JACOB G. LIPMAN 347 



material is most commonly applied in the row or hill with the aid of fertilizer-dis- 

 tributing machinery. The local application of the soluble salts establishes, in the 

 vicinity of the seed or plant, detached spots of soil in which the solution is more con- 

 centrated than it is in adjacent spots. All of this has an important bearing on the 

 activities of the soil bacteria and other micro-organisms. It has been shown experi- 

 mentally that where only small quantities of fertilizer are employed the farmer really 

 fertilizes the soil micro-organisms whose stimulated activities lead to a more rapid 

 decomposition of the soil organic matter and the release of increased amounts of 

 nitrates, phosphate, sulphates, and carbonates for the nutrition of the plants. Where 

 larger amounts of fertilizer are used the plant food removed by the crops may be more 

 or less completely replaced and the microbiological activities intensified at the same 

 time. It has been shown, particularly, that nitrogen salts help the micro-organisms to 

 utilize the more resistant forms of organic matter and to amplify thereby the supply 

 of available plant food. Similarly, when the soil reaction is right the use of mono- 

 calcic and di-calcic phosphates makes conditions more favorable for the fixation of 

 atmospheric nitrogen by Azotobacter and other nitrogen fixing bacteria. 



The consideration of commercial fertilizers in their relation to soil microbio- 

 logical activities should reckon also with certain specific influences of individual 

 salts. Thus ammonium salts when employed in relatively large amounts will give us 

 a somewhat different microbiological picture than will equivalent quantities of 

 nitrates. Different fertilizer salts will likewise exert an indirect influence on the 

 micro-organisms by modifying plant growth and through it soil reaction, moisture 

 relations, and the diffusion of gases. On the whole, the relation of commercial fertiliz- 

 ers to soil-microbiological activities is not fully understood. Fertilizer practice is 

 changing, new fertilizer salts are constantly appearing, and new methods of applica- 

 tion are being developed. Nevertheless, present-day information is sufficiently ac- 

 curate and extensive to justify the statement that chemical fertilizers are a potent 

 means of modifying soil-microbiological activities and through them the quantity 

 and quality of the harvested crops. 



Irrigation and drainage are both significant soil-fertility factors when considered 

 in their relation to microbiological activities. Like higher plants, bacteria and other 

 micro-organisms will become less and less active as the water supply is reduced below 

 the optimum. In the case of plants, wilting will occur when the thickness of the 

 moisture film on the soil particles is unduly reduced. It has been shown by Briggs 

 and Shantz that in coarse sand, wilting will take place at 0.9 per cent of moisture; in 

 fine sand at 2.6 per cent; in fine sandy loam at 9.7 per cent; and in clay loam at 16.3 

 per cent. In heavy clay soil, wilting wiU take place with a still higher percentage of 

 water, while in peat soils there is danger of wilting when the supply of water is re- 

 duced below 40 per cent. The differences just noted may be readily accounted for by 

 the extent of the inner surface in soils of different texture. The finer grained the soil, 

 the greater its inner surface and the thinner the moisture film produced by any given 

 quantity of water. Hence a light rain, or a relatively small amount of irrigation water, 

 would start microbiological activities and plant growth in a sandy soil, whereas a much 

 larger supply of water would be required to produce like results in a dry silt or clay 

 soil. On the other hand, during periods of drought, crops on the lighter soils would 



