MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 229 



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 their 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 inner surface of one 

 cubic foot 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 

 three or four acres. These differences are to be expected, since, as is 

 shown by Lyon and Pippin, 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 concen- 

 tration 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. 



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 scarcely 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 

 at the surface causes the film of capillary water to stretch more and more. 

 Finally it becomes very thin, breaks, and ceases to be continuous. The 



