PLANT-FOOD AND AGRICULTURE. 227 



The land must contain its fertile elements diffused throughout 

 the root area, so that the plant may be provided with food at each 

 absorbing rootlet during each period of its growth. To accom- 

 plish it, we must have some knowledge of the action of each of 

 the principal elements of fertilizers in the soil. 



The elements ordinarily requisite to be supplied to our land are 

 phosphoric acid, potash, and nitrogen ; the other elements of 

 plant-growth being either present in the soil in sufficient abund- 

 ance, or not concerning us as necessarily being applied in the 

 mixtures which we must use, in order to furnish the above named 

 substances. 



Phosphoric acid is one of the principal compounds of bone, in 

 the form of a phosphate of lime, and is furnished to commerce 

 either in bone, phosphate guanos, phosphate rock, or in the wastes 

 of commerce. It occurs in three forms : an insoluble condition, 

 in which it is unassimilable by the plant, a soluble form, wherein 

 it is readily absorbed, and an intermediate form, the reduced or 

 reverted form, of the soluble, wherein its adaptibility as plant- 

 food is questioned, but which we esteem of some value, on ac- 

 count of the condition in which it occurs, and the conditions under 

 which it is usually presented to the plant rootlets. It is commer- 

 cially prepared from bone or phosphate rock, by the addition of 

 sulphuric acid, which, uniting with a portion of the lime with 

 which the phosphorus is combined, leaves a soluble form. The 

 reaction may be expressed as below : 



Bone ash. Sulphuric acid. Superphosphate of lime. Sulphate of lime, 



3CaO, PO5 4- 2(H0, SO3) = 2 H 0, Ca P O5 + 2 (C a S O3) 



Three atoms of lime, f Diluted Sulphuric acid. Superphosph ite of lime, Gypsum, 

 combined with Phos- > united with water, 



phoric acid. j 



This product, the superphosphate, is the valuable portion of the 

 fertilizers placed on the market under this name, and it may com- 

 pose from ^ per cent, to 23 per cent, of their mass by weight of 

 soluble phosphoric acid. We thus see the importance of the sale 

 of fertilizers by guaranteed analysis under government control. 

 Ordinarily, a good superphosphate will contain about 10 per cent, 

 of soluble phosphoric acid, and a few per cent, of nitrogen, in 

 addition to insoluble and reverted phosphoric acid. We can esti- 

 mate its value on the basis of 12 cents a pound for the soluble 

 phosphoric acid, from 4 to 6 cents a pound for the reverted, and 

 1 or 2 cents for the insoluble. Yet these figures are subject to 

 changes. 



