CHAPTER IV 



SOIL MOISTURE: ITS CLASSIFICATION, MOVEMENT 

 AND INFLUENCE ON ROOT DISTRIBUTION 



Within certain limits tiie size and general character of top growth is 

 influenced bj^ the root system that supports it. Similarly the size and 

 distribution of the root system depends to an important degree on the 

 moisture content of the soil. 



CLASSIFICATION OF THE WATER IN SOILS AND PLANT TISSUES 



The physicist finds it desirable to distinguish between water in the 

 solid, liquid and vapor form; the chemist distinguishes between free 

 water, water of crystallization and water of constitution. Similarly it is 

 convenient for the student of soils and plant physiology to classify water 

 according to the form in which it is held in the soil or in plant tissue and 

 the consequent uses to which it may be put. No one classification has 

 proven most satisfactory for all purposes. Attention is here directed to 

 those that seem more useful in explaining the response of the plant to 

 varying water content. 



The Response of Water to the Force of Gravity and the Evaporating 

 Power of the Air. — The water of the soil is held in three conditions; (1) free, 

 or gravitational water, (2) capillary water and (3) hygroscopic water. The 

 free or gravitational water is that which moves down through the soil 

 under the influence of gravity. It is the surplus water that drains away 

 after heavy rains or heavy irrigation, finding its way eventually through 

 underground channels to streams or springs or to the so-called ground- 

 water level. Capillary water, on the other hand, does not move down- 

 ward in response to the force of gravity, but adheres to the soil particles 

 in the form of films of varying thicknesses. It does not drain away freely 

 with the seepage water, though there is some reduction in its amount 

 within a given soil depth if there is a material lowering of the water 

 table of the soil. However, it may be lost through evaporation from the 

 surface soil. Hygroscopic water is the moisture that is to be found in 

 air-dry soil exposed to a moist or saturated atmosphere. Like capillary- 

 water it exists in the form of thin films adhering to the surface of the soiL 

 The films, however, are much thinner than those of capillary moisture 

 and the soil retains this hygroscopic water with great tenacity. The 

 capillary and hygroscopic moisture together may be regarded as a product 

 of what the soil particle can hold against the pull of gravity on the 

 one hand and the evaporating power of the air on the other. 



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