CHAPTER VI 



CONDITIONS DETERMINING THE USE OF SOIL 

 MOISTURE BY PLANTS 



THE discussion in the preceding chapters has taken 

 no account of the effect on plants of soil moisture. Yet, 

 the plant is a most important factor, for it uses immense 

 quantities of water throughout the season, and the rate 

 of use is very difficult to control. It becomes necessary, 

 therefore, to investigate the relationship of the plant to 

 the water added to the soil in irrigation. The relation- 

 ship is of particular importance, because, under irriga- 

 tion, the farmer may apply different quantities of water, 

 at stated times, throughout the growing season. That is, 

 under irrigation a soil-moisture control is possible, which 

 is not possessed by any other system of agriculture. 



The essential question in agriculture is always, "To 

 what extent can the farmer control the conditions of 

 plant-production?" Where water is the critical factor, 

 as in irrigation, it is of first importance to know how the 

 absorption of water from the soil by plants may be con- 

 trolled. Once this is known, systems of farming may be 

 planned whereby the scanty water supply may be made to 

 reclaim the largest possible area of land, or to produce 

 the largest yield of high-quality crops. 



This chapter is devoted to a discussion of the condi- 

 tions that determine the rate at which water is taken 

 from the soil by plants. The rate at which water is used is 

 ordinarily different from the total quantity used by the 



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