The Soil and Plant Nutrition

One of the essential functions of the soil is to provide the plant with the nutrient elements necessary for it, which are absorbed by the roots in mineral form, even nitrogen, although it is stored in organic form. In most cultivated soils the permanent organic reserves of nutrient elements play only a very minor role, the humus generally being small in quantity and consisting of the most stable, hence scarcely mineralizable compounds. The question of nitrogen nutrition is very different in forested systems, where the ‘humus type’ that reflects all the aspects of the nitrogen cycle plays an essential role; in cultivated soils its role is minor. In very acid soils, and also in highly calcareous soils, phosphorus evolves under the influence of alternate wetting and drying towards crystalline forms, which are largely isolated from the cycle: this is the process of reversion.