24 BULLETIN 699, U. S. DEPARTMENT OF AGEICULTUKE. 



equal aiiiounts of phosphoric acid to the soil, the applications usually 

 being at a rate supplying phosi^horic 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 j&rst method of comparison has been the one 

 emploj^ed 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 compari,son 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 appli- 

 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 



