The discovery that the leguminous plants can through the nodule forming bacteria fix the free nitrogen of the air, has thrown a new light on green manuring, and the plants adapted to this purpose. The legumes have all the advantages of the other plants as humus formers, and at the same time increase the amount of nitrogen in the soil, and conse

quently should be used for this purpose whenever possible. They are as a rule deeper rooted plants, and are supposed to bring up mineral food from the subsoil, and leave it where it will be within reach of the more shallow rooted plants. Of the legumes, the crops most often recommended are red clover, the lupines, cowpea, crimson clóver, soy bean, the ordinary field bean and field pea; red clover being probably the one most generally used. These plants have been found to produce good results even when the

crop was harvested, and the stubble only plowed under. At the Rothamsted Experiment Station it has been estimated that 50 pounds or more of nitrogen per acre is added to the soil in the roots and stubble of clover alone.

Catch Crops for Green Manuring.-Where it is not advisable to devote an entire season to the growth of a crop for green manuring, good results may often be obtained by growing "catch crops" between the profit crops. The use of cover crops on orchards, and as a protection to the land during the winter, are modes of green manuring. As far as possible leguminous plants should be used for this purpose. The assertion is frequently made that by good tillage, and a judicious use of leguminous crops, the fertility of the soil may be maintained indefinitely without the use of fertilizers of any kind. The writer feels that this point has yet to be demonstrated, but no one doubts that these plants are of great value in the conservation of fertility.

Danger from Green Manuring.-While green manuring is a valuable method of increasing the humus supply of the soil it is not unattended by danger. In a dry season, for instance, the growth of a crop to plow under may result in lowering the moisture content of the soil to a point that is detrimental to the succeeding crop. There is also danger in such a season that there may not be sufficient moisture in the soil to bring about the decomposition of the organic matter which is turned under, resulting in serious injury to the physical condition of the soil. If a crop is plowed under during a dry season the ground should

be rolled, or otherwise firmed, so as to renew capillarity as far as possible.

Green Manuring Not Advisable on Stock Farms. -Green manuring as a general practice is not to be recommended in any style of stock farming. The

Crimson clover as an orchard cover-crop. The cover-crop is one method of green-manuring, and the crimson clover is a good example of the nitrogen-gathering plants used for this purpose

crops which are most valuable as green manures are also of great value as feeds, and it will be found more profitable to feed them to the animals and return the manure to the field, as will be shown later. On the whole, it may be said that green manuring will prove desirable in any system of farming (including truck farming) where the crops are sold from the farm, and

especially if all the crops produced are much alike in food requirements. On the other hand, if the farmer is engaged in animal husbandry the crops are of such great value as feeds that turning them under must be considered a wasteful practice.

CHAPTER X

ROTATION OF CROPS

Origin of Rotations.-It is the common experience of farmers in those parts of the world where the land has been cultivated for a long time, that the fertility of the soil is maintained for a much longer time by growing a variety of crops instead of producing one crop continuously. The adoption of a system of rotation of crops has been the outgrowth of accident rather than the result of an understanding of its underlying principles. The system of alternating years of barefallow and wheat may be said to be a two year rotation and was the first to be adopted. History teaches us that this was later followed by a three year rotation consisting of fallow, wheat, beans or oats; and still later, when the value of clover and fallow crops became evident, this rotation gave way to the now famous Norfolk rotation of turnips, barley, clover and wheat, the typical English rotation. The Norfolk four year course represents the more common type the world over, consisting as it does of cereals alternating with hoed crops and leguminous crops.

Plants Differ in Food Requirements.-There are many arguments to be advanced in favor of growing a variety of crops on the soil. The different crops vary in their food requirements and in their ability to procure this food from the soil. Where one crop is grown