696 THE POPULAR SCIENCE MONTHLY. 



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. Fa- 

 vored by position and circumstances, he has been enabled to carry out on 

 a large scale most important operations. Ills 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 vari- 

 ous 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 

 Tears 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 am- 

 monia, 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 labora- 

 tory 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 ma- 

 nures, 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 AS'hat 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 

 sup))]ies 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 in- 



