INFLUENCE OF MOISTURE 



61 



TABLE 9 



Influence of Moisture Content of Soil on Numbers of Bacteria 



(from Engberding) 



Influence of Moisture. — Bacterial development is gener- 

 ally at a maximum when the moisture content is relatively high 

 (Table 9). The maximum development of the aerobic bacteria, 

 which are concerned with some of the most important soil proc- 

 esses, takes place when the moisture content is between 50 and 70 

 per cent of the moisture-holding capacity. This corresponds to 

 the moisture content best adapted to development of most culti- 

 vated plants. Although bacterial activity in air-dry soil, with 2 to 

 5 per cent moisture, is almost negligible, drying of soil has a striking 

 effect on the growth of both microbes and higher plants, after the 

 dried soil has been remoistened. Drying renders the soil sub- 

 stances much more available for decomposition, due to physical 

 and chemical changes in the soil organic matter, to the killing of 

 many microbial cells by the desiccation process, and to modifica- 

 tions in the nature of the soil population. Whatever the respon- 

 sible factors may be, the intermittent drying and moistening of the 

 surface soil is quite beneficial in rendering nutrient elements 

 available for plant growth. Fig. 43, taken from laboratory studies, 

 indicates the course of microbial development following the 

 moistening of a dried soil. Over the same period of time, the 

 soil which had not been desiccated would have shown no pro- 

 nounced increase or decrease in abundance of bacteria or fungi 

 or in the formation of carbon dioxide. 



In humid regions the root systems of plants are confined much 

 nearer to the surface than in semi-arid zones, consequently the 

 plant residues are more generally concentrated near the surface 



