384 DRY LAND FARMING 



When one crop is grown for successive years on the 

 same land in the absence of commercial fertilizers, the 

 plant food in the soil gets out of balance. The crop 

 grown will draw more heavily on one element of plant 

 food than on another, consequently the supply of that 

 element becomes too much reduced for profitable pro- 

 duction. The result will not be changed though the 

 other elements of plant food in the soil are present in 

 sufficient quantity. For instance, should successive 

 wheat crops reduce the nitrogen in the soil below a 

 given quantity, full crops of wheat will not be obtained, 

 though the supply of phosphoric acid and potash should 

 still be ample. To maintain the equilibrium it becomes 

 necessary to add nitrogen. This can be most cheaply 

 and effectively done by introducing a legume into the 

 rotation, as clover or alfalfa. These crops may also be 

 made to add to the supply of phosphoric acid and potash 

 in the cultivated portion of the soil, when they are fed 

 to animals and the manure is applied to the land that 

 grew them, but such increase is at the expense of these 

 products in the subsoil. 



When cereals or cultivated crops only are grown 

 on land the supply of the humus gradually decreases. 

 Such decrease results in the loss of that mechanical con- 

 dition which is most favorable to production. Such loss 

 of condition may take various forms. Heavy soils be- 

 come more and more impacted, insomuch that they are 

 not easily plowed. When dry they become cloddy and 

 call for the exercise of much labor to pulverize them 

 when preparing a seed bed. On the other hand, light 

 lands become lighter to the extent frequently of lifting 

 with the winds. These conditions are the immediate out- 

 come of too great a reduction in the humus supply, and 

 the best remedy is to introduce into the rotation a grass 

 or clover crop, the roots and stubbles of which will 

 supply the need. 



