White sweet clover comprises a very large percentage of the acreage seeded to sweet clover at the present time. On this account this species ordinarily is referred to simply as "sweat clover." The yellow biennial species is designated as yellow sweet clover, and the annual yellow species as bitter clover, sour clover, or annual yellow sweet clover. This usage has been adopted in this bulletin.
Fig. 1.—Seeds and seed pods of three species of Melilotus and seeds of alfalfa: 1. White sweet clover; 2, biennial yellow sweet clover; 3, annual yellow sweet clover, or sour clover; 4, alfalfa. The small figures in each drawing show the natural size of the seed. The venation and shape of the seed pods are important characters in distinguishing the different species of sweet clover.
The cultivation of sweet clover should be preceded by a thorough understanding of the requirements for obtaining a stand. It can not be grown successfully on all soils, as many assume from seeing it growing in uncultivated places. Neither will it thrive in many sections of the country without careful preparation of the seed bed. Sweet clover will not grow successfully in acid soils unless lime is applied, but it will make a good growth in soils too low in humus to grow red clover, provided the soil is neutral or alkaline.
Sweet clover is an excellent plant to precede alfalfa, as the large roots do much toward breaking up and aerating the subsoil. Contrary to the belief of many, it will not inoculate the soil for alfalfa unless inoculation is applied to the sweet clover. If, however, the soil contains but few inoculating germs, the sweet clover will serve as a medium to inoculate it thoroughly.
A number of species of sweet clover are found throughout the world, and most of them are native to temperate Europe and Asia as far east as Tibet,