| Nitrate formed during | Four months. | Twelve months. | Two years. |
|---|---|---|---|
| Leached natural soil | .012 | .0420 | .061 |
| Extracted soil | None. | .0030 | .0042 |
It will be noted that in the course of four months, nitrification had not sensibly set in the extracted soil; while in the leached natural soil the nitrate-content had reached to three-fifths the amount originally present, and in the course of a year the nitrate-content of the latter was more than double that of the original (unleached) soil; while that in the extracted soil had only reached one-seventh of the same. At the end of two years we find a still farther increase of nitric nitrogen in both, the ratio between the two remaining about the same (1:14). At the same time the ratio of increase attained at first had materially diminished in the water-leached soil, probably on account of the accumulation of the niter itself.
It thus appears that although the nitrogen of the unhumified organic matter constituted about 40% of the total in the original soil, it would during the entire year have contributed only to an insignificant extent to the available nitrate-supply; while the fully humified “matière noire” contributed fourteen times as much. During the ordinary growing-season of four or five months the unhumified organic matter would have yielded practically nothing to the crop.
Functions of the unhumified Vegetable Matter.—The chief utility of the unhumified matter in the soil consists of course in its gradual conversion into true humus, in the course of which it evolves carbonic gas to act on the soil minerals; while at the same time it helps to render the soil more porous and thus facilitates the action of the aerobic bacteria, for which it serves as food. Hence the addition of vegetable matter to soils not already too “light” is always advantageous, so long as it does not introduce injurious, non-humifiable ingredients, like turpentine in the sawdust of resinous pines. But it is always advisable to first use such matter as litter for stock, in order to better prepare it for the processes of humification, under the influence of ammoniacal fermentation, such as occurs in the decay of green plants or animal matter. A portion of the ash ingredients also is quickly utilized by solution in the soil-water.
Matière Noire the Only Guide.—According to these results it is clear that in order to gain any tangible indications with respect to crop-bearing, it is the nitrogen in the humus proper, the matière noire only, that should serve as the basis; and that as a current source of nitrogen to the plant, the unhumified matter is hardly entitled to more consideration than the “insoluble silicates.” For, the favorable conditions for nitrification under which the above experiment was conducted, will very rarely be even approached under field conditions.
What are the Adequate Nitrogen Percentages in the Humus?—The nitrification of the matière noire being, apparently, the main source of plant-nutrition with that element under ordinary conditions, the question naturally arises as to what may be considered an adequate nitrogen-content of that substance, so as to permit a full supply of nitrates to the crop.
The data extant on this subject are rather scanty, and thus far have all been obtained at the California Experiment Station.[121] But they seem to be very cogent in proving that the growth of crops removed from the soil causes a rapid depletion of the nitrogen in the humus-substance, and that so soon as the nitrogen-percentage in the same falls below a certain point, the soil becomes “nitrogen-hungry;” so that the application of nitrogenous fertilizers is needed and is very effective. The data in the table below, as well as the figure of a culture experiment (No. 52 below), illustrate this point.
ADEQUACY AND INADEQUACY OF
NITROGEN CONTENTS OF HUMUS.
| Collection Number. | Kind of Soil. | Locality. | Per cent. Humus in Soil. | Per cent. Nitrogen in Humus. | Per cent. Nitrogen in Soil.[122] |
|---|---|---|---|---|---|
| 6 | Black Adobe. | Near Stockton, San Joaquin Co., Cal. | 1.05 | 18.66 | .196 |
| 1679 | “ | Virgin Soil, University Grounds, Berkeley | 1.20 | 18.58 | .203 |
| 1842 | “ | Ramie plot, Univ. Grounds, 10 years cultivated | 1.80 | 4.17 | .075 |
| 1841 | “ | Grass plot, Univ. Grounds, 10 years cultivated | 1.65 | 3.40 | .056 |
| 29 | Dark loam. | Sugar-cane land, Maui, H. T. | 10.90 | 3.15 | .347 |
| 27 | Dark loam. | Guava-land hills, near Hilo, Hawaii Island | 9.95 | 1.71 | .170 |
Nos. 6 and 1679 show the usual humus-and nitrogen-percentages in the “black adobe” or “prairie” soils of California. Nos. 1842 and 1841 represent the same soil as 1679, upon which, however, ramie and ray grass had respectively been growing, without fertilization, for about ten years; showing that while the humus-content of the soil has increased, the nitrogen-content of the humus has decreased in the case of ramie by 72.78%, in that of the grass by 76.78%; reducing the land to figures commonly found in the humid region. In the case of the ramie, the partial return through the leaves has resulted in a higher humus-content, together with higher nitrogen-percentage, than in the case of the grass, which in the several cuttings annually made, caused a greater depletion in nitrogen and a smaller accession of humus. The grass was very weak in its growth and partially dying out.