SOILS	267
If no red color has developed in the extract, the soil is already basic.	+.<
In this case, add from a burette tenth-normal alcoholic solution of hydro-	{
chloric acid until a pink color develops after standing several minutes, agitat-	»I*
ing after each addition and then allowing the soil to settle.    From the	! |
volume of acid used calculate the calcium carbonate equivalent of the soil.
If there is any indication of a green color developing after the disappearance
of red and after standing over night, add 5 cc more of base. If a green color	j.j j
should develop, it would require from 40 to 50 cc (corresponding to 4 to 5
tons of limestone) of base, in addition to that added to remove the red color.
Hopkins Method.—The acids of the soil (existing in equilib-
rium with partly hydrolyzed salts, as shown in equation
(1), page 265) are not easily extracted with water. If a solution
of potassium nitrate is added, such a reaction as the following	|fl
may occur:
H.A + KN0_3 *=» KA + HN0_3
Equilibrium   is   established with the weakly ionized acid pre-	*
dominating  but if the solution is removed and replaced by
more potassium nitrate solution, the reaction will proceed still
farther.    By  repeating this process several times, a result is
finally obtained, approximating complete extraction of the acid.
It has been found by working with a number of different soils
that the sum of the acid of such a series of extracts is about two
and one-half times that of the first extract.    In the Hopkins
method the assumption is made that the value of the first titra-
tion may be multiplied by 2.5 to give total acidity.    The method
seems to be more reliable with clay and loam than with muck	j   j
soils.	i^!|||
Determination of Acidity of Soil: Hopkins Method.—Place 100 gm of soil	j
and 250 cc of normal potassium nitrate solution in a 400-cc wide mouthed	j
bottle, stopper and shake continuously in a machine (Fig. 51) for three hours,	1
or every half hour for three hours by hand.    Allow to stand for fifteen hours.	j
Draw off 125 cc of the clear solution, using a pipette, boil for 10 minutes to	j
expel carbon dioxide, cool and titrate with tenth-normal sodium or potassium	I
hydroxide, using phenolphthalein as indicator.    Multiply the figure so	j
obtained by 2.5 and calculate the number of pounds of calcium carbonate	«
required per acre of 2,000,000 Ib. of soil.	i
The titrations of duplicate samples should not differ by more than 0.8 cc	»] *^!
for soil samples requiring less than 100 cc of sodium hydroxide.	" I
Active Plant Food.—The amount of nitrogen, phosphorus and
potassium that may be made available in a soil during a given	I
1