9 o THE POPULAR SCIENCE MONTHLY. 



priating their larger supplies of the same element, as will be seen from 

 the following experiments : 



" In alternating wheat and beans, the remarkable result had been 

 obtained that nearly as much wheat and nearly as much nitrogen 

 were yielded in eight crops of wheat in alternation with the highly 

 nitrogenous beans as in sixteen crops of wheat grown consecutively 

 without manure in another field, and also nearly as much as were 

 obtained in a third field in eight crops, alternated with a bare fal- 

 low." 



And again : " After the growth of six grain-crops by artificial ma- 

 nures alone, the field so treated was divided, and in 1873 on one half 

 barley, and on the other half clover, was grown. The barley yielded 

 37*3 pounds of nitrogen per acre, but the three cuttings of clover 

 yielded 151*3 pounds. In the next year, 1874, barley succeeded on 

 both the barley and the clover portions of the field. Where barley had 

 previously been grown, and had yielded 37*3 pounds of nitrogen per 

 acre, it now yielded 39*1 pounds ; but where the clover had previously 

 been grown, and had yielded 151*3 pounds of nitrogen, the barley suc- 

 ceeding it gave G9*4 pounds, or 30*3 pounds more after the removal 

 of 151 "3 pounds in clover than after the removal of only 37*3 pounds 

 in barley." 



We will now examine some of the evidence furnished by the Ro- 

 thamsted experiments, in regard to the sources from which the nitro- 

 gen of field-crops is obtained. 



As free or uncombined nitrogen cannot, as we have seen, be assimi- 

 lated by plants, we will next consider the supply of combined nitrogen 

 in the form of ammonia and nitric acid, existing in the atmosphere. 



From the earlier investigations of the rain-fall at Rothamsted and 

 likewise on the Continent, it was estimated that from eight to ten 

 pounds of combined nitrogen per acre was precipitated annually in the 

 rains of Western Europe. Later observations at Rothamsted show 

 that this estimate is probably too high, and Drs. Lawes and Gilbert, 

 after a full discussion of their records for twenty-seven years, fix the 

 probable amount at four to five pounds per acre. 



As this is only one fourth of the average annual yield of nitrogen 

 per acre of the unmanured wheat over a period of thirty-two years, 

 and but little more than one fourth of the average annual yield ob- 

 tained with barley over a period of twenty-four years, to say nothing 

 of the much larger yield of nitrogen in leguminous crops, it must be 

 admitted that it is an entirely inadequate source of supply of nitrogen 

 for vegetation. The nitrogen condensed by the soil from dew and 

 atmospheric vapor has not been definitely determined, and is not, 

 therefore, included in this estimate ; but it is probable that it is less 

 than that brought to the soil by the rain. On the other hand, it has 

 been shown by numerous experiments, including those at Rothamsted, 

 that free nitrogen is evolved in the decomposition of organic matter, 



