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 



