REPORT OF THE CHEMIST. 163 



These exist in very different proportions in different soils. The first 

 three, sand, day, and lime, represent more than 90 per cent, of the sub- 

 stance of most soils, and as one or the other jDredominates the soil is said 

 to be sandy, clayey, or calcareous. The most active constituents of the 

 soil, i)hosplioric acid, and the two alkalies, potash and soda, occur in very 

 small quantities, as do the other and less important constituents, mag- 

 nesia, chlorine, and sulphuric acid. 



Silica exists in different proportions in the various soils, mostly in an 

 insoluble state, and that most largely in the poorest sandy soils ; fer- 

 tile soils contain g-enerally a very small quantity of it in a soluble form. 

 Sandy soils contain from 70 to 90 per cent, of silica; even stiff clay soils 

 from 60 to 70 [)er cent. ; and calcareous or lime soils and marls from 20 

 to 30 per cent. 



Its value, as a source of plant food, consists in being in the form of 

 soluble silicates. In its insoluble state, like quartz sand, its action is 

 nearly mechanical, making the soil lighter for cultivation. Those soils, 

 derived from rocks of which feldspar is one of the constituents, will con- 

 tain some silica in a soluble form, whilst those derived from quartzose 

 rocks will contain it in the insoluble state. The hydrated silica, in the 

 analyses, represents that Avhich is gradually available for plant food. 



Alumina, or clay, is a silicate of aluminium, and it is derived from 

 the disintegration of feldspathic rocks and other similar silicates ; if ab- 

 solutely pure it would furnish nothing for ])lant food; as, however, this 

 is seldom tlie case, it furnishes a supply (jf potash frequently in con- 

 siderable quantities. Clay has the important i»roperty of absorbing and 

 retaining i)hosphoric acid, ammonia, potash, lime, and other substances 

 necessary for i)lant food. Clay soils contain on an average from 6 to 10 

 per cent, of alumina. In sandy soils it varies from 1 to 4 per cent., and 

 in marls, calcareous soils, and vegetable molds from 1 to 6 per cent. 



The presence of alumina in the soil is purely mechanical, as it is never 

 found in the mineral jiortions of plants, and the larger the percentage 

 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 analy- 

 sis used is but an imperfect indication of the amount of day 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 ]irolonged. 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 present. 

 This, however, is seldom the case. 



It is but liirely that the amount of silica dissolved satisfies the requirement for 

 combining with theaJamina into kaolinite, and in a very few cases there is an excess 

 of silica over that requirement. In numerous cases the silica falls so far below the 

 amount corresponding to the alumina as to raise a serious question as to the combina- 

 tion in wliich tlie latter occurs in the soil, the hi/draie (Gibbsito) being almost tlifc 

 only possible one, apart from zeolitic minerals. Perhaps this fact may serve to ex- 

 plain some of the otherwise incomprehensible variations in the physical properties of 

 soils whose chemical and mechanical analysis would seem to make them almost 

 identical. In some of the Tertiary and prairie soild of the .Southern States, more- 

 over, there seems to occur still another amorphous mineral, related to or identical 

 Avith saponite, which sometimes occurs in segregated masses and imparts to these 

 .ooila very peculiar and unwelcome properties in tillage. AVo are evidently, as yet, 

 very far from a full understanding of the mechanical constitution of soils. (E. W. 

 Ililgard, Tenth Census, U. S.) 



The lime 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- 



