EFFICIENCY OF FERTILIZERS 357 



sorption as one of the bases in a poly-silicate, and thus 

 held in a condition very sparingly soluble in pure water. 

 Other reactions take place, and a portion of the potas- 

 sium in some form is doubtless mechanically held by the 

 soil particles. While this added potassium is more 

 readily obtained by plants than that contained naturally 

 in many soils, it must become available largely by the 

 processes by which the natural supply is rendered soluble. 

 Ammonium sulfate undergoes a somewhat similar pro- 

 cess, while the nitrate of soda remains in a soluble form. 



It is evident, therefore, that the conditions which 

 contribute to the natural fertility of the soil also apply 

 to that added as fertilizers, with the possible exception 

 of the nitrate. 



Phosphate fertilizers may be rendered practically 

 insoluble in pure water, when added to the soil, and in 

 the presence of a large amount of iron and aluminum it 

 forms more or less ferric and aluminum phosphate, 

 which becomes soluble very slowly, even under the 

 action of soil-water and plant-roots. When converted 

 into tricalcium phosphate, the phosphorus becomes 

 soluble more readily; but, in any case, its rate of solu- 

 bility depends upon those conditions which are most 

 favorable to the solubility of the natural soil phosphates. 



It is generally recognized that a sandy soil responds 

 more promptly to the application of fertilizers than does 

 a clay soil. There may be two reasons for this: (1) 

 Absorption may not be so complete both on account of 

 the particles being larger, and because in many sandy 

 soils the particles are largely composed of quartz, which 

 does not have the property of forming combinations 



