CHAPTER XI 



ORGANIC CONSTITUENTS OF SOILS 



BY far the most valuable constituent of soils is the organic mate- 

 rial derived from -the plants and animals that have lived in and 

 on the soil. The term organic matter will he used to include all 

 material from organisms, to distinguish it from the term humus of 

 more restricted use. Humus refers, in its restricted meaning, only 

 to that portion of organic matter that is soluble in dilute alkali. 



Kinds of Organic Matter. Organic matter exists in the soil in 

 every stage of decay, from that whose cellular structure is still visi- 

 ble, to that very similar to coal. It may be divided into (a) active 

 or fresh, which decomposes readily; (b) the inert, which is 'usually 

 old and decomposes too slowly for the use of crops; and (c) the 

 coal-like material that oxidizes with extreme slowness, if at all, and 

 whose chief use is to impart a dark color to the soil (Figs. 74 and 

 To). The active is the most important and is that form which is 

 ordinarily supplied to the soil as manure and legumes. Under long- 

 continued, injudicious systems of cropping the active organic matter 

 is largely removed and the result is exhausted, " run-down " or 

 "worn-out" land. To maintain the productiveness the organic 

 matter must be supplied in considerable quantities and of a form 

 that will decay readily. It is equally essential to supply organic 

 matter in a more stable or less readily decaying form, as straw, 

 corn stalks or other non-leguminous material, since these benefit the 

 soil physically for a longer time than legumes. 



Amount of Organic Matter in Soils. The organic-matter 

 content of soils varies quite widely in the same locality. Even in 

 soils from which it has not been removed by erosion a distance of 

 a few rods may make a great difference in the amount. Soils con- 

 tain from a small fraction of a per cent to 90 per cent. Swamp 

 lands generally contain most, while sand soils contain least. 



How much organic matter a soil should contain is a question 

 often asked and one very difficult to answer. A soil may contain 

 five per cent of organic matter and be less productive than one with 

 only two per cent. Much depends upon its activity or rapidity of de- 

 composition. The chances for large yields are decidedly in favor of 

 the soil with a large organic content. A soil with a few tons of fresh 

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