292 MICROBIOLOGY OP SOIL' 



yond a certain point, the attraction between the molecules in the soil 

 grains and the more distant molecules of water is no longer great 

 enough to overcome the force of gravitation, and the excess of water 

 percolates downward. The water more or less readily moved by 

 gravitation is called hydrostatic water. 



For any given conditions of the soils the amount of hydrostatic, 

 capillary and hygroscopic water is directly dependent on the mechanical 

 structure. It is evident that the aggregate surface of the particles in 

 a fine-grained soil is much greater than that in a coarse-grained soil. 

 Actual determinations have shown that the aggregate irmer surface 

 of .02832 cm. (i cu. ft.) of coarse sand may be but a fraction of an 

 acre; whereas the same quantity of the finest clay may have an 

 inner surface equivalent to 1.2141-1.6188 hectares (3 or 4 acres). 

 These differences are to be expected, since, as is shown by Lyon and 

 Fippin, I g. of fine gravel may contain 252 particles; i g. of medium 

 sand, 13,500 particles; i g. of very fine sand, 1,687,000 particles; i g. 

 of silt, 65,100,000 particles, and i g. of clay, 45,500,000,000 particles. 



Since the soil water is spread as a film over the solid particles and 

 varies in amount with the fineness or coarseness of the soil, and since 

 the quantity of plant food going into solution is determined largely 

 by the amount of water in contact with the soil particles, it follows that 

 clay soils will, under the same conditions, contain more plant food in 

 solution than loam soils and still more than sandy soils. From the 

 standpoint of soil microbiology this is important, for the microorganisms 

 live and multiply in the film water surrounding the soil particles. The 

 concentration of salts in this film water as well as their composition 

 must of necessity affect bacterial activities. In the same way, methods 

 of tillage and cropping affecting the concentration and composition 

 of the film water will modify the chemical changes caused by bacteria 

 and other microorganisms. 



Effect of Drought and of Excessive Moisture. — Optimum 

 conditions for plant growth and the development of many important 

 soil bacteria are furnished when about half of the entire pore space is 

 filled with water. In light sandy soils the optimum moisture content 

 may be reached when the wet material contains sarcely more than 8 

 to 10 per cent of water by weight; while in silt and clay soils the 

 optimum may reach 16 to 20 per cent, or even more. 



Continued depletion of soil moisture by plant'roots and evaporation 



