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 
cowpeas, have already been considered, but field and 
garden 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. 
