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SOILS 235 1f

Approximate Determination of Optimum Moisture Content of Soils.—

Weigh three 25-gm portions of the 40-mesh air-dried soil and place them in

200-cc beakers or wide-mouth bottles. Add to the three portions, 5, 6 and

7 cc of water, respectively. Cover the bottles with watch glasses and

allow to stand for two days. Remove the soil and see if any sample is wet {;» t M

enough to form balls. If not, repeat the experiment with modified quantities 11 j

of water. The optimum moisture should be just a little less than this j j! j'

amount. jl^jlfi

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Total Nitrogen.—The relative amount of nitrogen in soils ;ij«j

varies greatly, although it is usually approximately in proportion bj * j

to the organic matter. A soil in Manitoba is reported to contain fl,)

as high as 20,100 Ib. per acre (1.005 per cent) while a sample jljij

from the " Jack Pine" plains of Michigan is said to contain only }jj \ I jjjl

740 Ib. per acre of 2,000,000 Ib. (0.0037 per cent) of soil (4| jf j |

Nitrogen is one of the most important of all elements in the
soil. It is absolutely essential to plant existence and it cannot
be taken from the abundant supply of the air by the plant
itself. Certain forms of soil bacteria cause the fixation of this
elementary nitrogen in the form of nitrates, which can then be

utilized by the plant. The chief purpose of nitrogen in plant t^\ j

economy is to provide for the construction of protein by the ,*j'

plant. The deep green color of plant leaves is often an indica- ;'j

tion of an abundance of available nitrogen. ,jn

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Determination of Total Nitrogen.—Place 10 gm (5 gm if a muck soil) f ]

of 40-mesh soil and 30 cc of concentrated sulphuric acid in a 500-cc Kjeldahl jf

flask. Proceed as described on page 151, and following. Calculate the «Jj s

per cent of nitrogen. 1

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Nitrate Nitrogen.—The amount of nitrate nitrogen present in jjjj

a soil depends mainly upon the amount and kind of vegetation, |||

and upon the degree of compactness, the temperature and the f J
water content of the soil. The most favorable temperature
seems to be about 35° and the most favorable water content is
one-third of its saturation. These factors largely determine

whether or not a soil is suitable for bacterial development. gH|

The amount of nitrate present in a soil at any one time is &**
seldom very large, ranging from zero to 1000 Ib. per acre (0.05

per cent). It is difficult to find more than a trace of nitrate sij

nitrogen in soil just under an old sod whereas in some western *'

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