294 NATUEE AND PROPEETIBS OF SOILS 



155. Fertilizer residues may be deyeloped in two gen- 

 eral ways: (1) by selective absorption by the soil; and (2) by 

 differential diffusion into the plant. Regarding the first case 

 (see par. Ml), it has already been established that soils 

 ordinarily absorb the basic ions more strongly than the acid 

 radicals, thus tending to leaye an acid residue in the soil solu- 

 tion. Sodium nitrate, ammonium sulfate, calcium nitrate, 

 potassium chloride and potassium sulfate, therefore, tend to 

 produce an acid residue, when they are first added to a soil. 



The final result, however, cannot be determined until the 

 action of the crop is known. If the crop especially utilizes 

 the cation or basic radical, it will intensify the selective ab- 

 sorption of the soil and a still more pronounced acid residue 

 will result. This would be the case with ammonium sulfate, 

 potassium sulfate, and potassium chloride. If, however, the 

 anion or acid radical is utilized to the greater extent, the ac- 

 tion of the soil absorption would be nullified and an alkaline 

 residue would tend to develop. This is especially true with 

 sodium nitrate when applied in large amounts over a term of 

 years, the physical condition of the soil becoming impaired 

 due to the presence of sodium carbonate.^ 



One other condition is possible. If the plants should use 

 the cation and anion of a fertilizer salt in equal proportions, 

 no residue would result. This seems to happen to an approxi- 

 mate degree with ammonium nitrate, potassium phosphate, 

 potassium nitrate, and ammonium phosphate. Such salts are 

 extremely valuable in long-continued experiments, where the 

 disturbing effects of fertilizer residues are to be avoided. 

 Monoealcium phosphate, the important constituent of acid 

 phosphate, needs especial consideration. When added to the 

 soil, it immediately reverts to the tricaleium form if active 

 calcium is present.^ Even with the large amount of gypsum 



^ Hall, A. D., The JSffect of the Long Continued Use of Sodium Nitrate 

 on the Constitution of the Soil; Trans. Cheni. Soe. (London), Vol. 85, 

 pp. 950-971, 1904. 



^CaH,(P0,)2 + 2CaH,(C03)a = Caa(PO.)2 + 4H,0 + 400^ 



