THE WATER REQUIREMENTS OF FRUIT PLANTS 13 



with respect to nutrient supply should be a constant and conscious aim 

 in scientific orchard management, though perhaps the water conservation 

 influence of optimum growing conditions may be more or less masked 

 by the increased requirements for the accompanying increased growth. 



The Wilting Point for Fruit Plants. — There seems to be some differ- 

 ence of opinion as to how near to the hygroscopic coefficient plants can 

 exhaust the water supply of the soil. Loughridge states that certain 

 plants can remove enough of the hygroscopic moisture of the soil to 

 maintain life though they cannpt grow under these conditions; Hilgard 

 states that soils of great hygroscopic power can withdraw from moist 

 air enough moisture to be of material help in sustaining the life of vegeta- 

 tion in rainless summers or in time of drought, though only a few desert 

 plants can maintain normal growth.^* In most plants, however, wilting 

 will occur before the moisture content of the soil has been reduced to its 

 hygroscopic coefficient. 



Wilting Coefficients. — The work of Briggs and Shantz^" has led them 



to conclude that the wilting coefficients for most soils equal their 



hygroscopic coefficient _,, , . .,, , . ^ 



— ^p o I n nio Thus a sandy loam with a hygroscopic coefli- 



cient of 3.5 per cent, would have a wilting coefficient of about 4.8 and a 

 clay loam with a hygroscop/c coefficient of 11.4 would have a wilting coeffi- 

 cient of 16.3 per cent. These investigators state, "The wilting coeffi- 

 cient is the same, within the limits of experimental error, for a plant in 

 all stages of development. In other words, the soil-moisture content 

 at the wilting point is not dependent to any material degree upon the 

 age of the plant. ... [It] is not materially influenced by the dryness of 

 the air, by moderate changes in the solar intensity, or by differences in the 

 amount of soil moisture available during the period of growth, "^o It 

 ranges for different soils from less than 1 per cent, in the coarsest sands 

 to as high as 30 per cent, in the heaviest clays. "The use of different 

 plants as indicators of the wilting point produces only a relatively small 

 change in the wilting coefficient of a given soil. Representing the mean 

 value of the wilting coefficient of a given soil by 100, a range from 95 

 to 105 approximately, would result from the use of different plants 

 as indicators, . . . The xerophytes tested gave a mean ratio inter- 

 mediate between the hydrophytes and mesophytes. This would indicate 

 that plants native to dry regions are unable to reduce the water content of 

 the soil to a lower point at the time of wilting than is reached by other 

 plants. . . . There is evidence that drought resistance in a plant is not 

 due to an additional water supply made available for growth by virtue 

 of a greater ability on the part of that plant to remove moisture from 

 the soil. "20 



Wilting Under Field Conditions. — The work of Briggs and Shantz on 

 wilting coeflficients of different soils was done, however, under fairly 



