436 Mineral Nutrition of Plants 



vineyards in the San Joaquin Valley of California that receive neither 

 rain nor irrigation during the entire growing season. On the other hand, 

 potato fields in the same area are irrigated daily, and successful main- 

 tenance of Ladino clover pastures in this valley requires that the soil 

 moisture content be kept at near-field capacity. The lower soil tempera- 

 tures prevailing in highly moist soils may be a factor in the need for 

 much more frequent irrigation of potatoes and Ladino clover. 



The criterion adopted as the measure of growth response is involved 

 in the evaluation of the effect of degree of soil moisture depletion on 

 growth. Adams, Veihmeyer, and Brown (2) studied the effect of vari- 

 ous irrigation regimes on growth and productiveness of cotton. The 

 plots maintained at the highest level of soil moisture produced the max- 

 imum vegetative growth of the plants, and vegetative growth declined 

 with decreasing soil moisture reserve at time of irrigation. Yet, there 

 were virtually no differences in yield of seed cotton per acre among 

 these various treatments. Thus, the plants made a physiological re- 

 sponse to increased level of soil moisture supply, but did not provide 

 a corresponding economic return. Hendrickson and Veihmeyer ($y) 

 observed the maximum vegetative growth of peach trees on plots that 

 had the most abundant water supply. However, the production of these 

 trees was not superior to those irrigated less frequently, and the keep- 

 ing quality of the fruit from the frequently irrigated trees was quite 

 inferior. Here again, maintenance of relatively moist soil produced the 

 maximum physiological response as regards vegetation, but the treat- 

 ment was actually an economic liability. Guayule has been observed to 

 decline in vegetative growth as soil moisture depletion prior to irriga- 

 tion is intensified, but rubber production was found to be increased 

 (129,64). 



The tenability of the concept that all soil moisture above the wilting 

 percentage is "equally available" to plants is conditioned, therefore, by 

 the criteria used in evaluating the results, in addition to the degree of 

 prevalence of modulating factors. Consequently, the question as to 

 whether or not the soil moisture content above the wilting percentage 

 will become limiting to full utilization of nutrients available to the 

 plant is correspondingly involved. Unquestionably, conditions fre- 

 quently prevail in which plant response to fertilization is limited by 



