Page:Popular Science Monthly Volume 22.djvu/100

90 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 fallow."

And again: "After the growth of six grain-crops by artificial manures 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 succeeding it gave 69·4 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 Rothamsted experiments, in regard to the sources from which the nitrogen of field-crops is obtained.

As free or uncombined nitrogen cannot, as we have seen, be assimilated 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 obtained 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,