_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 [49] Precipitation with an excess of acid does not greatly change the results. [50] In California soils this is mostly silica soluble in carbonate of soda. 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.