ACID, OB SOUR, SOILS 377 



tween the basic material and the absorptive substances 

 in the soil. Thus, a soil containing large quantities of 

 clay, and other absorbent substances requires more basic 

 material for the formation of calcium carbonate than 

 does a soil having less absorptive material. Further- 

 more, with the same original content of basic material, 

 the former soil requires a greater addition of lime to 

 overcome its sourness than does the latter. For this 

 reason a heavy soil usually requires a larger dressing of 

 lime to correct its acidity than does a light one. 



Even if a soil does not have its absorptive capacity for 

 bases satisfied, there is some formation of calcium car- 

 bonate constantly taking place, as is evidenced by the 

 removal of the bicarbonate of calcium in the drainage 

 water of soils that are distinctly acid. The benefits that 

 soils derive from the presence of calcium carbonate will 

 be mentioned later (pars. 454-457). It need only be said 

 here that its presence in insufficient quantity constitutes 

 a form of so-called acidity, or sourness, in soils. The 

 formation of calcium carbonate in a given soil increases 

 with the mass of base. The effect of an application 

 of lime, therefore, is to increase the quantity of car- 

 bonate_ formed, even when the absorptive capacity of 

 the soil is not satisfied. This is why even relatively 

 small applications of lime are beneficial to soils having 

 great absorptive capacity. 



283, Production of soiir soils. — Soils in a humid 

 region tend to become acid. This may be due to any one 

 or more of several causes: (1) removal of calcium and 

 other bases in drainage water; (2) removal of bases by 

 plants ; (3) formation of salts of the bases with organic 

 matter incorporated with soil ; (4) accumulation of acid 

 residues of fertilizers. 



