CHAPTER VI 

 SOIL WATER 



Of the great number of factors that influence the growth 

 of crops none is of more importance, or possibly of as much 

 importance, in its effect on the yield of crops as water. A soil 

 may contain too much water for the best growth of crops, or 

 it may have too little. On the one hand, we approach swamp 

 conditions, and on the other the desert state. Even in the 

 same locality and with equal rainfall one field may have 

 too much moisture and another too little. While the volume 

 of water contained in a soil depends more or less on the rain- 

 fall, it is not controlled entirely by it; for within a wide 

 range of atmospheric precipitation soils of the same type 

 may not vary greatly in their moisture content. This is 

 because there are other factors beside rainfall that serve 

 to regulate the supply of soil water. 



62. Forms of water in soils. — It has already been pointed 

 out that in every soil there are spaces between the particles, 

 or aggregates of particles and that the size and total volume 

 of these spaces vary with different soils. These spaces 

 may be completely filled with water or they may be nearly 

 empty. When the pore spaces are entirely filled with water, 

 three forms of water are found to be present : (1) hygro- 

 scopic, (2) capillary and (3) free or gravitational. These 

 forms differ in their relation to the soil particles. 



63. How the three forms of water differ. — No soil in a 

 natural state, that is as it exists in the fields or woods, is 

 ever perfectly dry. No matter how small the rainfall or how 



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