TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Process: The defined condition is particularly the first pH range, with pH of the mixture of organic compound being adjusted to within the second pH range before the mineral addition and fermentation step. The iron is particularly maintained at a trivalent state in the mineral solution in the mineral addition and fermentation step and the post-composting mineral addition step, especially under aerobic conditions. Before the mineral addition and fermentation step, a step of aqueous mineral solution impregnation is conducted by impregnating the mixture of organic compounds with the aqueous mineral solution, while maintaining within the first pH range and supplying oxygen at the same time. The first and second pH ranges are at pH not less than 5, and therefore the definite condition is at pH not less than 5. Iron is the most abundant metal in the mineral solution which is present as Fe3+ at pH not more than 3, Fe3+and Fe(OH)2+ at pH 3-4, Fe3+, FE(OH)2+and Fe(OH)2+ at pH 4-5, and Fe(OH)2, Fe(OH)2+ and Fe(OH)3 at pH not less than 5. The sum of magnesium and calcium in such mineral solution is not more than 30 wt.% of the iron amount. An inorganic acid extraction step is carried out to prepare the mineral solution by extracting a sedimentary rock soil chosen from basalt and andesite to give a slag containing pure iron (Fe(3+)) and non-crystalline adlerstein (alpha-FeOOH) and the mineral solution.Preferred Humus Complex: The inorganic acid extract particularly contains a sum of magnesium and calcium of not more than 30 wt.% of the content of iron. EXAMPLE - Organic waste was homogenized, pH adjusted to 7 with slaked lime before soaking with a mineral solution for fermentation and composting with further additions of the organic waste and mineral solution and water, with agitation at 43 degrees Centigrade to give a ferrihydrite humus complex: 1.5 g of the ferrihydrite containing 1 g iron; specific surface area being 5 (g) x 200 (m2/g) per ton of complex.