Wadleigh and Richards 443 



its greatest intensity when the upper limit of film water equaled twice 

 the maximum molecular water-holding capacity (presumably twice 

 field capacity); these results tend to be in line with those of Bhaumik 

 and Clark (9) for their heavier soils. 



Waksman (132) has reviewed the earlier work on the influence of 

 soil moisture on microbiological activity. Different organisms vary as 

 to the optimum soil moisture content for their activity. Thus, nitrifica- 

 tion is at its highest near moisture content of field capacity, and exces- 

 sive quantities of water are much more injurious than too low a mois- 

 ture content. It is quite evident that to whatever extent the mineral 

 nutrition of plants is dependent upon the activity of soil microorga- 

 nisms, soil moisture level will have an indirect effect on nutrition 

 through its influence on soil microbes. 



Another aspect of the relation of soil moisture to microorganism 

 activity is concerned with minor element nutrition. The mineral nutri- 

 tion of crops on sandy soils in Wisconsin is intimately connected with 

 the organic matter content of these soils. It has been reported to the 

 authors that during the drought year of 1946, boron deficiency symp- 

 toms became prevalent throughout the state. It could have been that 

 reduced microbial activity due to drought caused insufficient minerali- 

 zation of the boron present in the organic matter of the soils. 



MINERAL NUTRITION IN DRY SOILS 



During protracted periods of drought, the fertile surface soil may 

 dry to less than the wilting percentage. This may have drastic conse- 

 quences to shallow-rooted crops. Even though deep-rooted plants may 

 obtain adequate moisture under these conditions from the deeper hori- 

 zons, the question immediately arises as to the ability of plants to 

 absorb nutrients from the fertile top soil when it is drier than the wilt- 

 ing percentage. For example, boron deficiency symptoms usually be- 

 come predominant during a drought (g<y,22). There are two alternative 

 explanations for this: (a) Drying of soil may affect the availability of 

 borate, as is the case with potassium and phosphate; or (b) plants are 

 unable to absorb boron when the soil moisture is below the wilting 

 percentage and inadequate supplies of this nutrient are present in the 

 lower horizons. 



Breazeale (20) carried out a number of experiments to ascertain 



