A model of solute uptake by a growing root system is discussed in relation to the ability of a soil to support the nutritional requirements of plants. It is quantitative, and should apply to the absorption of nitrate, phosphate, and potassium by grasses and arable crops. The principles, if not the detail, are relevant to all soils.
The model was tested in an experiment in which the nitrogen and potassium taken up by a rape plant were measured. The results suggest that the model has identified the significant variables in the absorption process.
The principal factors affecting the supply of nutrient to a given plant are the total quantity of diffusible nutrient, the rate at which the nutrient can move, and the distance it has to travel to a root surface. The exact relevance of each factor for the different nutrients is readily determined.
The diffusion coefficients of potassium and phosphate are often low. Mass flow contributes little to their supply, and an adequate exploitation of the soil reserves depends on a well-developed root system. Nitrate, on the other hand, can usually move easily to roots, either by diffusion or mass flow. The whole rooting volume is depleted fairly quickly, and the amount absorbed depends solely on the quantity present in the rooting volume.
These ideas have practical significance. In particular, they suggest that the soil properties which influence root growth may deserve as much attention as soil chemical composition in the drawing up of fertilizer recommendations.