The effect of prolonged growing of alfalfa on the nitrogen content of the soil
1917
Swanson, C.O.
Kansas has a number of alfalfa fields which have been continuously in this crop for twenty to thirty years or more. The older fields are found in the central and western part of the state. Near these fields generally are fields which are in native sod used as pastures or as hay land, and fields which have been used continuously for grain growing for thirty to forty years or more. By sampling these fields in close proximity, data are secured from which the increase or decrease in the nitrogen content of the soil in alfalfa can be calculated. By assuming that the fields now in alfalfa had the same nitrogen content originally as the field now in native sod and that the average annual rate of loss before the alfalfa was seeded was the same as that of the fields used for continuous grain growing, the nitrogen content at the time the alfalfa was seeded can be calcuated. By comparison with the results of the three fields at the present time, calculation can be made of the increase or decrease of nitrogen content due to the growing of alfalfa. In no fields in alfalfa is the nitrogen content equal to that in native sod, except a few in the semiarid portion of the state, where it was greater. In most cases in the central and eastern parts of the state the nitrogen content of the alfalfa field is greater than that of the field used for continuous grain growing. By accounting for that lost before the alfalfa was seeded and comparing with the amount present in the soil now, it is found that on the whole the growing of alfalfa has not added to the amount present in the soil, except in a few fields in the semiarid portion of the state. All that the alfalfa has done has been to prevent further losses or, in other words, to maintain an equilibrium.
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