“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.”