4. The mere mechanical effect of the extensive root system can scarcely be over-estimated. As soon as germination begins the plant starts its tiny roots downward on the search for moisture. Roots four feet long have been found on alfalfa but four months old; roots nine feet long have been found below alfalfa but nine months old. After the taproot reaches a few inches below the surface, it sends out smaller roots that have a lateral growth of but a few inches, when they too, take a downward course for moisture and for mineral elements needed for the growth above. These first smaller roots decay and others start out from the taproot lower down. These decay and still others start. The decaying roots add humus to the soil, and the openings left by them form a wonderful system of channels for the penetration of air and water into the soil. The erstwhile compact earth is honeycombed and air and water penetrate the graves of the dead roots until, when the alfalfa field is ready to be used for a different crop, the soil has been wonderfully changed not only in its chemical elements but in its physical character. 5. The regular deposit of alfalfa leaves, from the cuttings, under the best care, has been estimated at one-half ton or more per acre every year. As these leaves contain a great percentage of protein, it can readily be seen that they make a heavy contribution to the soil’s fertility. VALUE OF STUBBLE AND ROOTS When in his system of rotation the farmer is ready to plow up his alfalfa, he has another inestimable contribution to the land’s fertility in the stubble and roots. It is not recommended to plow under any considerable growth as a green manure, as the hay crop is too valuable. Its market value would buy more fertilizers than the same growth is worth for humus. After a field has stood for five or six years, the roots have added largely to the humus content. Prof. W. P. Headden of Colorado, estimated that the fertilizing value of the stubble and 6¹⁄₂ inches of roots plowed under is about $20 per acre, while the value of the stubble and entire root system is not less than $35 per acre. The New Jersey station estimated that the amounts of plant food gathered by an acre of alfalfa in two years were equivalent in nitrogen to that contained in 3500 pounds of nitrate of soda; in phosphoric acid to that contained in 600 pounds of boneblack superphosphate, and in potash to the amount contained in 1200 pounds of muriate of potash, or equal to what would have cost $124. EFFECTS ON SUCCEEDING CROPS