The soil-enriching effect of the growth of clover and other legumes has been recognized from the earliest days of agriculture. Even ancient Roman authors emphasized their \value In regular rotations. Since then the use of leguminous crops to Increase the yields from soils more or less worn out has gradually gained In favor. By the early part of the eighteenth century, clover had replaced the older bare fallow as a means of restoring fertility and It was a very common crop. At that time It was practically the only Important fertiIizer known and It received much attention and many theories were advanced about Its growth and action.