moisture  through- 
out the  year. 

Experiments  with  canning  peaches 
showed  that  maintaining  readily  avail- 
able moisture  in  the  soil  up  to  and  in- 
cluding harvest  time  did  not  injure  either 
the  shipping  or  canning  quality. 

On  the  other  hand,  lack  of  moisture 
for  several  weeks  before  harvest  pro- 
duced peaches  of  tough,  leathery  texture. 

Under  similar  conditions  of  dry  soil, 


[19] 


pears  frequently  have  a  high  pressure 
test,  indicating  later  maturity  than  those 
kept  watered ;  but  this  difference  in  hard- 
ness tends  to  disappear  in  storage.  Delay 
in  maturity  may  be  serious  in  districts 
where  early  shipping  is  desired. 

Quality  in  prunes,  as  measured  by  the 
specific  gravity,  is  apparently  not  greatly 
affected  by  the  irrigation  treatment.  It 
seems  to  be  associated  with  climatic  con- 
ditions during  the  summer. 

The  drying  ratios  of  prunes  do  not 
seem  to  be  materially  affected  by  the  ir- 
rigation treatment.  They  are  chiefly  de- 
pendent on  the  amount  of  fruit  on  the 
trees.  Years  of  large  crops  have  high  dry- 
ing ratios  while  those  of  light  crops  have 
low  ratios. 

Cracking  of  prunes  on  the  trees  occurs 
from  time  to  time  to  a  serious  extent. 
Experimental  results  and  observations 
over  many  years  indicate  this  trouble  not 
to  be  due  to  irrigation;  it  probably  has 
its  origin  in  the  climatic  condition  dur- 
ing the  growing  season. 

Benefits  from  Good  Irrigation 
Practice 

The  benefits  derived  from  good  irriga- 
tion practice  are  cumulative.  Increased 
crops  result  chiefly  from  increased  sizes 
of  trees  which  in  turn  depend  on  the 
trees  being  kept  healthy  and  vigorous. 
One  of  the  chief  factors  in  keeping  trees 
vigorous  is  an  irrigation  plan  providing 
readily  available  soil  moisture  at  ALL 
times. 

There  appears  to  be  no  irrigation  for- 
mula that  will  QUICKLY  improve  crops 
of  deciduous  fruits.  Sometimes  the  bene- 
fits from  irrigation  may  be  slow  in  ap- 
pearing, and  are  only  apparent  after 
several  years. 

On  the  other  hand,  immediate  re- 
sponses to  changes  in  irrigation  treat- 
ments result  when  the  watering  of  trees 
is  suspended.  These  responses  are  not 
only  immediate,  but  they  are  often  in- 
jurious. 


IRRIGATION   DURING   THE 
GROWING   SEASON 

Assuming  a  mature  orchard  with  the 
trees  24  feet  apart  on  the  square  system, 
and  with  the  majority  of  roots  in  the  up- 
per 5  feet  of  soil,  there  are  2,880  cubic 
feet  of  soil  from  which  each  tree  may 
obtain  water.  This  volume  of  soil  is  es- 
sentially a  reservoir  that  contains,  when 
it  is  filled  to  its  field  capacity,  a  definite 
amount  of  readily  available  moisture. 

An  example  taken  from  actual  meas- 
urements :  a  peach  orchard  in  one  of  the 
largest  peach-growing  sections  on  clay 
loam  soil  with  a  field  capacity  of  25  per 
cent,  two-thirds  of  which  is  readily  avail- 
able, contains  approximately  260,000 
pounds  of  dry  soil  in  the  2,880  cubic 
feet.  A  25  per  cent  moisture  content  of 
this  soil  is  65,000  pounds  of  water  or 
1,040  cubic  feet.  Two-thirds  of  this,  or 
about  700  cubic  feet  of  water,  is  readily 
available  to  the  trees,  and  this  quantity  is 
equivalent  to  a  depth  of  about  15  inches 
of  water. 

In  other  words,  when  the  PWP  is 
reached,  an  application  of  water  3  inches 
deep  would  be  required  to  wet  each  foot 
of  such  soil. 

Of  course,  if  the  entire  5  feet  of  soil 
is  not  reduced  to  the  PWP,  15  inches  will 
not  be  needed. 

While  an  application  of  15  acre-inches 
to  the  acre  may  seem  too  great  when 
ordinary  irrigations  are  considered,  it 
must  be  remembered  that  ordinarily 
water  is  applied  before  all  the  readily 
available  moisture  is  exhausted,  or  the 
soil  is  not  wet  to  5  feet. 

Another  experiment  with  clay  loam 
soil  showed  it  to  have  a  field  capacity 
of  about  25  per  cent,  half  of  which  was 
readily  available.  Only  about  500  cubic 
feet  of  water  could  be  used  readily  by  the 
trees  and  it  would  require  approximately 
10^2  inches  of  water  to  replenish  the 
supply. 


[20] 


Use  of  Water  by  Trees  During 
the  Growing  Season 

After  the  leaves  are  formed,  the  trees 
begin  to  draw  upon  the  soil  moisture 
and  continue  to  do  so  until  it  is  reduced 
to  slightly  beyond  the  PWP.  After  irri- 
gating, this  process  is  repeated.  In  prac- 
tice, of  course,  the  orchard  should  be 
irrigated  before  the  trees  wilt. 

The  number  of  times  this  cycle  of 
events  takes  place  during  the  growing 
season  depends  upon  the  size  of  the  trees 
(the  coverage  of  ground  by  the  trees), 
the  climatic  conditions,  and  the  kind  and 
depth  of  the  soil. 

The  total  amount  of  water  that  com- 
parable trees  will  use  will  not  be  greater 
on  a  clay  soil  than  it  is  on  a  sandy  soil 
if  both  are  fertile  and  have  readily  avail- 
able water  at  all  times.  Usually  on  sandy 
soils,  however,  water  must  be  applied 
more  frequently  and  in  smaller  amounts 
than  on  clay  soils. 

With  the  coming  of  warm  weather,  the 
readily  available  water  is  quickly  used 
by  the  trees.  It  should  be  replenished. 

Weeds  May  Indicate 
Irrigation  Needs 

You  may  be  able  to  judge  when  your 
trees  need  water  because  of  your  close 
association  with  them  and  your  daily 
observation  of  their  condition. 

When  wilting  or  other  evidences  of 
lack  of  readily  available  moisture  are 
hard  to  detect  in  the  trees  themselves, 
you  may  rely  on  some  of  the  broad- 
leaved  weeds  which  may  be  left  as 
indicator  plants  in  various  places  in  the 
orchard.  Generally,  such  weeds  are  deep- 
rooted  enough  to  indicate  by  their  wilt- 
ing a  lack  of  readily  available  water  in 
the  soil  occupied  by  the  roots  of  the  trees. 

The  soil  at  this  time  will  show,  under 
examination,  its  condition  of  dryness, 
and  the  grower  may  become  familiar 
enough  with  it  to  recognize  when  the 
moisture  content  is  close  to  the  PWP.  At 
other  times  he  may  anticipate  when  this 


condition  will  be  reached.  This  could 
avoid  possible  injury  to  the  trees  by 
actual  wilting. 

Where  only  small  streams  of  water  are 
available,  the  time  necessary  to  cover  the 
orchard  may  be  so  long  that  the  trees 
which  are  irrigated  last  may  be  decidedly 
affected  before  they  receive  water.  It  is 
very  important,  therefore,  to  anticipate 
when  the  PWP  will  be  reached  so  that 
irrigation  may  be  started  in  time. 

Examples  of  Water  Needs  in 
California 

Here  are  some  examples  of  water  needs 
in  various  California  fruit-growing  areas, 
based  on  field  experiments: 

Mature  peach  trees  in  the  Sacramento 
and  San  Joaquin  valleys  showed  that  the 
interval  between  depletions  of  the  readily 
available  moisture  in  the  upper  5  feet 
of  soil  in  the  summer  varied  from  3 
weeks  in  a  sandy  soil  to  6  weeks  in  a 
clay  loam  soil. 

On  shallow,  hardpan  soils,  trees  should 
be  irrigated  more  frequently  with  smaller 
applications  of  water  than  on  deep  soils. 

Mature  prune  trees  on  loam  soils  in  the 
Santa  Clara  Valley  exhausted  the  readily 
available  soil  moisture  in  from  4  to  6 
weeks  during  the  hottest  part  of  the 
season. 

In  the  central  coastal  region,  under 
low  evaporating  conditions,  one  irriga- 
tion during  the  growing  season  was  suf- 
ficient for  apple  and  pear  trees  on  deep 
fine-textured  soil. 

Citrus  on  the  coastal  plain  areas  of 
San  Diego  County,  in  years  of  normal 
rainfall,  need  a  summer  irrigation  of  12 
to  15  acre-inches  of  water.  The  irrigation 
interval  ranges  from  four  weeks  on  the 
lighter  soils  to  a  maximum  of  six  weeks 
on  the  heavier  soils. 

In  the  interior  valleys  of  San  Diego 
County  and  the  intermediate  areas  of 
Orange  County,  the  seasonal  transpira- 
tion use  by  mature  trees  in  good  condi- 
tion ranges  from  18  to  22  acre-inches 


21] 


per  acre.  July  and  August  are  the  months 
of  heaviest  use. 

In  San  Bernardino  County  the  use  of 
water  by  a  citrus  grove  with  a  heavy  sum- 
mer crop  of  weeds  increased  the  seasonal 
use  by  8  acre-inches  of  water  per  acre. 
In  this  orchard  a  maximum  use  of  7% 
acre-inches  per  acre  was  observed  during 
July  which  required  irrigation  at  two- 
week  intervals. 

Evergreen  trees  use  water  later  in  the 
fall  and  earlier  in  the  spring  than  decidu- 
ous trees.  They  even  use  some  moisture 
on  clear  warm  days  during  the  winter. 

Depth  of  Roots 

In  most  of  our  experiments,  the  dis- 
tribution of  roots  of  deciduous  trees  has 
been  such  that  a  uniform  use  of  water 
has  occurred  in  the  top  5  to  6  feet  of  soil. 

Studies  in  the  irrigation  of  citrus 
groves  show  that  an  average  of  not  more 
than  5  per  cent  of  the  moisture  used  was 
taken  from  the  fifth  foot  of  soil,  which 
indicates  that  most  of  the  roots  were 
above  this  depth.  In  fact,  in  soils  less 
than  3  feet  in  depth  50  to  60  per  cent 
of  the  roots  probably  are  in  the  first  foot 
of  soil  or  below  the  cultivated  layer. 

On  the  other  hand,  walnut  trees  on  a 
fairly  uniform  soil  extracted  the  moisture 
to  a  depth  of  12  feet  or  more. 

Influence  of  Water  Tables 

The  presence  of  a  high  water  table 
may,  in  some  cases,  result  in  upward 
movement    rapid    enough    to    replenish 


CITRUS 


DECIDUOUS        WALNUTS 


water  in  the  upper  layers.  In  others,  the 
upward  movement  may  not  be  sufficient 
to  take  care  of  the  needs  of  the  trees. 
Thus,  it  has  been  observed  in  some  cases 
that  frequent  surface  irrigations  are 
necessary  on  certain  types  of  soil  even 
when  the  water  table  is  fairly  close  to 
the  surface. 

Marked  fluctuation  of  the  water  table 
during  the  growing  season  may  produce 
harmful  results.  Under  these  conditions, 
a  high  water  table  should  not  be  relied 
upon  to  supply  moisture  during  the  grow- 
ing season,  and  drainage  may  be  neces- 
sary. In  addition  harmful  concentration 
of  salts  may  accumulate  if  the  water  table 
is  near  the  surface. 

Use  of  Cover  Crops 

Cover  crops  in  the  orchard  during  the 
growing  season  do  not  conserve  soil 
moisture. 

The  combination  of  trees  and  cover 
crops  needs  more  water  during  the  grow- 
ing season  than  trees  alone  if  the  trees 
do  not  completely  cover  the  ground. 

The  reduction  of  evaporation  losses 
due  to  shading  the  soil  by  the  cover  crop 
is  negligible  when  compared  with  the 
amount  used  by  the  plants.  Furthermore, 
lessened  transpiration  by  the  trees  be- 
cause of  the  increased  relative  humidity 
brought  about  by  cover  crop  transpira- 
tion is  very  slight. 

Experiments  with  alfalfa  in  a  mature 
peach  orchard  on  a  sandy  soil  indicate 
that  the  orchard  needed  about  50  per  cent 
more  water  for  the  season  with  cover 
crops  than  without  them.  The  increase 
in  water  use  by  the  cover  crop  occurred 
in  the  spring  and  fall  when  the  trees  were 
defoliated. 

Maintenance  of  Readily 
Available  Water 

The  moisture  content  in  the  soil  during 
the  growing  season  ordinarily  fluctuates 
between  the  field  capacity  and  the  PWP. 

If  the  soil-moisture  content  goes  above 
the  field  capacity  and  remains  there  for 


[22] 


any  great  length  of  time,  the  trees  may 
be  seriously  affected.  In  several  experi- 
ments, however,  prune  trees  were  kept 
with  standing  water  around  them  for 
relatively  long  periods,  with  apparently 
no  serious  effects.  Other  trees,  such  as 
pears  on  French  root,  have  been  known 
to  withstand  saturated  soils  for  long  pe- 
riods without  apparent  injury;  but  it  is 
safer  to  avoid  this  condition. 

Both  the  leaves  and  the  fruit  are  af- 
fected when  the  soil  moisture  is  reduced 
to  the  PWP.  Fruit  on  trees  on  dry  soil 
grows  more  slowly  than  fruit  on  trees 
having  readily  available  moisture.  It  is 
exceedingly  important,  therefore,  to  see 
that  the  soil-moisture  content  does  not 
remain  at  or  go  below  the  PWP  for  more 
than  a  few  days. 

The  trees  will  not  be  affected,  however, 
if  the  soil  is  irrigated  when  it  already 
contains  readily  available  water. 

If  possible,  wet  the  soil  at  each  irriga- 
tion to  the  depth  in  which  most  of  the 
roots  lie  even  though  the  lower  layers 
still  contain  some  readily  available  mois- 
ture. It  is  less  expensive  to  wet  this  depth 
at  this  time  than  later. 

Wetting  the  soil  to  a  depth  of  5  or  6 
feet  will  usually  be  sufficient  with  most 
deciduous  trees,  and  to  a  shallower  depth 
with  citrus  trees.  If  there  is  an  impervious 
layer  within  the  depth  mentioned,  use 
just  enough  water  to  wet  the  soil  above 
this  layer. 

"Overirrigation"  is  a  term  often  used 
to  mean  frequent  irrigation  resulting  in 
the  maintenance  of  readily  available 
moisture  at  a  high  level.  Actually,  over- 
irrigation  results  when  enough  water  is 
used  on  deep  soils  to  cause  percolation 
below  the  roots,  or  waterlogging. 

Leaching  may  take  place  if  irrigations 
are  too  frequent  or  too  great  in  amount. 
Leaching  may  be  necessary  where  the 
salinity  of  the  soil  is  high  and  must  be 
kept  in  balance. 

The  amount  of  water  to  be  used  at  each 
irrigation  varies  with  the  kind  and  depth 
of  soil  to  be  wetted,  and  with  its  mois- 


ture content  at  the  time  of  irrigation.  If 
water  is  applied  before  the  soil  moisture 
content  has  reached  the  PWP,  less  water 
will  be  required  to  wet  a  certain  depth. 
The  apparently  deeper  penetration  of 
water  obtained  in  some  early  irrigations 
over  later  ones  is  due  to  irrigation  before 
all  the  readily  available  moisture  is  ex- 
hausted. 

SEASONAL  IRRIGATION 

Spring.  In  some  cases,  irrigation  dur- 
ing the  spring  is  desirable. 

If  the  winter  rainfall  has  been  insuf- 
ficient to  moisten  the  soil  to  a  depth  of 
6  feet  or  more,  this  may  be  made  up  by 
spring  irrigation.  Again,  if  a  cover  crop 
has  been  allowed  to  grow  so  late  that  the 
readily  available  soil  moisture  is  almost 
depleted,  spring  irrigation  may  be  neces- 
sary. If  a  cover  crop  has  not  depleted 
the  soil  moisture,  the  first  irrigation  may 
be  delayed  until  the  readily  available  soil 
moisture  is  nearly  exhausted,  particu- 
larly, if  only  one  irrigation  can  be  given 
before  the  crop  is  harvested. 

Fall.  Many  deciduous  orchards  in  Cali- 
fornia are  allowed  to  remain  in  a  dry 
condition  for  a  long  period  each  fall. 

As  long  as  leaves  remain  on  the  trees 
and  can  function,  some  transpiration 
takes  place  if  evaporation  conditions  are 
favorable. 

Very  often  after  the  crop  is  picked, 
either  no  further  water,  or  only  one  irri- 
gation, is  given.  As  a  result,  the  trees  may 
reduce  the  soil-moisture  content  to  the 
PWP,  and  then  remain  in  a  wilting  con- 
dition for  a  long  time.  This  affects  some 
kinds  of  trees  more  than  others.  If  it  is 
necessary,  however,  to  omit  one  irriga- 
tion from  the  regular  schedule,  the  one 
in  the  fall  may  be  eliminated  with  less 
danger  of  serious  injury  than  one  in  mid- 
summer. 

Late-season  irrigation.  No  evidence 
was  found  to  support  the  belief  that  trees 
watered  late  in  the  season  continue  grow- 
ing and  do  not  mature  their  young 
growth   and  buds   in  time  for  them  to 


[23] 


withstand  winter  temperatures.  No  in- 
jury that  could  be  attributed  to  lack  of 
maturity  has  been  produced  in  our  ex- 
periments on  prune,  peach,  or  apricot 
trees,  or  on  grapevines  by  watering  late 
in  the  season. 

With  citrus  fruits  it  is  particularly  im- 
portant to  maintain  a  supply  of  readily 
available  water  during  the  fall.  In  order 
to  secure  best  results,  trees  should  have 
readily  available  moisture  in  the  fall  as 
well  as  during  the  other  seasons. 

As  a  rule  it  is  necessary  to  wet  the 
soil  in  the  fall  only  to  a  depth  sufficient 
to  supply  the  needs  of  the  trees  until 
rains  begin.  For  example,  if  the  orchard 
is  irrigated  late  in  September  or  October, 
only  2  or  3  feet  of  soil  need  be  wetted. 

Irrigation,  also,  is  necessary  for  plant- 
ing certain  cover  crops  that  seem  to  grow 
best  when  established  early  in  the  fall. 

Winter.  In  some  districts  winter  irri- 
gation is  practiced  in  deciduous  or- 
chards. This  is  unnecessary  if  the  winter 
rainfall  is  sufficient  to  wet  the  soil  to 
the  depth  containing  most  of  the  roots. 
If  the  rainfall  has  been  insufficient  for 
this  purpose,  irrigation  during  the  win- 
ter is  desirable.  There  must  be  readily 
available  moisture  present  during  the 
winter  months  even  though  the  trees  use 
little  water  at  this  time  of  the  year. 

Winter  irrigation  rests,  in  part,  upon 
the  desire  to  fill  up  the  soil  reservoir  with 
cheap  water  for  use  in  the  growing  sea- 
son. As  we  have  seen,  the  soil  can  only 
be  filled  to  its  field  capacity  and  any  ad- 
ditional water  above  that  required  to  wet 
the  soil  occupied  by  the  roots  moves 
down,  when  drainage  is  unrestricted,  and 
may  be  lost  by  deep  percolation  unless 
it  is  later  recovered  by  pumping. 


When  drainage  is  restricted,  however, 
winter  irrigation  may  cause  unfavorable 
soil-moisture  conditions,  because  of  the 
accumulation  of  free  water  above  the 
hardpan,  particularly  in  the  low  places  in 
the  orchard. 

INFLUENCE   OF   IRRIGATION 
ON   ROOT   DISTRIBUTION 

Our  experiments  also  do  not  support 
the  belief  that,  by  withholding  irrigation, 
trees  may  be  made  to  send  their  roots 
deeply  into  the  soil;  that  light  irrigation 
tends  to  encourage  shallow  rooting;  and 
that  irrigating  on  one  side  of  the  tree 
only  will  result  in  confining  the  roots  to 
that  side.  These  ideas  are  not  correct. 

Our  experiments  show  that  if  soils  are 
wet  only  to  a  certain  depth,  and  if  the 
soil  below  this  depth  is  at  the  PWP,  the 
roots  will  be  confined  within  the  wetted 
area. 

On  the  other  hand,  plants  which  are 
normally  deep-rooted  cannot  be  made  to 
keep  their  roots  in  the  upper  layers  of 
soil  if  those  at  lower  depths  have  a 
readily  available  supply  of  moisture  and 
if  no  other  adverse  condition  for  root 
development  lies  below. 

If  the  soil  is  wet  to  the  full  depth  to 
which  the  roots  would  normally  go  at 
the  beginning  of  the  growing  season,  then 
later  applications  of  water  during  the 
summer  will  have  no  influence  on  the  ex- 
tent of  the  distribution  of  the  roots,  un- 
less they  be  frequent  enough  to  produce 
conditions  that  are  unfavorable  for  root 
growth. 

The  presence  of  water  in  amounts 
above  the  field  capacity,  a  condition  often 
called  waterlogging,  may  injure  the  roots 
of  some  trees. 


[24] 


CULTIVATION  OF  ORCHARDS 


Losses  of  Moisture  from  Soils 

Does  soil  cultivation  (to  form  a  soil 
mulch)  save  moisture?  Our  experiments, 
as  well  as  those  of  others,  on  the  losses 
of  water  from  soil,  and  the  effect  of  cul- 
tivation on  these  losses,  very  clearly  show 
that  cultivation  of  itself  does  not  con- 
serve moisture. 

The  losses  of  moisture  stored  in  the 
soil  are  caused  by  extraction  by  the  roots 
of  trees  and  other  plants  in  the  orchard, 
and  by  evaporation  directly  from  the  soil 
surface.  Experiments  show  that  the 
amount  of  water  used  in  transpiration 
comprises  a  major  portion  of  the  total 
losses  from  the  soil  under  California  con- 
ditions. 

A  study  of  uncropped  soils,  both  culti- 
vated and  uncultivated,  in  tanks  and  in 
field  plots,  showed  that  tillage  of  the  soil 
did  not  save  water.  The  soil  dried  out 
to  the  same  extent  and  depth  whether 
cultivated  or  not.  It  was  also  found  that 
about  half  of  the  moisture  that  was  lost 
within  80  days  after  the  application  of 
water  was  lost  within  the  first  week.  This 
means  that,  even  if  cultivation  did  reduce 
evaporation,  it  would  not  be  effective  be- 
cause so  large  a  portion  of  the  loss  occurs 
before  the  surface  soil  is  dry  enough  to 
be  properly  cultivated. 

Moisture  Losses  by  Evaporation 

The  loss  of  moisture  by  evaporation 
during  periods  longer  than  those  usual 
between  irrigations  was  confined  to  rela- 
tively shallow  depths  of  soil,  because  the 
movement  of  moisture  by  capillarity 
from  moist  to  drier  soil  is  extremely  slow 
in  rate  as  well  as  slight  in  extent.  A  large 
portion  of  the  loss  was  in  the  upper  4 
inches.  A  much  smaller  amount  was  lost 
from  the  next  4  inches.  Moisture  below 
these  upper  8  inches  of  soil  was  lost  at 
an  extremely  slow  rate. 


In  California  where  water  is  applied 
in  such  amounts  that  considerable  depths 
of  soil  are  wetted,  the  loss  by  evaporation 
from  the  surface  layers  is  a  small  portion 
of  the  total. 

Where  a  water  table  is  relatively  close 
to  the  surface,  evaporation  losses  may 
be  greater  than  those  indicated. 

Cracking  in  Soils 

These  experiments  were  made  on  dif- 
ferent soils,  including  clays  which 
cracked  badly  on  drying  when  crops  were 
grown  on  them,  but  which  cracked  only 
to  very  shallow  depths  when  kept  bare. 
Cracking  is  the  result  of  drying,  which 
in  turn  is  brought  about  mostly  by  the 
extraction  of  water  by  plants.  In  most 
soils,  cracking  does  not  take  place  until 
the  moisture  content  is  reduced  below 
the  field  capacity.  In  a  few  others,  prin- 
cipally adobe  soils,  cracking  may  start 
before  the  soil  is  drained  to  its  field  ca- 
pacity. In  this  case,  cracking  occurs  while 
the  soil  is  still  too  wet  to  be  cultivated 
safely. 

The  loss  of  water  by  evaporation  from 
the  small  cracks  in  the  soil  takes  place  at 
such  a  slow  rate  that  probably  nothing 
would  be  gained  by  covering  them. 

With  large  cracks,  however,  which 
form  before  the  readily  available  mois- 
ture is  exhausted,  some  water  may  be 
saved  by  the  mulch  where  it  is  possible 
to  cultivate   the   soil  without  puddling. 

Cultivation  and  Water  Distribution 

The  studies  showed  that  cultivation 
had  no  influence  on  the  distribution  of 
water  in  the  soil. 

We  found  no  evidence  that  stirring  the 
surface  soil  influenced  the  upward  move- 
ment of  water.  The  part  cultivation  has 
been  supposed  to  play  in  preventing  the 
upward  rise  of  moisture  is  based  upon 
the  theory  that  moisture  can  move  in  the 


[25] 


GROUND  SURFACE 


Water  does  not  move  rapidly  either  upward,  sideways,  or  downward  by  capillarity,  and 
it  will  stay  until  removed  by  plants.  This  graph  is  taken  from  actual  measurements  in  an  irrigation 
furrow.  The  soil  is  loam.  After  the  water  disappeared  from  the  irrigation  furrow,  a  trench  was  cut 
across  it  and  the  line  of  demarcation  between  moist  and  dry  soil  was  noted.  The  trench  was  then 
covered.  Fifty-six  days  later,  it  was  opened,  a  new  face  was  cut,  and  the  line  of  demarcation  was 
again  determined,  but  the  moisture  movement  was  too  slight  to  measure. 


soil  in  all  directions  through  capillarity 
and  that  by  cultivation  the  upward  move- 
ment is  lessened.  The  loose  dry  soil  is 
assumed  to  act  as  a  blanket,  shutting  off 
evaporation.  The  loosening  of  the  soil 
reduces  the  number  of  points  of  contact 
between  the  particles,  and  is  supposed  to 
lessen  the  capillary  pulling  power. 

Since  evaporation  losses  are  confined 
so  largely  to  a  shallow  surface  layer,  and 
since  movement  by  capillarity  is  ex- 
tremely slow,  especially  when  the  soil  is 
not  in  contact  with  free  water,  movement 
of  moisture  from  a  lower  depth  does  not 
take  place  rapidly  enough  to  replace  that 
lost  by  evaporation. 

Cultivation  and  Yield 

Numerous  experiments  have  been 
made  to  measure  the  effectiveness  of  cul- 
tivation by  means  of  yields  produced. 
Results  of  many  of  these  experiments  are 
valueless.  They  contain  too  many  varying 
factors.  But  where  cause  and  effect  can 
be  segregated,  the  increased  yields  result- 


ing from  cultivation  can  be  attributed  to 
the  removal  of  weed  competition. 

Deep  Tillage 

Deep  tillage  (subsoiling,  deep  plowing, 
subsoil  dynamiting)  has  been  found  in- 
effective to  materially  increase  crop 
yields. 

An  added  objection  to  deep  tillage  in 
orchards  is  the  probable  injury  owing  to 
root  pruning. 

On  the  other  hand,  subsoiling  or  blast- 
ing before  planting  may  be  desirable 
under  special  conditions  where  particular 
kinds  of  hardpan  are  present.  These  exist 
when  the  hardpan  may  be  broken  up  eco- 
nomically by  these  methods,  and  it  will 
not  resume  its  original  impervious  condi- 
tion upon  being  wetted  again,  and  where 
the  soil  is  pervious   and  fertile  below. 


Cultivation  and  Soil  Aeration 

There  is  abundant  evidence  that  til- 
lage, of  itself,  does  not  increase  yields. 
Therefore,   the   idea   that   cultivation   is 


[26] 


beneficial  for  soil  aeration  and  results 
in  increased  fertility  and  yields  does  not 
seem  justified. 

Experiments  indicate  that  sufficient 
aeration  ordinarily  takes  place  in  or- 
chard soils. 

Experiments  in  California  have  shown 
that  rapid  nitrification  takes  place  be- 
low the  depths  affected  by  tillage. 

On  the  other  hand,  unfavorable  con- 
ditions for  aeration  result  if  water  is  ap- 
plied frequently  enough  to  fill  the  pore 
space  in  the  soil  and  maintain  this  satu- 
rated condition  too  long. 

Experiments  of  others  show  that  crop 
yields  are  not  increased  by  stirring  the 
surface  of  the  soil,  and  that  cultivation 
does  not  increase  aeration  in  the  soil  oc- 
cupied by  the  roots  of  the  trees. 

Frequent  cultivations  may  change  the 
soil  structure  so  that  infiltration  of  water 
is  retarded. 

The  Purposes  of  Orchard 
Cultivation 

Cultivation  in  orchards  should  be  di- 
rected toward  certain  useful  purposes. 
Some  of  these  are  to: 

1.  Remove  noxious  weeds  and  weed 
competition. 

2.  Facilitate  subsequent  orchard  oper- 
ation, such  as  irrigation,  harvesting, 
brush  removal,  and  spraying. 

3.  Incorporate  cover  crops  and  manure. 

4.  Prepare  the  soil  as  a  seed  bed  for 
cover  crops. 

5.  Facilitate  the  control  of  certain  pests. 

6.  Aid  in  the  absorption  of  water  where 
tillage  or  other  orchard  operations  have 
produced  an  impervious  condition  of  the 
soil. 

Weeds,  during  the  growing  season,  and 
cover  crops,  if  allowed  to  grow  too  late 
in  the  spring,  are  serious  competitors 
with  the  trees  for  moisture  and  nutrients. 
Cultivation  is  the  best  means  of  remov- 
ing this  competition. 

Several  orchards  operations  are  greatly 
facilitated  by  having  the  soil  in  proper 
condition. 


Better  levees  or  furrows  can  be  made 
when  there  is  sufficient  loose,  dry  soil  on 
the  surface  than  where  the  surface  is  hard 
or  cloddy. 

Picking  of  such  crops  as  prunes  and 
almonds  is  much  easier  from  a  loose,  fine 
surface  than  from  among  clods  or  weeds. 

Spraying  and  brush  removal  are  made 
easier  when  irrigation  levees  are 
smoothed  down   and  furrows  filled  up. 

On  steep  slopes  contour  cultivation 
may  stop  water  from  running  off  and  be- 
ing wasted. 

Plow  Sole 

Plow  sole  is  a  more  or  less  impervious, 
dense  layer  of  soil  formed  just  below  the 
depth  of  tillage. 

Ordinarily,  a  plow  sole  will  form  if 
the  soil  is  cultivated  while  too  wet. 

You  can  lessen  the  possibility  of  a  plow 
sole  forming  if  you  limit  the  necessary 
cultivations  to  a  time  when  the  soil  is 
in  such  a  condition  that  it  will  not  be 
puddled  by  the  implement. 

There  is  no  accurate  way  to  determine 
how  dry  a  soil  must  be  before  it  can  be 
cultivated  without  forming  a  plow  sole. 
Experience  with  each  soil  is  your  best 
guide. 

Since  cultivation,  in  the  absence  of 
weeds,  has  no  influence  in  conserving 
moisture,  much  is  to  be  gained  by  keep- 
ing off  the  ground  until  there  is  least 
danger  of  forming  a  plow  sole. 

Experience  has  shown  that  leaving  the 
soil  untilled  is  sometimes  the  best  remedy 
in  overcoming  a  plow  sole. 

Repeated  Cultivation  and 
Soil  Permeability 

Cultivation  of  a  compacted  surface 
layer  may  increase  permeability  of  soil 
to  water,  but  this  lasts  only  for  a  very 
short  time. 

Repeated  cultivation  tends  to  decrease 
soil  permeability.  Keep  all  tillage  opera- 
tions in  the  orchard  as  shallow  and  as 
infrequent  as  necessary  to  accomplish 
the  useful  purposes  mentioned. 

27] 


Weed  Control  by  Oil  Sprays 

The  use  of  oil  sprays  for  controlling 
weeds  has  been  followed  in  citrus  or- 
chards for  a  number  of  years,  and  to  a 
limited  extent  in  deciduous  orchards. 

Where  this  method  has  been  used, 
growers  report  improved  soil  conditions. 
No  harmful  effects  have  been  observed 
to  date. 

It  should  be  remembered,  however, 
that  the  oil-spray  method  has  been  used 
for  only  a  relatively  short  period  in  de- 
ciduous orchards. 

Economy  in  Orchard 
Irrigation  and  Cultivation 

During  the  past  several  years,  many 
California  fruit  growers  have  applied  the 
principles  set  forth  in  this  circular.  In  so 
doing  they  have  materially  changed  pre- 
vious practices  in  their  orchards. 

In  general,  cultivation  has  become  less 
frequent  and  shallower.  Furrows  or  levees 
are  often  used  for  two  or  more  irriga- 
tions, instead  of  breaking  them  down  and 
making  new  ones  each  time  water  is  used. 

The  sides  of  levees  used  more  than 
once  may  have  to  be  cultivated  lightly 
between  irrigations.  This  is  not  to  break 
down  the  levee,  but  to  provide  soil  which 
fills  the  cracks  in  the  levee  resulting  from 
drying.  By  following  this  procedure, 
levees  may  be  used  one  or  more  times 
even  on  clay  soils. 

Because  of  the  lessened  number  of 
cultivations  there  has  been  considerable 
saving  in  many  cases  in  the  cost  of  cul- 
tivation. 

A  Rational  Plan 

Disk  the  orchard  in  the  spring;,  to  elim- 
inate weeds.  Leave  the  soil  with  enough 
loose  surface  soil  to  construct  furrows 
or  levees  later  in  the  season.  If  it  rains 
before  the  first  irrigation,  you  may  have 


to  cultivate  again.  But  do  not  till  the 
soil  merely  for  the  sake  of  stirring  it.  Do 
not  cultivate  again  until  after  the  first 
irrigation  unless  the  weeds  are  too  nu- 
merous and  large.  In  some  cases,  the 
orchard  is  cultivated  after  the  first  irriga- 
tion. In  others,  it  is  cultivated  according 
to  the  amount  of  weed  growth  and  cost 
of  water. 

You  may  leave  the  original  furrows  or 
levees  for  several  irrigations  if  it  costs 
less  to  replace  the  water  used  by  weeds 
than  it  does  to  remove  them.  Follow  the 
same  general  procedure  with  later  irri- 
gations; irrigate  only  when  the  readily 
available  soil  moisture  is  about  ex- 
hausted. 

Ordinarily  cultivate  and  smooth  the 
orchard  before  harvest  to  facilitate  pick- 
ing of  crops  such  as  prunes  and  almonds, 
and  to  avoid  jolting  fresh-fruit  crops  by 
hauling  them  over  levees. 

If  you  use  tree  props,  cultivate  the  or- 
chard early  enough  before  harvest  to 
permit  the  placing  of  the  props. 

Usually  the  soil  in  deciduous  orchards 
is  dry  after  harvest.  Irrigation  is  then 
necessary.  After  the  last  irrigation  do  not 
cultivate  unless 

•  a  cover  crop  requiring  seed-bed  prepa- 
ration is  used 

•  it  is  necessary  to  break  down  the  levees 
for  spraying  or  removing  pruning  brush 
during  the  winter. 

In  all  cases  plan  the  irrigation  sched- 
ule so  that  it  does  not  interfere  with 
spraying  and  harvesting  operations. 

IN  CONCLUSION,  and  briefly  stated, 
the  most  important  purpose  of  cultiva- 
tion of  orchard  soils  is  to  remove  weed 
competition. 

The  purpose  of  irrigation  is  to  provide 
readily  available  moisture  in  the  soil 
throughout  the  year. 


Co-operative  Extension  work  in  Agriculture  and  Home  Economics,  College  of  Agriculture,  University  of  California,  and  United  States  Department  of  Agriculture 
cooperating.  Distributed  in  furtherance  of  the  Acts  of  Congress  of  May  £  and  June  30.  1911.  George  B.  Alcorn,  Director,  California  Agricultural  Extension  Service. 


30m-l,'60(A5135)JF 


[28]