The nature and composition of soil organic matter

The comparatively marked initial variations in the nature of soil organic matter taken from different locations are related closely to the vegetative cover and climatic conditions that prevailed for hundreds of years prior to their being used for the growing of cultivated crops. These conditions as commonly found in virgin forest, grassland, prairie and desert soils are broadly outlined in Chapter 7 and need not be repeated here. 

Cultivation hastens decay processes and the crop residues are rapidly converted into 

Bog soils, commonly described as peat or muck, are discussed in some detail in Chapters 29 and 30, but need to be mentioned here because most of the chemical studies of humus have been made on humus extracted from these soils rather than from mineral soils. This is in some respects unfortunate because humus from the two sources is not exactly the same, although similar. Bog humus is used in such studies chiefly for practical reasons it is more readily obtained in quantity from peat or muck soils and contains much less clay. In mineral soils much of the 1—5% humus usually present is there in stable combinations with the inorganic portion of the soil and cannot be removed except by very drastic chemical treatments that may markedly decompose or otherwise change the organic matter. 

CHEMICAL NATURE OF SOIL ORGANIC MATTER Some early concepts of humus and its formation 

Much effort has been exerted during the past 50—75 years in attempts to determine the chemical nature of humus and its mode of formation. Waksman (1938) has summarized the earlier studies adequately, but it does seem advisable to make specific reference to the paper by Schreiner and Dawson (1927) and to Waksman s proximate analysis method. 

---