352 



THE SMALL GRAINS 



3 







50 inches, Rotmistrov calls the root- 

 inhabited layer. Observations show 

 that this layer under cropping becomes 

 more humid or drier, according to the 

 time of year or nature of the season, so 



N he also calls it the periodically humid 



layer. Under this layer, to a further 

 depth of 6 feet, is an intermediate dry 

 layer, containing only unavailable water, 

 that is, 10 per cent or less. Below 

 this still lies the permanent humid 

 layer, containing 2 per cent to 3 per 

 cent of available water in the upper 

 ^ portion, and 7 per cent to 8 per cent in 

 ft the lower portion, and extending to the 

 ^ ground water. After summer tillage or 

 after an intertilled crop, the soil mois- 

 tens from the surface entirely through 

 ^ the intermediate dry layer to the 

 ^ ground w T ater, resulting in an uninter- 

 rupted moist layer. In continuous 

 cropping, the intermediate layers is al- 

 ^ ways dry, and toward July the peri- 

 odically humid layer loses its reserve 

 moisture. Figure 109 shows the dis- 

 tribution of moisture in the soil at 

 ^ different stages of a 4-year rotation. 

 ^ 379. Comparative value of summer 

 N tillage. In the experiments of the 

 X United States Department of Agricul- 

 ture previously discussed (370, 373), it 



FIG. 109. Distribution of moisture in the soil at 

 different stages of a 4-year rotation. 



