MAINTAINING SOIL FERTILITY 83 



air as free nitrogen. Experiments have shown that in the first 

 two or three decades after new land is brought into cultivation, 

 the loss of nitrogen from these two causes may be two or three 

 times as large as the loss through the removal of the crops 

 grown on the land. 



91. Organic matter and nitrogen. The organic matter and 

 the nitrogen of the soils of the corn belt have been decreased 

 approximately one third and in many cases one half since they 

 were first put into cultivation mainly through intensive tillage 



FIG. 36. How soils wash 



The greatest waste of soil fertility is the waste of the soil itself. Heavy rain falling upon 



soils from which the vegetation has been removed wash away much of the best soil. 



(Photograph from Bureau of Soils, United States Department of Agriculture) 



incident to the growing of corn. Indeed, the loss of organic 

 matter is responsible for the decreased productivity of most so- 

 called "worn " or depleted soils. This is probably because a soil 

 which is low in organic matter is usually in poor physical con- 

 dition ; the bacterial activity in such a soil is lessened, and the 

 soil loses its "life," as the farmer says. 



92. Removal of plant food in crops. The loss of plant-food 

 elements through the removal of the crops grown on the soil 

 can be readily understood. If crops are continually removed 

 and nothing is returned, the supply of available food is soon 

 depleted, and finally the total amount of plant food in the soil 



