Page:Popular Science Monthly Volume 22.djvu/98

88 that have arisen in the developmental progress of science, but simply to call attention to some of the leading lines of investigation at Rothamsted which have had an influence in correcting our theories of vegetable nutrition and soil-exhaustion and in improving our methods of agricultural practice.

The legitimate aim of all systematic, exact experiments is to lay a foundation of well-ascertained and closely related facts on which may be developed a superstructure of science to supersede the theoretical speculations which form an important part of the prelude of scientific discovery. In this work of reconstruction, Drs. Lawes and Gilbert have for many years occupied a prominent position, and a full account of their labors would involve in the record a history of agricultural science for the past half-century.

From an agricultural stand-point one of the first steps in the study of the laws of plant growth and nutrition is to ascertain the relative influence of the soil and the air in the supply of plant-food, as they are the only sources from which plants obtain the elements which enter into their composition.

The atmosphere is a mixture of gases, of which more than three fourths is nitrogen, and less than one fourth oxygen, with something less than one part in ten thousand of carbonic acid. In addition to these there are traces of ammonia and a variable quantity of vapor of water.

As the carbon, which forms about one half of the dry substance of plants, is all derived from the minute proportions of carbonic acid found in the atmosphere, it has been assumed that the comparatively small amount of nitrogen required by plants could be readily obtained from the abundant stores of this element in the atmosphere, and that wide-leaved plants, like clover and beans, could more readily assimilate it than those with narrow leaves, like the grasses.

Experiments by Boussingault and the elaborate researches at Rothamsted, however, show that free nitrogen, the most abundant constituent of the air, is not assimilated by plants. The atmospheric sources of nitrogen for plant-growth must, therefore, be limited to the minute quantities of combined nitrogen in the form of ammonia and nitric acid.

Important data as to the amount of nitrogen in various field-crops, grown under a variety of conditions, and the sources from which it is obtained, are furnished in the Rothamsted field-experiments.

For a period of thirty-two years, wheat, on plots without manure, yielded an annual average of 20·7 pounds of nitrogen per acre. The yield, however, declined from an average of more than twenty-five pounds during the first eight years to an average of but sixteen pounds during the last eight years of the experiment.

Barley, for a period of twenty-four years, on plots without manure, yielded annually an average of 18·3 pounds of nitrogen per acre, with