342 SOIL FERTILITY 



prominent role in determining the growth and distribution of plants and, in conse- 

 quence, also of micro-organisms. The fact that the latter carry on their activities 

 largely in the moisture films surrounding the soil particles, that these films represent 

 solutions of varying composition and concentration, that the organic and inorganic 

 colloidal material in the soil is the seat of many chemical reactions, and that the 

 intensity of biological changes is directly reflected in the intensity of chemical changes, 

 is evidence that chemical and biological soil factors are intimately related. 



Organic matter is the direct source of food and energy of the heterotrophic soil 

 organisms. It is the indirect source of energy for most of the autotrophic organisms. 

 For this reason too much importance cannot be attached to organic matter as a link 

 in the chain of soil fertility. Aside from the organic matter of recent origin found on 

 the surface of the soil and within its tilled portion, there are older residues in the sur- 

 face and subsoil of plants, soil-infesting insects, and micro-organisms. The quantity 

 of organic matter will ordinarily vary from less than i per cent in light sandy soils 

 to more than 5 per cent in heavy silt or clay loams. In muck and peat soils the organic 

 matter content may range up to 85 per cent. An acre of surface soil 6| inches deep 

 is usually assumed to weigh 2,000,000 pounds; hence, at 2 per cent there would be 

 present about 20 tons of organic matter in this quantity of soil. In the next layer of 

 the same depth the quantity of organic matter is ordinarily only half as great. It 

 becomes relatively still less in the following layers. There is also a marked difference 

 in the quality of the organic matter at different depths, particularly as to the carbon- 

 nitrogen ratio. Everything being equal, the greater the depth from which the organic 

 matter is taken, the narrower its carbon-nitrogen ratio. This is due primarily to 

 variations in the proportions of carbohydrates and proteins brought about by micro- 

 biological activities. As would be expected, the older residues in the subsoil represent 

 material that had been used over and over again for the building of microbial cells. 



Excessive concentrations of soluble salts injure higher plants and depress the 

 activities of soil micro-organisms. In extremely acid soils soluble salts of aluminium 

 and iron often produce toxic effects. In some abnormal soils compounds of boron, 

 copper, nickel, arsenic, and manganese may be present in unduly large amounts and 

 exert toxic action. Certain organic compounds, toxic to higher plants, have been 

 isolated from soils. Some of these compounds may be injurious also to micro-organ- 

 isms. In modern fertilizer practice large amounts of chemical fertilizer are often ap- 

 plied in the row or hill. When such treatment increases beyond a certain point, the 

 concentration of the soil solution, seed germination, plant growth, and microbio- 

 logical activities may be interfered with. Altogether, therefore, toxic or injurious 

 substances may arise in the soil either as the result of the weathering of the rock 

 material or the chemical changes brought about by cropping and the use of com- 

 mercial fertilizers. 



Both positive and negative influences are exerted by micro-organisms in the soil. 

 The transformation by them of various organic substances involves the production of 

 large quantities of carbon dioxide, the formation of ammonia and nitrates, and the 

 mineralization of organic matter. As the end products of decay and fermentation, 

 carbonates, phosphates, sulphates, and other simple salts are again made available for 

 the use of plants. Nitrogen fixing bacteria, both symbiotic and non-symbiotic, may 



