Page:Popular Science Monthly Volume 22.djvu/99

Rh a decline from an average of twenty-two pounds over the first twelve years to an average of 14·6 pounds over the next twelve years. Mineral manures, containing no nitrogen, applied to barley and wheat did not materially increase the yield of nitrogen in the crop.

A succession of root-crops (with three years of barley intervening after the first eight years), dressed with a complex mineral manure, yielded an average of 26·8 pounds of nitrogen per acre, per annum, over a period of thirty-one years; with a decline from an average of forty-two pounds over the first eight years to 13·1 (in sugar-beets) over the last five years. Afterward, with the change of crop to mangolds, the yield of nitrogen was somewhat increased.

Beans, for a period of twenty-four years, yielded an average of 31·3 pounds of nitrogen without manure; and, with a complex mineral manure, an average of 45·5 pounds of nitrogen per acre. The decline in yield of nitrogen was, however, from an average of 48·1 pounds over the first twelve years to only 14·6 pounds over the last twelve years, when unmanured, and from an average of 61·5 pounds over the first twelve years to but 29·5 pounds over the last twelve years, when mineral manures were applied.

An annual average yield of nearly two hundred pounds of nitrogen per acre was obtained in the clover grown for twenty-seven years in succession on a plot of old garden-soil that was exceptionally rich in nitrogen at the beginning of the experiment. As in the case of other crops, there was a marked decline in the average yield of nitrogen in the last half of the period, and there was also a great reduction in the stores of nitrogen contained in the soil.

The leguminous crops, beans and clover, it will be seen, contain a larger amount of nitrogen per acre than the gramineous crops, wheat and barley. In the Rothamsted experiments it was, however, found that manures containing nitrogen benefited the gramineous crops, while they had but little, if any, influence upon the growth of leguminous crops. The chemical composition of the crop was not, therefore, an index of the manurial constituents required to promote its growth.

When turnips, barley, clover, or beans, and wheat were grown in rotation for twenty-eight years, the average annual yield of nitrogen per acre was 36·8 pounds, and in the mixed herbage of permanent grass-land, when unmanured, the annual yield of nitrogen averaged thirty-three pounds per acre.

The larger average yield of nitrogen per acre in the crops in rotation and in the mixed herbage of the permanent grass-land, as compared with the yield of nitrogen in gramineous crops when grown separately, is not entirely due to the larger amount of nitrogen in the leguminous species themselves, but also to their influence upon the gramineous species which are able to take up and assimilate more nitrogen when the highly nitrogenous leguminous crops have been