Manuring and Rotations 75 
Potash is closely associated with photosynthesis in the 
formation of sugar in the leaves, whereas phosphoric 
acid is required in large quantities in the formation of 
seeds. This may explain in part the high potash re- 
quirements of sugar-beets, since work must be carried on 
in the leaves in producing sugar. 
WAYS OF MAINTAINING SOIL FERTILITY 
Various means may be used in maintaining the pro- 
ductivity of the land. Probably no system is complete 
that does not provide for the return to the land of at least 
a part of the mineral matter removed by the crop. This 
may be accomplished by the use of barnyard manure or 
by the addition of the substances in the form of com- 
mercial fertilizers. The plowing under of green-manure 
may also help in making available elements contained in 
the soil in large quantities, but in a condition that the 
crop cannot make use of them. In cases in which legumes 
are used for green-manure, there is also a direct addition 
of plant-food in the shape of nitrogen. Every good sys- 
tem of keeping the soil productive will include a rotation 
so arranged that the maximum returns will be secured 
and that will, at the same time, maintain the soil in 
good condition. Under most conditions, the practical 
method of maintaining the fertility of sugar-beet soil will 
combine all the ways mentioned. Farm-yard manure 
will be supplemented by the wise use, in a commercial 
form, of elements necessary to balance the needs of the 
crop on any particular soil; and crop rotations will be 
practiced in which some legumes will be plowed under as 
