196 THE PRINCIPLES OF SOIL MANAGEMENT 



impaired, should be selected. In semi-arid regions, 

 where dry farming farming without irrigation where it 

 is usually required is practiced, it is sometimes advis- 

 able to grow but one crop in two years, because the 

 annual rainfall is not sufficient to produce a profitable 

 crop each season. This practice, of course, implies 

 those conservation practices which safeguard the rainfall 

 as it collects, by appropriate tillage methods. 



The loss of water by direct evaporation from the soil 

 may be excessive, and result in direct reduction of the 

 crop yield. This type of loss is so familiar that examples 

 hardly need be cited. In the results with the Rotham- 

 sted rain gages, about 50 per cent of the annual rainfall 

 was regained in the drainage water. Since the gages 

 bore no crop, the remaining 50 per cent must have 

 been lost by evaporation. And it will be noted that in the 

 summer months under warm temperature this loss was 

 greatest, amounting to 75 per cent of the rainfall. 

 Correspondingly, in the semi-arid and arid sections of 

 the country, where there is little or no drainage, the rain- 

 fall is all lost by evaporation. Investigations indicate 

 that about 70 per cent of the precipitation on the land 

 surface is derived from evaporation from the land sur- 

 face. Even in the humid sections, where the annual 

 rainfall is ample for maximum crop production, the crops 

 are frequently reduced even below the profit point by 

 prolonged periods of dry weather in the growing season, 

 during which the loss from the plants, coupled with the 

 loss from the soil, exhausts the soil supply. If we refer 

 to page 135, we note that the water absolutely needed 

 for crop production, and including the necessary losses 



