“We have seen when the arable land is laid down in grass an accumulation of nitrogen takes place in the surface soil. This accumulation may be slow or rapid, according to the treatment of the field, but in the case of an ordinary meadow the accumulation does not pass a certain point. After a certain number of years no further rise in nitrogen appears in the soil, although the external conditions of the meadow remain precisely the same as they were when the former accumulation of nitrogen took place. The influence of crop residue, Prof. Warington says, “where the ammonium salts were applied without ash constituents the produce was the smallest, and so was the nitrogen in the soil, and this nitrogen, like the crops, was a diminishing quantity. Where superphosphate was supplied with the ammonia the crop was considerably increased, and so was the nitrogen of the soil, which has shown little change in sixteen years. Where the ammonia was used with a full supply of phosphate and potash the produce was the largest; the nitrogen, too, of the soil was largest, and shows a tendency to rise.” “We see here at once a relation between the amount of the crop and the rise or fall of the nitrogen in the soil. The quantity of nitrogenous matter in a surface soil can only be maintained when the crop grown on the soil reaches a certain annual amount. There is, in fact, an annual waste of the nitrogenous capital of the soil, and if the proportion of the nitrogen of the soil is to be maintained there must be an equal annual addition of fresh nitrogenous organic matter. This is furnished to the soil in the form of crop residue, consisting of dead roots, leaves and stubble of a former crop, and the dead matter of weeds. When this crop residue is of large amount, as in the cultivation of red clover or in any case of green manuring, or when smaller residues are left untouched by the plow and allowed to accumulate, as in the case of a pasture, the conditions for an increase in the nitrogen of the soil are present. When, on the other hand, the crop residue is nil, as in the case of a bare fallow, or very small, as upon unmanured land, there is either none, or an insufficient replacement of the annual loss of organic matter in the soil, and the nitrogen of the soil consequently falls.” “The proportion of nitrogen in a soil can only be maintained when the supply of ash constituents (phosphates and potash) is sufficient to furnish the necessary amount of crop and crop residue.”
The nitrogenous organic matter contained in soils is for the most part an insoluble substance, a fact of the greatest importance for the maintenance of the fertility of the soil. While in this condition it is of little use to the higher order of plants among which our ordinary crops are included. To become available as plant food, it must be oxidized and rendered soluble, but as soon as this step is effected it becomes liable to be lost by drainage. Not many years ago we would have been satisfied in explaining the oxidation which occurs in soil as due to a simple contact with oxygen. We now take a different view of these changes. We know that the organic matter of a soil is split up and oxidized by means of living agents. A fertile soil is, in fact, teeming with life of many kinds. Many of these living agents are quite invisible to our eyes, and yet are performing changes on a great scale, upon the accomplishment of which the growth of our food crops depends.
The living agents which attack the organic matter of soil may be classed as (1) animal life—worms and insects; (2) fungi; (3) bacteria. The worms, beetles, larvae, etc., in a surface soil feed on the recently dead vegetable matter left by the crop or weeds which previously had possession of the soil. The carbon of this vegetable food is oxidized in their bodies and exhaled as carbonic acid, while the nitrogen is excreted in simple forms of combination. The fungi also feed on the nitrogenous organic matter of soil; carbon is oxidized in their cells and exhaled as carbonic acid, while their dead nitrogenous tissues restore to the soil a great part of the nitrogen which they had assimilated.
The conditions which favor the complete oxidation effected by bacteria are aeration of the soil by tillage, the presence of a suitable amount of water and of calcium carbonate, and a high temperature.
It has been mentioned above of the natural limits to the accumulation of nitrogen in the soil. Prof. Warington thinks he can now perceive some of the causes of such limits.
“The addition of organic matter to a soil either as crop or weed residue, or as farmyard manure, at once makes that soil a suitable home for the animal life—the fungi—and the bacteria, whose function it is to reduce organic matter to the condition of inorganic matter. An increase of organic plant residue or manure thus creates some of the conditions favorable to its own destruction. The rate of oxidation in the soil is now no longer what it was; the oxidizing agents have increased with the material to be oxidized. If, therefore, a soil is laid down in pasture or receives an annual dressing of farmyard manure, the nitrogen in that soil will only increase so long as the annual increment of organic matter exceeds the annual decrement by oxidation. If this increment is a limited quantity it will be met before long with an army of destroyers competent to effect its destruction. The richest soils are thus the most liable to waste and demand the greatest exercise of the farmer’s skill to preserve their condition. When the conditions of the soil are changed, when the pasture is plowed up or the arable land is left without manure, there is at first a rapid loss of nitrogen, but the rate of loss soon diminishes. The organic matter most easily attacked has disappeared. The army of oxidizing organisms has been reduced to starvation. A partial equilibrium is established when the annual destruction of organic matter amounts to little more than the annual residue of crop and weeds; but an absolute equilibrium is reached only when the annual loss by nitrogen is equaled by the atmospheric supply. In every case nature seeks to establish an equilibrium.”
For the American farmer to obtain good heavy crops, and consequently, large crop residues, there is only one way to do it. Invest some money in ground phosphate rock, and after applying it, plant a legume—cow peas, red clover, etc., and the investment will pay you the biggest dividend you ever received in your life.
Alfalfa-Growing in the South
By Joseph E. Wing, Mechanicsburg, Ohio.
