406 DRY LAND FARMING 



not removed from the soil save as it is taken up by the 

 crops where proper methods of cultivation are followed. 

 In humid areas, the loss from the leaching of fertility 

 down through the soil in the form of nitrates is present 

 more or less every season. This loss is proportionate to 

 the abundance of the nitrates, and to the superabun- 

 dance of the water in the soil. Some fertility may be 

 removed through the washing of the soil. The loss from 

 this source may be felt more or less on soils that have 

 much slope, providing these are cultivated. On fairly 

 level lands this may be almost entirely prevented by a 

 judicious system of tillage (see p. 91). 



Rodents have doubtless exercised an important influ- 

 ence on the increase of the available fertility in dry 

 areas by the extent to which they have burrowed into 

 the soil and carried it to the surface. The channels 

 thus made in the process of burrowing were followed 

 by air and moisture and these have aided in the reduc- 

 tion of the soil. The combined influence of sun and rain 

 furthered decay in the soil that was brought to the sur- 

 face. This decay was favorable to the growth of plants, 

 which in turn added to the available fertility. This 

 process has been going on for long centuries, hence 

 the influence of these toilers has doubtless been material 

 in the liberation of soil fertility. 



The loss of fertility. Many claim that the supply 

 of fertility in dry areas will never be lost. This view 

 is doubtless based on the observation that grain crops 

 have been grown on the same land for periods of, say, 25 

 to nearly 50 years without any diminution in the yields, 

 and in the absence of any form of fertilization other 

 than that furnished by the straw of the grain crop or 

 crops that were grown upon the land. These results 

 have been obtained in Utah and also in some other 

 parts of the semi-arid belt. This, however, only demon- 



