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less decomposed organic matter. Besides the nitrogen, the humus either contains phosphoric acid and potash in highly available forms or assists in rendering them available, for the crop is enabled to obtain much more of these substances from a soil rich in humus than from one in which the humus content is low.

The presence of decomposing organic matter in the soil is an important factor in making the mineral elements of plant food available. During decay certain acid substances known collectively as humic acids are produced, and these undoubtedly have a solvent action on the mineral matters of the soil, tending to make them more available to the plant. Perhaps quite as important a factor is the large amount of carbonic acid formed during the process of fermentation. This carbonic acid dissolved in the soil water is of prime importance in the production of soluble plant food, and it also has a beneficial effect on the physical condition of the soil, especially if the soil contains a large amount of clay.

Experiments conducted in Minnesota and North Dakota have shown conclusively that as the humus content of the soil is decreased by constant cultivation and cropping (especially if planted continuously to one crop like wheat) the nitrogen content of the soil, the amount of moisture that it will retain, and the crop production are likewise decreased.

Humus in Soil Decreased by Tillage. All the methods for making potential plant food available which have been so far discussed tend to decrease the amount of humus in the soil. Tillage, drainage and

bare fallowing increase the amount of food available to the crop, because they present ideal conditions for the decomposition of the organic matter in the soil, but dependence upon these methods alone will eventually result in injury through loss of humus. This fact is strikingly shown in the following table adapted from a Minnesota bulletin.

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Humus Increased by Green Manuring.-The lesson to be learned from the above table is that while intelligent use should be made of improved methods of tillage, etc., these should be supplemented by some means of maintaining the supply of humus. The method that first suggests itself. is Nature's own way of growing a crop to be afterwards incorporated with the soil. This is the process known as green manuring. Plowing under green crops raised for that purpose is one of the oldest means of improving the fertility of the soil. It was advocated by Roman writers more than two thousand years ago, and has been in more or less common use among progressive farmers ever since.

The value of green manuring depends primarily upon the fact that it increases the amount of humus in the soil, a point that has been shown to be of great importance.

Two Classes of Plants for Green Manuring. The crops used for this purpose may be of two kinds, viz: those which add nothing directly to the soil, and those which increase its nitrogen supply. Of the first class the crops generally recommended are buckwheat, spurry, mustard, rye, rape, etc. Plowing sod land

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Oats as a green-manure. Two classes of plants are used as green-manurethose which gather nitrogen and those which do not. Oats is a good example of the latter class.

may be said to be a species of green manuring, and as such would be included in this class. These crops while they add no element of plant food to the soil are beneficial because they gather food from the soil that would not be available to the less hardy plants, and on their decay leave it in forms suitable to the succeeding crop. As mere humus formers they are of great value.

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

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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 clover, 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.

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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

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