No. 6. DEPARTMENT OF AGRICULTURE. 403 



lime takou up by the ijlaut.s, aud especially by their leaves, as having 

 a iiual use beyond the plant organism, in that of the ailinial browsing 

 upon it. 



Turning now to some of the uses of lime in the soil, let us consider 

 in the first place, its power to neutralize acids. 



Acidity may be caused in a soil hy the oxidation of sulphide, such 

 as iron pyrite^s; the occurrence of such acidity is principally confined 

 to soils lying below certain exposed coal beds and to alluvial bottoms 

 affected by the acid water from mines. Acidity is most frequently 

 due to the presence of an excess of humus acids formed upon the 

 decay of pre-existing vegetation; thi^s excess may arise from the 

 presence of a very large amount of humus in the soil, as in swamps 

 aud sometimes in forests and old meadows; or it may occur w^here 

 there is no large amount of humus, but where the soil contains little 

 basic material capable of neutralizing even the small amount of 

 humus acids present. Hence both in lowlands where the accumula- 

 tion of food supply brought by drainage from the surrounding hills 

 has produced a heavier growth of vegetation, and where standing 

 water has prevented its rapid and complete decay, and on. uplands 

 deprived of lime and other basic constituents by the very drainage 

 that enriched the lowlands, may injury from acidity appear. The 

 third cause of acidity is in the nature of the fertilizers applied; not 

 only do acid phosphates tend to produce a slight aciditj-, but neutral 

 salts, such as potassium sulphate and muriate, ammonium sulphate 

 and calcium sulphate, whose bases the plant usually assimilates more 

 largely than their acids, also give rise after a time, to a pronounced 

 acid condition unless the soil contains a large excess of basic mate- 

 rials or unless fertilizers of opposite tendency, such as nitrate of 

 soda or neutral phosphates are sufficiently employed. 



Whatever the source of the acidity, most agricultural plants are 

 injuriously affected by it. W. Maxw^ell (14) has recently experi- 

 mented to determine the susceptibility of various orders of agricul- 

 tural plants to an excess of acids. He used weak solutions of citric 

 acid for the purpose. An excess of 0.1 to 0.02 per cent, of this acid in 

 a soil quickly killed clovers and members of the mustard family, to 

 which the cabbage, rape, turnip, cress and radish belong, while 

 lupines, vetches and beans vegetated but produced no seed; wheat 

 and barley were badly injured, and corn blossomed but did not ear. 

 Pearl millet, on the other hand, seemed to prefer a sour soil and with- 

 stood the action of even one per cent, of citric acid. Other evidence 

 on the same subject is presented in Bulletin Gl. 



Not only do our crops sutler direct injury from the presence of 

 acidity, but fail also to derive the benefit of the full activity of the 

 lower organisms inhabiting the soil. It is to-day quite generally 



(14) Jahresbincht Agrix-Chem., 1888, 42. 



