CEREAL CULTIVATION GREAT PLAINS 351 



increased to 16.8 per cent by September 11 the next year, 

 a gain of 7 per cent, equivalent to about 6.5 inches of 

 water. As the rainfall during the interval was 19.7 inches, 

 about 33 per cent of it was stored. (2) In another in- 

 stance of summer tillage following oats, similar records 

 and figuring show that 24 per cent of the season's rainfall 

 was retained. (3) In a third field, summer-tilled follow- 

 ing sorgo, during the entire period from harvest to the 

 next seeding date only 2.5 inches of water were accumu- 

 lated, and from March to August there was only 1 per 

 cent of increase. The rainfall intervening was 12.2 inches, 

 of which 21 per cent was retained. Even with this small 

 quantity of stored moisture, this field and one other given 

 similar treatment, both sown with winter wheat, were the 

 only fields that produced any small grain in 1911. 



The quantity of water which can be stored varies 

 greatly in different seasons, but is from 10 per cent to 

 33 per cent at this station. With a dry soil surface and 

 high evaporation, as much as a half inch of rain may- 

 be of no use in water storage, though of much benefit to 

 a crop. 



378. Moisture layers of semi-arid soils. In the Rus- 

 sian Chernozem, the same conditions of semi-aridity exist, 

 and the same methods of dry-farming are practiced as in 

 our Great Plains. As the soil is very dark because of its 

 high percentage of humus, summer tillage is called the 

 " black fallow," in contrast to green-manuring and uncul- 

 tivated fallow. Rotmistrov (1913, pp. 28-34) has made 

 soil moisture studies at the Odessa Experiment Farm, 

 and in a report of his work has, by illustrative terms, 

 presented some of the facts concerning the storage of soil 

 moisture in an interesting way. 



The feeding depth of cereal roots, extending to about 



