MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 2 29 



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 the amount of hydrostatic, capillary and 

 hygroscopic water in soils is directly dependent on their mechanical 

 composition of the soil. 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 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 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 



