154 COBN CROPS 



had the weeds shaved off gave as good results as an aver- 

 age of five years as when carefully cultivated, and similar 

 results have been secured at other stations. (See p. 206.) 

 On other occasions cultivation has apparently conserved 

 sufficient moisture to improve the yield. The underlying 

 principles have not been clearly worked out. 



It seems apparent that a well-cultivated surface, with 

 a good store of organic matter, will take up a moderate 

 rain more readily and store a large percentage of it deep 

 enough to protect from surface evaporation than will a 

 hard and uncultivated surface ; also, that when this mois- 

 ture is stored continued cultivation will decrease the rate 

 of loss from the upper 10 inches of surface. 



EEOSION 



108. Effect of erosion. — Erosion affects the agricul- 

 tural value of land in the two ways : first, by producing 

 gullies and large ditches, thus increasing the expense of 

 crop cultivation and resulting in the actual loss of some 

 land; second, by reducing available fertility, through 

 removing the surface. 



In the latter case, the damage to productivity depends 

 on the soil. In heavy clay soils, much of the available 

 fertility seems to be in the surface 6 inches. On such 

 soil productivity is often reduced for many years by turn- 

 ing up too much subsoil at one time with the plow. On 

 the other hand, as pointed out by King,i in many regions, 

 especially of low rainfall, the subsoil, even to several 

 feet deep, is as productive as the surface soil. In a case of 

 such surface, erosion would work little or no damage. 

 However, in most of the regions where erosion is severe, 



' King, F. H. The SoU, p. 29. 



