88 THE POPULAR SCIENCE MONTHLY. 



versies 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 meth- 

 ods 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 Ro- 

 thamsted, however, show that free nitrogen, the most abundant con- 

 stituent 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 



