FACTORS IN CROP-ADAPTATION 501 



In arid soils some plants are able to withstand alkali 

 conditions where others quickly succumb. There 

 may be toxic substances in the soil injurious to one 

 plant and not to another. These may arise from the 

 growth of other crops, and so determine the plants which 

 may be associated with that crop. This bears on crop- 

 rotation. 



355. Requirements for growth supplied by the soil. 

 The internal conditions of different soils may be very 

 different. On a puddled clay soil saturated with water, 

 only a few plants may thrive. On a dry sandy soil, 

 only certain other plants can secure the essentials for 

 growth. On a very shallow soil, shallow-rooted, early- 

 maturing crops may be grown where trees would utterly 

 fail. On soils subject to midsummer drought, early- 

 maturing crops may be grown where late-maturing 

 crops would fail. Thus, the soil conditions are the arbiter 

 in the selection of crops to be produced. The distribu- 

 tion of different crops and types of agriculture is a re- 

 flection of this adaptation. Many failures result from 

 failure to recognize these relations. 



Full knowledge for the accurate adaptation of crops 

 to soil, or soil to crops, is yet to be gained. Such infor- 

 mation is often not to be derived by definite experi- 

 mentation. It comes of long experience. But many 

 striking examples of adaptation are known. They are 

 governed by soil conditions broadly considered, rather 

 than by any single factor. One of the most general 

 of these relations is the adaptation of early truck crops 

 to light, sandy soil; of grass, to heavy soil. Certain 

 varieties of apple grow to their highest perfection on 



