Page:Popular Science Monthly Volume 28.djvu/714

696 nitrogen brought down by rain or that absorbed by the soil or the plant, constitutes but a very small proportion of the total amount they assimilate, and that the soil itself (or manure) is practically the main source of their supply. Indeed, it is a question whether on arable land as much or more may not be lost by drainage or otherwise than is supplied by the atmosphere." The field experiments on which these conclusions rest have formed Sir John Lawes's principal work. Favored by position and circumstances, he has been enabled to carry out on a large scale most important operations. His general plan has been to select fields in a condition of agricultural exhaustion, that is, in a state in which a fresh supply of manure was needed to fit the soil for the growth of another crop. Upon this exhausted soil each of the most important crops in the rotation was grown year after year upon the same spot, in plots without manure, and in other plots in which various kinds of manure, but usually the same to each, were applied yearly. Thus it became possible to determine the point of relative exhaustion or excessive supply of any of the constituents of the manure. The details of this method are given an exemplary explanation in Mr. Lawes's "Report of Experiments on the Growth of Barley for Twenty Years on the Same Land," published in 1874, when the experiment was still in progress. The field had been divided into plots of about one fifth of an acre each. Some of these had never received any manure during the twenty years; the others received some one or more of the food constituents which barley requires. Thus, one was manured with phosphates, a second with alkalies, a third with ammonia, a fourth with ammonia and phosphates, a fifth with ammonia, phosphates, and alkalies, etc., every year in succession. At harvest the crops were carefully weighed, and were then analyzed in the laboratory under the superintendence of Dr. Gilbert, when the amounts of dry matter, ash, and nitrogen, were determined.

"The advantages of this systematic mode of experimenting," says an English review of the report, "are very great. Carried on in the same manner for so many years, these experiments answer questions relating to the exhaustion of the soil, to the permanent effect of manures, to the effect of season upon the produce. With the aid of the laboratory investigations they teach us what proportion of the various ingredients supplied in the manure is recovered in the crop, and how the composition of the plant is affected by the various conditions of the soil. In conjunction with analyses of the soil and of the drainage water, we learn what becomes of the manures supplied, how deeply they have penetrated into the soil, what is the loss suffered through drainage, etc. A single field experiment, thus thoroughly and patiently carried out, touches half the domain of agricultural chemistry, and supplies information of the most solid and valuable kind."

Mr. Lawes addressed himself with great skill and success to the task of perfecting the methods of analysis; but, even after all his