III. THE SOIL 49 



It appears to have been demonstrated that the organic matter of 

 soils, usually termed " humus," is highly complex in constitution, that 

 it is closely associated or combined with certain mineral constituents 

 of the soil, especially with portions of the potash, lime, phosphoric 

 acid, sulphur and iron ; that nitrogenous compounds are invariably 

 present, mainly in the form of protein -like substances, 1 and that it, or a 

 portion of it, is possessed of acid properties and can form compounds, 

 known as humates, with bases. 



A number of substances obtained from soils have been identified 

 recently, by American investigators, especially Schreiner and Shorey, 

 to some of which, e.g., dihydroxystearic acid and picoline carboxylic 

 acid, powerful toxic effects upon plants are ascribed (vide Chap. IV.). 

 But whether these substances were actually present in what may be 

 regarded as normal "humus" or were derived from the unchanged or 

 little changed residues from the crops recently growing in the soil, 

 appears to the writer to be doubtful. The same remark also applies 

 to several other substances isolated by American investigators from 

 soils, e.g., agroceric acid, paraffinic acid, lignoceric acid, agrosterol, 

 pentosans, xanthine, fatty glycerides and resin acids. Probably, in a 

 soil which had just borne a crop of mustard, a careful examination 

 would disclose the presence of allyl isothiocyanate, derived from the 

 plant debris. 



In fact, " humus " can only be regarded as a complex mixture of 

 various decomposition products formed in the various stages of the 

 complicated chain of reactions attending the ultimate conversion in the 

 soil, of all the organic compounds of vegetable (and animal) tissues into 

 the simple compounds, carbon dioxide, water and nitrates. It has 

 thus no permanent or definite chemical composition or constitution. 

 The more knowledge we can obtain as to the composition and functions 

 of its various transition compounds, the better, but it is evident that 

 at any point in the process of decay, the composition of the substance 

 will be highly complex and must depend largely upon the nature of 

 the original organic material from which it has been derived. 



CLASSIFICATION OF SOILS. 



Soils are usually divided by practical agriculturists into 



Sandy soils, containing less than 10 per cent clay and less than 3 

 per cent calcium carbonate. 



Loams, containing from 40 per cent to 70 per cent clay and less 

 than 3 per cent calcium carbonate. 



Clay soils, containing from 70 per cent to 95 per cent clay and 

 less than 3 per cent calcium carbonate. 



Marls, containing from 5 per cent to 20 per cent calcium carbonate, 

 the rest mainly clay. 



1 Suzuki (Bull. Coll. Agric., Tokio, 1907, 7, 513; Jour. Chem. Soc., 1908, ii. 

 Abstracts, 127) obtained, by the action of strong hydrochloric acid on humus, various 

 amino-acids, characteristic of the products of hydrolysis of proteids, e.g., alanine, 

 leucine, amino-valeric acid, aspartic acid, histidine and tyrosine. Schreiner and 

 Shorey (Jour. Biol. Chem., 1910, 8, 381) also separated various cleavage products 

 of protein from soils. 



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