depth of from ten to fourteen feet furnishes good drainage for the spaces between the drains.
The nitrogen applied in the manures that is not taken up by the crop or stored up in the soil, or lost in the waters discharged by the tile-drains, may therefore be fully accounted for in the amount that must be carried, under the conditions of the experiments, by the drainage-waters to the lower strata of the subsoil, without entering the drains. The estimated losses of nitrogen by drainage, based on the amounts detected in the waters discharged by the drains, may therefore with good reason be increased by the amount not accounted for in the crop and in the accumulations of the soil.
Practically, then, in the light of the Rothamsted experiments, we may look upon the soil as the great source of the nitrogen of plants, as the atmosphere can furnish but a small proportion of the needed supply, and this is more than counterbalanced by the losses from drainage.
In connection with this imperfect outline of some of the leading lines of investigation that have been so successfully prosecuted at Rothamsted, it would be interesting to examine the data that indicate the relations of nitrogen to other elements of plant-growth, as supplied in manures and assimilated by crops when cultivated in succession or in rotation with other species; but these, with other cognate topics, must be omitted, as we can not at this time undertake anything like an exhaustive discussion of the results of these valuable experiments.
The great importance of physiological researches and the comparatively subordinate influence of purely chemical methods in solving the great problems of agricultural science, have been so fully illustrated in the experiments at Rothamsted that we must accept them as the basis of a new departure in the development of a consistent science of rural economy. In the light of these experiments the generally accepted theories of soil-exhaustion must be reconstructed, and the action and relative value of manures must be investigated from a new stand-point.
The exhaustion of a soil can no longer be estimated by the constituents removed in the crop, Wheat and oats, with other cereals, are generally considered as exhausting crops, and a summer fallow is looked upon as a means of increasing or restoring the fertility of the soil; but the grain-crops when grown by themselves, and the summer fallow itself, are alike the occasion of a loss of fertilizing materials, and in precisely the same way. In both cases there is a long interval in which there are no living roots of plants in the soil to take up the nutritive materials as they are transformed into the soluble form, and they are lost by percolation to the lower strata of the subsoil out of the reach of vegetation.
Many of what are called restorative plants feed in the deeper layers of the soil, and they may, by their scattered foliage and thick roots,