22 



The presence of alumina in the soil is purely mechanical, as it is 

 never found in the mineral portions of plants, and the larger the per- 

 centage of it present the more difficult the soil becomes to cultivate,, 

 offering a greater or less resistance to the implements of tillage. 



The percentage of alumina, as found by the method of chemical an- 

 alysis used, is but an imperfect indication of the amount of clay in the 

 soil. The amount of alumina continues to increase long after the rest of 

 the important substances have been dissolved, if the digestion in hot 

 dilute acid be prolonged. If this was combined as a hydrous silicate 

 the amount of hydrated silica found, by boiling the insoluble residue 

 with sodium carbonate, should bear a certain ratio to the alumina pres* 

 ent. This, however, is seldom the case. 



" It is but rarely that the amount of silica dissolved satisfies the re- 

 quirement for combining with the alumina into kaoliuite, and in a very 

 few cases there is an excess of silica over that requirement. In numer- 

 ous cases the silica falls so far below the amount corresponding to the 

 alumina as to raise a serious question as to the combination in which 

 the latter occurs in the soil, the hydrate (Gibbsite) being almost the only 

 possible one, apart from zeolitic minerals. Perhaps this fact may serve 

 to explain some of the otherwise incomprehensible variations in the 

 physical properties of soils whose chemical and mechanical analysis 

 would seem to make them almost identical. Ii- some of the Tertiary and 

 prairie soils of the Southern States, moreover, there seems to occur still 

 another amorphous mineral, related to or identical with Saponite, which 

 sometimes occurs in segregated masses, and imparts to these soils 

 very peculiar and unwelcome properties in tillage. We are evidently r 

 as yet, very far from a full understanding of the mechanical constitu- 

 tion of soils." (E. W. Hilgard, Tenth Census U. S.) 



The Ume or calcareous matter, generally occurring in the state of car- 

 bonate, varies in soils from about 90 per cent, and under in limestones 

 and marls, to mere traces in some other soils. Clays and loams gener- 

 ally contain from 1 to 3 per cent, of the carbonate. Less than 1 percent- 

 may be regarded as a defective quantity. In the lightest sandy soils 

 the percentage of lime should not fall below 0.100, in clay loams not 

 below 0.250, and in heavy clay soils not below 0.500. (Hilgard.) Where 

 a soil is deficient in lime, the little there is of it is present in combina- 

 tion with the organic acids, and is more abundant on the surface than 

 in the subsoil. It preserves the particles of clay in a seperate coagu- 

 lated condition, and thus allows them to exercise their absorbent powers 

 on various salts, which otherwise would escape their action. It also 

 promotes the decomposition of vegetable matter and the formation of 

 nitrates in the soil. 



Most green crops are often subject to disease, when grown on soils 

 deficient in lime, even when they have been well manured. Up to a 

 certain stage the cereal or other crops appear to thrive well, but as the 

 season advances they sustain a check, and yield a poor harvest. This 



