Grandeau Method: Matière Noire.—The only method now known which accomplishes this separation, practically excluding the unhumified while fully dissolving the humified matter—is that of Grandeau: the extraction of the soil, first with dilute acid, in order to set the humic substances free from their combinations with lime and magnesia; and their subsequent extraction with moderately dilute solutions of ammonia (or other alkali hydrates). Upon the evaporation of the ammonia solution the humus is left behind in the form of a black lustrous substance (“matière noire” of Grandeau) much resembling the crust of soot formed in flues from wood fires. As it contains a variable amount of ash, it must be burnt and the ash subtracted from the first weight.
Amounts of Humus in Soils.—While in peat, marsh and muck lands the humus-content may rise above twenty per cent, in ordinary cultivated lands it rarely exceeds about five per cent, and very commonly falls below three per cent, even in the humid regions. In properly arid soils we find a very much lower average, rarely exceeding one per cent, and frequently falling to .30 and even less. This scarcity of humus manifests itself plainly in the prevalently light gray tint of the arid soils.
Meadows and woodlands generally show the highest humus-content in their surface soils, gradually increasing while in that condition; while when taken into cultivation the humus-content gradually decreases, owing to the free aeration and consequent “burning-out” caused by tillage. Hence the humus must be from time to time replaced by the use of stable manure, or green-manure crops, to prevent injurious changes in the tilling qualities of the land. Not only humus as such, but according to Schloesing also the insoluble colloid humates, produce in the soil a loosening effect or tilth (Germ. Bodengare), which apparently cannot be brought about by any other substance.[48]
Humates and Ulmates.—That the insoluble humates of lime, magnesia, iron, manganese and alumina are present in most soils is conclusively shown by the composition of the solution obtained by the extraction of soils with weak acid, as above mentioned in connection with the quantitative determination of humus according to Grandeau; since these bases are almost always extracted by the weak acid. When the brown solution of alkali humate obtained in this process is carefully neutralized with sulfuric or hydrochloric acid, or is mixed with solutions of the above bases, flocculent, insoluble precipitates are formed, while the solution is discolored. Similar precipitates may be obtained with other metallic solutions, notably with that of copper, which precipitates the humus-acids most completely. Doubtless these compounds contribute greatly to the conservation of the humus-content of soils, protecting it to a certain extent from oxidation, and also preventing excessive acidity. The brown tint of certain subsoils in the northern humid regions have been shown by Tollens and others to be due not to ferric hydrate, as had been supposed, but to calcic, magnesic and aluminic humates. None of the mineral bases or acids present can be detected in the humic solution by the usual reagents.
Mineral Ingredients in Humus.—That the mineral plant-food ingredients present in the humus extracted by the Grandeau process, and which remain as ash when the matière noire is burned, are capable of nourishing plant growth, was directly shown by Grandeau, Snyder and others. The former was inclined to consider that those substances were mainly thus taken up by plants, under natural conditions. This theory, however, has not been sustained by subsequent investigations; the mineral plant-food thus extracted is not a measure of the immediate productiveness of the soils, as demonstrated by Snyder, and the residual soils are not sterile. It is also still doubtful to what extent the mineral bases and acids are naturally combined with the humus-substances, it being contended by some that they are brought into organic combination by the acid and ammonia extraction. The investigations of Snyder and Ladd, above referred to, prove however to some extent at least that the humus-substances are naturally combined with them, and that probably they are largely made available to plants through the direct and indirect action of the humus compounds. This subject is farther considered in [chapter 19].
The nature and amounts of these mineral substances are well exemplified in the subjoined full analysis by Snyder, of the ash of the humus and humates extracted from a compound sample of prairie soils of Minnesota, which had been thrown down from the ammonia solution by simply neutralizing the liquid:[49]
ASH OF HUMUS FROM MINNESOTA
PRAIRIE SOILS.
| Insoluble matter[50] | 61.97 |
| Potash (K₂O) | 7.50 |
| Soda (Na₂O) | 8.13 |
| Lime (CaO) | 0.09 |
| Magnesia (MgO) | 0.36 |
| Peroxid of Iron (Fe₂O₃) | 3.12 |
| Alumina (Al₂O₃) | 3.48 |
| Phosphoric acid (P₂O₅) | 12.37 |
| Sulfuric acid (SO₃) | .98 |
| Carbonic acid (CO₂) | 1.64 |
The large amounts of the soluble alkalies potash and soda thrown down with the humic matters are very striking, as is the very large proportion of phosphoric acid. Lime and magnesia had, of course, been mainly eliminated by the preliminary acid treatment.
Functions of the Unhumified Organic Matter.—The unhumified plant debris in the soil are not to be regarded as useless, even aside from their potential conversion into active humus. Not only do these remnants of vegetation lighten the soil, rendering it more pervious to air and water, but in their progressive decay they give off carbonic gas, which is active in soil-decomposition; and they serve as nourishment to the soil bacteria upon which its thriftiness so greatly depends. [See below, chapter 9].