100 NATURE AND PROPERTIES OF SOILS 



organic matter is plant "[issue. 1 Some of this matter ac- 

 cumulates from the above-ground parts of plants that have 

 died and fallen down to become mixed with the surface soil; 

 the remainder is a result of root extension and subsequent 

 decay. The organic matter of the surface soil is derived from 

 the tops and the roots of plants growing on it, while that of 

 the subsoil is very largely a result of root extension and sub- 

 sequent decomposition. 



Since soil organic matter has its origin very largely from 

 the higher plants, it is advisable to consider the general chem- 

 ical nature of such material. 2 About 75 per cent, of average 

 green plant tissue is water. The dry matter is made up of 

 carbon, oxygen, hydrogen, and mineral material in the ap- 

 proximate ratio of 6, 5, 1 and 1 respectively. The preponder- 

 ant elements of normal plant tissue are evidently carbon, 

 oxygen, and hydrogen. (See Fig. 20.) 



It is usual in classifying the compounds in plants to group 

 them under the following heads: (1) carbohydrates, (2) 

 fixed oils and waxes, (3) volatile oils and resins, (4) organic 

 acids and their salts, and (5) nitrogenous compounds. 3 The 



1 It must not be inferred that higher plants are the only source of soil 

 organic matter. Assuming that the weight of one bacterial cell is 

 .000,000,002 of a milligram and that in each gram of a normal fertile 

 soil, weighing 2,000,000 pounds to an acre-seven inches, there are 

 100,000,000 of such organisms, the weight of bacteria alone would be 

 400 pounds to the surface acre. This is a very conservative estimate, 

 800 pounds probably being more nearly correct. Considering the molds, 

 fungi, algae, actinomycetes, insects, and earthworms, there are probably 

 2000 pounds of living material in every acre of normal soil exclusive 

 of plant roots. These organisms in their functioning supply no insig- 

 nificant portion of the soil organic matter. 



a For a fuller discussion see: Ingle, Herbert, Manual of Agricultural 

 Chemistry, Chap. X, London, 1913. Also, Stoddard, C. W., The Chem- 

 istry of Agriculture, Chap. Ill, Philadelphia and New York, 1915. 

 Also, Thatcher, R. W., The Chemistry of Plant Life, New York, 1921. 



3 1. Carbohydrates — Sugars, starch, cellulose, legnin, inulin, gums, 

 pectins, and pentosans. 



II. Fixed oils and waxes — Castor oil, corn oil, cottonseed oil, linseed 

 oil, and the like. 



III. Volatile oils and resins— Oil of mustard, of cloves, of pepper- 

 mint, etc. Rosin, myrrh, balsam, etc. 



