96 THE BOOK OF CORN 



or, in other words, these products contain those con- 

 stituents which are likely to be deficient, and are valu- 

 able mainly for this reason. 



In the third place, it must be remembered that 

 manures, both direct and indirect, differ in respect to 

 the character of the constituents that are contained in 

 them. The plant can obtain its food only in a soluble 

 form, and all materials containing plant food in an 

 insoluble form must change to the soluble state, and 

 the different materials containing these constituents 

 differ widely in the rate at which they change from an 

 insoluble form to a soluble and available one, even 

 when used under the same conditions of soil and cli- 

 mate. Those materials containing constituents that are 

 soluble are in most cases called immediately available, 

 and those containing them in an insoluble state vary in 

 their value according to the rate at which they will 

 change from an insoluble to a soluble form under the 

 average conditions of soil, season and climate. 



In the farm manures, for example, the solid por- 

 tion contains the chief constituents, nitrogen, phos- 

 phoric acid, in an insoluble form, while the liquid por- 

 tion contains them in a more soluble state, and experi- 

 ments have demonstrated that the nitrogen, particularly 

 in the liquid portion, is more easily obtained by the 

 plants than that in the solid. Of the same amount of 

 nitrogen in the solid and the solid and liquid portions 

 combined, the immediate crop will use three times as 

 much of the latter as of the former. It is, therefore, 

 not altogether a question of manure, but of the quality 

 of the constituents contained in it. The same principle 

 is true in regard to the nitrogen in the different arti- 

 ficial products — there is a wide range of availability. 

 Nitrate of soda contains it in its most soluble and avail- 

 able form, and the returns from a unit of nitrate nitro- 



