432 Mineral Nutrition of Plants 



in waterlogged soils (/J/, fl, 134). There is a rapid loss of applied 

 nitrate under these conditions, but only a fraction of it is recoverable 

 as ammonia. Willis and Sturgis (134) observed that large quantities 

 of nitrogen as ammonia are lost from waterlogged soil high in nitrogen 

 and maintained at a high temperature (ioo° F.), or from soils high 

 in organic matter. These results indicated that such a soil will tend to 

 reach an equilibrium at which it will maintain a low soluble-nitrogen 

 content against losses induced by high temperatures and alkaline re- 

 actions. De and Sarkar (37) found that much of the difference in 

 nitrogen between the amount of nitrate applied to a waterlogged soil 

 and that recoverable as ammonia was due to nitrogen assimilated by 

 the increased population of microorganisms. Wallihan (131) confirmed 

 this and pointed out that this condition explains the relatively low loss 

 of nitrogen when waterlogged soils are drained. He found that there 

 was a relatively rapid rate of nitrate production following drainage of 

 such a soil, providing further evidence that the denitrification process 

 actually prevents excessive losses of an important plant nutrient from 

 waterlogged soils. On the other hand, the nitrogen so stored may be 

 withheld from crop plants, rice for example, growing on such a soil. 

 In fact, Willis and Sturgis (134) have emphasized the generally poor 

 response observed to applications of nitrogen on rice. They attribute 

 much of this effect to loss of ammonia on denitrification, but the com- 

 petition for nitrogen by microorganisms is also undoubtedly involved. 

 A further effect of wet soils on microorganisms arises from the fact 

 that wet soils tend to be cold (6). That is, the higher the moisture 

 content of a soil the higher its heat capacity. This means that wet soils 

 warm up more slowly during the spring months. Since relatively low 

 soil temperatures depress microbiological activity, nutrient availability 

 dependent on this activity will be correspondingly depressed. 



MINERAL NUTRITION UNDER SOIL MOISTURE VARIATIONS BETWEEN 

 FIELD CAPACITY AND THE WILTING PERCENTAGE 



The mineral nutrition of plants within the available range (see 

 p. 412) of soil moisture is conditioned by (a) the extent to which 

 growth and, consequently, mineral utilization might be limited by 

 water supply, (b) the effect of change in thickness of the moisture 



