THE NEED OF CROP ROTATION 147 



from the soil. Potatoes remove large quantities of potash; 

 timothy takes nitrogen ; small grains take phosphoric acid. 

 If any one of these crops be grown year after year on 

 the same field, the soil there will lose most of the avail- 

 able food for that kind of plant. 



But though the land that has grown potatoes for three 

 or four years gives only decreasing crops of that plant, it 

 may give excellent grain crops. An " exhausted " soil 

 often is exhausted only for a particular crop. Nearly all 

 plants, however, require at least a small amount of each 

 kind of plant food ; so if exhaustion of any sort goes too 

 far, it becomes difficult to use the land profitably at all 

 until its fertility has been built up again by slow and 

 costly processes. 



In colonial times tobacco was raised on the rich lands 

 of Virginia and Maryland each season until field after 

 field was exhausted and abandoned. A little later, in the 

 central West, and later still, in the more distant West, a 

 like sin against the soil was committed with wheat. 



Of course some attempts were made on the colonial to- 

 bacco lands and on the western wheat fields to supply the 

 needed plant food by fertilizers. But it was found, to the 

 surprise of the farmers, that this checked the decrease in 

 crop returns only partially. As will be more fully ex- 

 plained a little further on, the fact is that food exhaustion 

 is not the only evil that follows from using the soil con- 

 tinually for the same crop. 



It is possible, however, to grow crops on the same land 

 every year for centuries, and at the same time to improve 

 its condition steadily instead of exhausting it. To do 

 this requires fertilizers, but quite as much even more 

 it requires that successive crops vary in kind. That 

 is, it requires a wise crop rotation. In a rude way, even 

 some barbarous peoples have learned the need of a " rota- 



