24 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
equal amounts of phosphoric acid to the soil, the applications usually 
being at a rate supplying phosphoric acid sufficient for one year’s 
crop. (3) The several phosphates have been applied in quantities 
representing equal money values at the particular time and in the 
particular locality in which the experiment was conducted. 
Unfortunately, the first method of comparison has been the onef 
employed in many field experiments, and while the results obtained 
are of some value, the method is very illogical, both from a scientific 
and economic viewpoint, since there are wide differences in the phos- 
phoric acid-content of the various phosphates and the cost of these 
phosphates differs greatly. The only points in favor of such a 
method of comparison are that the amount of labor expended in 
spreading equal quantities of phosphatic material is the same and 
that all experiments conducted according to this method are to a 
certain extent comparable. 
If the role which the more soluble phosphates play in the soil was 
simply that of supplying plant food, and the cost of phosphoric acid - 
in its various forms was practically the same, the second method of 
comparison would be the logical one to follow. Moreover, all experi- 
ments conducted according to this method are also to a certain extent 
comparable. But water-soluble phosphates perform other functions 
in the soil which are often as important as the direct supplying of 
plant food to the growing crop. Not only do they affect the solubil- 
ity of the soil minerals, but they influence and stimulate bacterial 
life and alter the physical condition of the soil. Because of the ease 
with which they are distributed in the soil and the quantity of soluble 
sulphates with which they are usually associated (in acid phosphate), 
they can not but be more energetic in their action than equal amounts 
of phosphoric acid in the form of relatively insoluble phosphates, 
and therefore a comparison of the two classes of phosphates based 
on equal applications of phosphoric acid is almost certain to be favor- 
able to the soluble variety, even though the cost of the less soluble 
phosphates is usually considerably less. 
The third method of comparing the various classes of phosphates 
has much to recommend it, since the effectiveness of the insoluble 
phosphates must depend largely on their thorough distribution in the 
soil, a distribution which can only be brought about by heavy apph- 
cations of very finely ground material. Moreover, the relative value 
of two fertilizer materials must in the end be determined by the 
financial returns obtained from equal investments in the two forms. 
In many localities the price per ton of finely ground raw rock phos- 
phate is about one-half that of acid phosphate or basic slag and one- 
third that of bone meal. Since the average rock phosphate contains 
about twice as much phosphoric acid as acid phosphate, the same 
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