SOILS AND FERTILIZERS 457 



difference in actual plant food, the high grade materials as a rule 

 being cheaper, pound for pound of actual plant food. 



FERTILIZING MATERIALS. 



The term form, as applied to a fertilizing constituent, has refer- 

 ence to its combination or association with other constituents, which 

 may be useful, though not necessarily so. The form of the constit- 

 uents, too, has an important bearing upon its availability, and hence 

 upon its usefulness as plant food. Many materials containing the 

 essential elements are practically worthless as sources of plant food, 

 because the form is not right; the plants are unable to make use of 

 them they are unavailable. In many cases the form of these ma- 

 terials is changed by proper treatment and they become valuable, 

 not because the element itself is changed, but because it then exists 

 in such form as readily to feed the plant. 



Commercial mixed fertilizers are prepared by mixing together 

 acid phosphate, nitrogenous materials, and potash salts in the pro- 

 portion necessary for the composition desired. In some fertilizers 

 the phosphoric acid comes partly from bone or tankage. These 

 fertilizers contain only a small proportion of their available phos- 

 phoric acid in the water-soluble form and they contain a consider- 

 able quantity of insoluble phosphoric acid. Fertilizers made with 

 acid phosphate alone contain two-thirds or more of their phosphoric 

 acid in the water-soluble form and only about one-sixth of the total 

 quantity present is insoluble. Acid phosphate is considered as a 

 more desirable source of phosphoric acid in mixed fertilizers than 

 bones or tankage. 



After the proper materials have been weighed out, they are 

 mixed thoroughly and passed through a coarse sieve to insure a 

 uniform degree of fineness. Any other material which does not 

 pass through the sieve is ground up and added to the mixture. Some- 

 times the ingredients must be ground up before they can be mixed, 

 as they may have become hard and lumpy. 



A large number of different substances containing the fertil- 

 izer elements necessary for the growth of crops are found stored in 

 nature, in rocks, soils, or deposits of various kinds. Refuse-prod- 

 ucts in several important manufacturing industries furthermore fur- 

 nish one or more of these elements. These substances are placed 

 on the market and sold under the general name of commercial fer- 

 tilizers. They may be of mineral, vegetable, or animal origin, and 

 supply nitrogen, phosphoric acid, or potash, or mixtures of these 

 ingredients. 



Substances of this kind have been applied to cultivated soils 

 from time immemorial, and experience has taught farmers that the 

 results secured by applying them may fully equal and even excel 

 those realized from the use of barn yard manure. A number of the 

 commercial fertilizers are easily soluble in water, and, as a result, 

 quick-acting, while others have to go through a process of decompo- 

 sition in the soil before they can be of much 'benefit to crops. 



Grade of Materials. Materials that do not show a wide variation 

 in composition, and in which the constituents are practically uniform 



