TILLAGE 155 



Rotation of crops. In some sections of our country there 

 are farms upon which no other crop than wheat has ever 

 been grown ; on others, cotton or corn have been grown con- 

 tinuously. When the same crop has been grown upon one 

 piece of land for a series of years, however, there is a possi- 

 bility that the soil may be depleted of some necessary element 

 and fail to give adequate returns. Other plants, needing 

 different proportions of the minerals, would be able to thrive 

 where the first crop would fail. Again, certain toxic sub- 

 stances excreted by one set of plants may accumulate in the 

 soil until they put an end to the healthy growth of these 

 plants long before the food materials are exhausted, though 

 these toxic substances are not harmful to other crops. It has 

 been found, also, that when the same crop is grown in the 

 same place for any length of time, the insect and fungous 

 pests that trouble it greatly increase and the weeds associated 

 with the crop multiply. In meadows, daisies often overspread 

 the grasses, and wild mustard thrives in grainfields. Ragweed 

 comes in with the wheat, and srnartweed and foxtail grass grow 

 with the corn. The advantages of a change or rotation of 

 crops are thus seen to be many, and everywhere that modern 

 methods obtain, some .kind of rotation is practiced. A still 

 further advantage of crop rotation is that it permits the cul- 

 tivation of deep and shallow-rooted crops alternately, and 

 thus the whole soil is laid under tribute. The usual rotation 

 consists of a grain crop such as oats or wheat, a cultivated 

 crop such as corn, a forage crop such as clover or alfalfa, 

 and, in addition, the field may be used as pasture for a year 

 or more. Somewhere in the rotation it is usually planned to 

 have a legume crop, which, when finally plowed under, en- 

 riches the soil by the addition of much nitrogen. Even in 

 small gardens the benefits to be derived from a rotation of 

 crops are worth securing. In nature there is also a more or 

 less well-defined rotation. Ponds dry up and new forms of 



