440 Mineral Nutrition of Plants 



a Sassafras sandy loam, the lower the yield of potatoes. Adding increas- 

 ing amounts of superphosphate had a moderately beneficial effect, how- 

 ever. The effect of these two fertilizing materials on potato yields under 

 these conditions was directly related to their effect on the solute con- 

 tent of the soil solution. Correspondingly, Rahn (96) found that during 

 a "dry" year, fertilization with manure produced a much higher yield 

 of melons than did mineral fertilization, but that during a "wet" year 

 there was no difference in effect on yield from these two sources of 

 fertility. 



The need for taking into account soil moisture supply in adjusting 

 the fertility program is well recognized in the Hawaiian Islands. Night- 

 ingale {go, 8g) emphasized in his studies on the nitrogen, phosphorus, 

 and potassium nutrition of pineapples that the capacity of the plant 

 to utilize efficiently the available supplies of these nutrients was con- 

 ditioned by soil moisture supply and other environmental factors. 

 Clements and Kubota (31, 32) have developed a technic for following 

 the status of moisture, nutrient, and sugar content of sugar cane over the 

 course of its development, and of adjusting the irrigation and fertility 

 program in accordance with the trend in the status of the plants. 

 Dr. Clements discusses this technic in an accompanying paper. 



The effect of change in thickness of the 

 moisture films on nutrient availability 



The discussion in the fore part of this paper pointed out that as the 

 thickness of moisture films on the soil particles decreases, the intensity 

 with which the water is retained on the particles by surface force action 

 increases. Buehrer and Rose (24) discuss the physical properties of ad- 

 sorbed water. The initial layer of water adsorbed on clay particles is 

 presumably held by a "pressure" of several thousand atmospheres (pF 

 6-7). The characteristics of water retained under such high pressure 

 differ from those of "free" water. There is found to be a tremendous 

 drop in the dielectric constant and presumably a decrease in its solvent 

 power because of the greatly decreased polarity. This implies, therefore, 

 that with diminishing thickness of moisture films, there is a corre- 

 sponding decrease in the proportion of water in the film with normal 



