THE WATER REQUIREMENTS OF FRUIT PLANTS 9 



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are often several successive years in which the total precipitation does not 

 exceed 6 inches."^ 



Another interesting application of the same principle has been recorded in 

 South Dakota. Cottonwoods planted rather close together for windbreak or 

 shelter belt purposes, thrive for a number of years, but eventually a stage is. 

 reached when they begin to die from crowding. If wider spacing or thinning is 

 practiced their longevity is increased correspondingly.^* 



Factors Influencing the Water Requirements of Plants. — It is advisable 

 at this point to review some of the data available on the economy with 

 which the plant uses water. From what has been said regarding the total 

 water requirements of the plant it is evident that only an extremely 

 small percentage is finally held by the plant as a constituent of the proto- 

 plasm or is used in the manufacture of chemical compounds. The 

 greater portion of the water has been required to meet evaporation. 

 Since the water requirement is a ratio between the water used and the 

 plant material produced, it is evident that all other factors favoring the 

 nutrition of land plants will tend to decrease their water requirement and 

 that all factors tending to increase water loss through transpiration 

 will increase it. Experimental evidence bearing on the factors affect- 

 ing, nutrition is available, but the effects of factors altering water loss 

 have not been so thoroughly studied. 



Nutrient Supply. — Table 7 show^s the mean water requirements of 

 oats and wheat as influenced by fertilizer treatments and Table 8 presents 

 data showing the effects of various amounts of nitrogen upon the water 

 requirement of the plant. 



It is a reasonable assumption that when the soil solution is poor in 

 any indispensible element more water must be taken up by the plant to 

 obtain an ample amount of this element. However, this is true only 

 within certain limits, because of the abiUty of plants to withdraw from 

 the soil nutrient materials in proportions quite different from thofee in 

 which they occur there. Attention has been called to the considerably 

 higher water requirement of plants in the very rainy climate of Munich, 

 Germany, than in the drier portions of northern Germany or in Wis- 

 consin. It is suggested that as the moisture approaches the extreme in a 

 wet soil the soil solution is diluted; hence conditions are presented that at 

 least in a way are comparable with those found in a "poor" soil. More 

 water is required to absorb a given amount of nutrients. Possibly in this 

 case the poor aeration attendant upon a soil moisture content above the 

 optimum may also affect the water requirement. The effects of a very 

 dry soil, which Hkewise increases the water requirement, is attributed by 

 Briggs and Shantz^^ to the restricted area which the active roots and root 

 hairs occupy under these conditions. It seems a strange perversity of 

 fate that the soil conditions and soil treatment which are most likely to 

 result in a restricted root system, such as heavy soils, hardpan, water- 



