Uptake of P by plants is very sensitive to its concentration in the soil solution. This concentration (P-plant available) is often very low but is buffered by various P fractions in the solid phase through complex desorption–adsorption, dissolution–precipitation and mineralization–immobilization processes. These processes are in turn influenced by the presence and activity of plant roots and environmental conditions. P-plant available is often only a few kg per hectare (roughly between <2 and 20 kg P per hectare). P-plant available is the amount direct available to crops (within year-level).