14 FORAGE CROPS 
tent of the valuable element nitrogen. The prin- 
cipal reasons for the rotation of crops are based on 
these facts. The purpose of a rotation is to pro- 
long the period during which profitable grain and 
other crops may be produced with the natural 
supplies of plant-food. 
Under present conditions in this country, the 
area that is devoted to the growing of hay is rela- 
tively large, although the average yield per acre is 
comparatively small. It is probable that the main- 
tenance of fertility by means of rotations is not so 
greatly influenced by the growth of leguminous hay 
crops as would be the case if, aside from their 
value as forage, there were a better understanding 
of their usefulness in soil improvement. The low 
average yield per acre of hay may be ascribed to 
several causes, chief among which are: 
1. Too extensive systems of practice, which do 
not provide for sufficient available plant-food. 
2. Lack of general information concerning the 
habits of growth, usefulness and composition of 
many grasses and legumes, which are well adapted 
to special conditions, including lack of knowledge 
of the best combinations of these plants. 
3. Defective systems of rotation, which do not 
provide for a proper succession of cereals and le- 
gumes, or which leave the land bare for long periods. 
4. Too little care in the preparing and manuring 
of the land for seeding, 
