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much potash a given crop will take out; but this will nut give you any idea of how- 
much nitrogen that crop will take out. We have been somewhat successful in 
measuring by chemical methods the amount of potash and phosphoric acid which a 
given crop will take from the soil. Chemists have tried various experiments and 
have obtained from them interesting data, but none of these experiments will give 
an entirely satisfactory answer to the nitrogen problem. However, from experi- 
ments with soils taken from various parts of the United States we obtained some 
reasonable idea as to how much nitrogen the soil will yield. These results assume, of 
course, that these soils furnished a reasonable amount of other plant foods; other- 
wise there would have been no crop at all. 
Another thing that we observe is this: Economy of production, as far as plant food 
is concerned, is attained always with a maximum crop — that is to say, a small crop 
takes a larger percentage of plant food from the soil in proportion to the organic 
matter than does a large crop. All our data go to prove that the farmer, if he seeks 
economical results, will produce a maximum crop, because for such a crop a propor- 
tionately less quantity of plant food is required. That is another point most defi- 
nitely brought out in these researches of ours into the elements of soil fertility. 
In our analytical work on the availability of plant food we use the methods of the 
Association of Official Agricultural Chemists, but in making the solutions we do not 
always use the official methods, because they are designed only for particular cases. 
Our solutions for this special investigation were made with dilute hydrochloric acid, 
using a large quantity of soil and a large excess of reagents, but the same for each 
soil. These are placed in a revolving apparatus where the contents are kept mixed 
all the time and the temperature is constant. 
We must not forget that temperature is an important function in solubility of all 
kinds — sometimes increasing it, sometimes diminishing it; therefore, in experiments 
to determine comparative solubility, the temperature must always be constant. All 
our experiments to determine solubility were made at what we considered an average 
temperature for summer, when the upper layers of the soil have probably a tempera- 
ture of 40°. Of course, the temperature may be much higher in some localities and 
lower in others, but in general when the soil is exposed to the full sunlight the tem- 
perature of the surface would be about 40°. 
In conducting these investigations a number of men in the Bureau of Chemistry of 
the Department of Agriculture — Mr. Ewell, Air. Moore, and others — collaborated. Mr. 
Ewell was particularly concerned in the investigation of the power of the soil to 
induce nitrification. We all understand that organic nitrogen is available only as the 
nitrifying organism prepares the food for the use of the plant. We must cook for the 
plant and prepare its food the same as for any other living being, and the nitrogen 
must be prepared especially for the use of the plant. The following experiments and 
methods were employed for this purpose: 
Samples of soils were taken from different localities and kept in sterilized tubes 
in such a way that no organism could be introduced into them from the time they 
were taken until the experimental work had begun. These were sent to the labora- 
tory and the nitrifying power of the soil was studied systematically in each case. In 
these experiments we used simply an ammonia salt — not wishing to study particu- 
larly the organism which produces ammonia, but rather that element in the soil 
which produces nitric acid from ammonia. Carefully protected ammoniacal mate- 
rial, to which no additional organism had been added, was used, and the material 
was then subjected to the ordinary processes of nitrification, with the results displayed 
upon these charts. 
The nitrous organism must begin its work before the nitric organism can act. At 
first the nitric organism is usually a little behind, but it soon overtakes its colaborer, 
so that after a few weeks the nitrous acid completely disappears. The work of the 
nitrous organism is overcome by the more vigorous action of the nitric ferment. 
In our experiments it was found that in nearly all the cases the lack of nitrifica- 
tion was not so much due to the absence of organisms as to the lack of a nitrifiable 
base. By the addition of lime to a soil which had a weak nitrifying power the latter 
was greatly increased. This shows the importance, especially in clover growth, of 
having some substance present in the soil, like lime, which will neutralize the nitric 
acid formed, and prevent it from exercising any toxic effect on the plant; for, 
strange to say, some of these plant foods do exercise a toxic effect on plants if allowed 
to accumulate, and acids, of which nitric acid is one, are illustrations of that fact. 
Thus the application of lime to a soil not only modifies the physical conditions, but 
also favors the development of nitrifying organisms. The result of these experi- 
ments is to demonstrate that in order to conserve the fertility of the soil the nitrify- 
ing organism must be cared for and must have something to work upon. The nitro- 
gen in the soil in organic matter is useless unless it is converted into nitric acid. 
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