ACID, OB SOUE, SOILS 381 



several decades, after which it becomes more and more 

 difficult to obtain a crop until the attempt must finally 

 be abandoned. The change from forest to tillage has 

 opened the way for an acid condition of soil, through the 

 loss of bases carried off in the crops and the destruction 

 of humus by tillage. The dissipation of humus is doubt- 

 less the more serious source of loss. Instances may be 

 cited in which a farm has been so managed as to main- 

 tain the humus supply and the ability of the soil to pro- 

 duce red clover, although surrounding farms, on which 

 humus has been depleted, have completely failed to grow 

 this crop. 



Apparently humus holds the basic constituents of the 

 soil in a form in which they function as rather easily 

 soluble salts, instead of locking them up as insoluble 

 silicates. A given quantity of base in a soil is therefore 

 more effective in preventing acidity by combining with 

 weak acids, and possibly in forming carbonates, if the soil 

 is well supplied with humus than if it is lacking in that 

 constituent. 



287. Effect of fertilizers on soil acidity. — That the 

 continued use of ammonium sulfate on land may result 

 in producing a sour condition has been shown by a num- 

 ber of investigators. The absorption and nitrification 

 of the ammonia of that salt, and its final utilization by 

 plants, leaves sulfuric acid, which combines with calcium 

 and escapes in the drainage water. This may occur even 

 when this fertilizer is used in quantities not excessive, 

 but continued for many years, as has been shown by 

 Gardner and Brown ^ at the Pennsylvania Experiment 



1 Gardner, F. D., aad Brown, B. E. The Lime Require- 

 ment of the General Fertilizer Plats as Determined Penodieally.. 

 Rept. Pennsylvania Agr. Exp. Sta., 1910-1911, pp. 25-60. 



