CHAPTER VIII 



THE FORMS OF SOIL-WATER AND THEIR 

 CHARACTERISTICS 1 



A soil, in order to function as a medium for plant growth, 

 must contain a certain amount of water. This moisture pro- 

 motes the innumerable chemical and biological activities of the 

 soil, it acts as a solvent and carrier of nutrients, and in addi- 

 tion it functions as a nutrient itself. The amount, character, 

 and control of the soil-moisture must evidently be reckoned 

 with in any study of soil and plant relationships, whether they 

 are of a practical or a theoretical nature. The productivity 

 of a soil is often a direct function of its moisture condition. 



85. Forms of soil-water. — As has already been demon- 

 strated, a soil of a given volume weight has a definite pore 

 space which may be occupied largely by air or by water, or 

 shared by both, as the case may be. Of course, an ideal soil 

 for growth is one in which there is both air and water, the 

 proportions depending on the texture and the structure of 

 the soil and the character of the crop. Assuming for the time 

 being, however, that the pore space is almost entirely filled 

 with water, or, in other words, that the soil is saturated, three 

 forms of water are found to be present — hygroscopic, capillary 

 and gravitational. These forms differ not only in the amount 

 and proportion of the solutes which they carry but also in the 

 positions that they occupy in their relation to the larger soil 

 particles and the accompanying colloidal complexes. 



1 Keen, B. A., Belations Existing Between the Soil and Its Water 

 Content; Jour. Agr. Sci., Vol. X, Part 1, pp. 44-71, Jan., 1920. A 

 good review of the subject. 



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