Green Manure The most import ant method of maintaining the soil's fertility is by the use of green manure crops, such as clover, cowpeas, soy beans and alfalfa. These crops add to the supply of nitrogen in the soil under favorable conditions and when plowed under improve its tilth. When harvested they con stitute a valuable feed for live stock. The addi tion to the plant food supply of the soil by the growth of leguminous crops is accomplished in two ways: Firstly, by the presence and growth of certain organisms peculiar to these crops, inducing the development of root tuber cles. These organisms have the power of draw ing upon the free and unavailable nitrogen of the atmosphere and converting it into an avail able and useful condition for plant food. Sec ondly, from the fact that these crops root deeply as a rule, drawing upon plant food in the soil, which is not in position for use by ordinary crops. This food is assimilated by the plants, so that when the crop is plowed under the de composition of the roots and stems leaves this plant food where it can be gotten at and used by corn or other plants.
Cowpeas and soy beans are leguminous crops, introduced into this country recently from Asia, and are coming to be grown on a large scale as green manure crops for the prep aration of the land for corn.
In summing up the important points as re gards the condition of the soil for corn, it may be said that continuous cropping without rota tion or manuring seems inadvisable, and results in the exhaustion of the fertility of the land to such an extent as to render it an expensive process to bring it back to a state of productive ness. Commercial fertilizers are expensive agents for maintaining the food supply for corn, and cannot be successfully used except for local or peculiar conditions or for the correction of some unusual occurrence, as the acidity of cer tain bog soils, or the small alkaline areas in the fields in certain sections of the Middle West. The most successful and profitable plan adapted to most conditions seems to be a rotation of corn with some legume, preferably one which can be used to supplement corn as a ration for live stock, and the feeding of all crops followed by the return to the soil of the plant food in the shape of composted manure.
Zea mays originated in all proba bility in Mexico. From Mexico it was carried north by the Indians by means of barter and trade, so that when the early explorers of Amer ica visited the section now included in the United States they found considerable areas un der a crude system of corn cultivation by the Indian tribes. Upon the development of the vast sections of the Mississippi Valley corn became the principal crop, and is now recog nized as the leading American cereal. From
America this crop has been carried to all con tinents of the world, and is grown to a greater or less extent in tnost of the leading countries, especially in those in which the climate and soil conditions are similar to those of our Missis sippi Valley region. Corn flourishes best in those sections having an annual rainfall of about 30 incites, or an artificial sdpply by irri gation, and a season of about 120 days for the maturing of the larger types. The crop seems to reach its best development in the temperate regions, where a part of the season, the time of greate.st growth, has warm days and nights. Under such conditions the plants grow with great rapidity. Corn readily responds to cli matic conditions; namely, by taking a late variety to a region of short seasons, an early variety can be developed by selection, and vice versa. For this reason, we have races of corn which vary greatly in their characteristics, due to the adaptation to climatic conditions. It is not a good policy suddenly to remove a race suited to a peculiar set of conditions to very different conditions. Under such circumstances the crop may fail to mature, or may develop some quality detrimental to its value. It is probable that by selection races may be im proved for any given corn region, and that by continued breeding and selection, these races may be further improved without resource to the importation of seed from other sections. However, it seems that there are certain con ditions it' which corn naturally reaches its highest and fullest development, and it is prob able that in those sections most advance will be made in the permanent improvement of corn. It may be advisable for corn-growing sections not specially suited for corn growing occasion ally to secure a small supply of breeding seed from the best corn regions, which after a few years' growth will have become suited to the new conditions and may have a better type than those which have been grown in the less favor able localities.
One of the important things which has been neglected for the most part, in so far as climatic conditions are concerned in corn cul ture, is the systematic comparison of different races from different regions, in order to find those races which are the best naturally suited to local conditions. It is probable that before breeding is begun, it would be advisable to make such a test in order to determine the best breed ing stock for permanent improvement.