CROP ROTATION 473 



of the soil. Some crops, as bunch onions, spinach and 

 lettuce, leave practically no refuse on the ground, while 

 others, as turnips, beets and potatoes, furnish consider- 

 able quantities of vegetable matter. Another class of 

 plants, the legumes, serve as nitrogen traps, in addition 

 to supplying humus of the most valuable character. The 

 leguminous plants, however, vary greatly as soil im- 

 provers. The manurial crops as the clovers, vetch and 

 coiypeas, have already been considered, but field and 

 gar(i6n types of peas and beans deserve greater consider- 

 ation in crop rotations because of their value as soil 

 improvers. 



685. Relation to toxics. — Experiments made by the 

 United States Department of Agriculture indicate that 

 the roots of plants (at least of some species) exude tox- 

 ics or substances which may not be injurious to other 

 plants, but are poisonous to themselves. While there 

 is no proof that this is true concerning vegetables, it is 

 probable, and the principle should be regarded when 

 planning rotations for all types of vegetable gardening. 



686. Relation to insects and diseases. — Numerous allu- 

 sions have been made on preceding pages concerning the 

 importance of rotation to avoid losses from the ravages 

 of insects and plant diseases. A multitude of insects and 

 disease spores pass the winter in the ground or are pro- 

 tected by refuse on the surface. If the host plants are 

 grown annually these pests are likely to become more 

 and more destructive. This is one of the strongest argu- 

 ments for crop rotation in vegetable gardening. Ordi- 

 narily, three to five years should elapse between crops of 

 the same species, although less time is sufficient in many 

 instances. 



687. Relation to other factors. — Several additional fac- 

 tors should be taken into account, as the profits of the 

 various crops that might be grown, crop adaptation, and 

 the physical and chemical properties of the soil. 



