Volume 2.3 Phosphorus
Phosphorus (P) exists in the soil in organic and inorganic forms that enable plants to convert solar energy into chemical energy that is used to synthesize sugars, starches, and proteins. However, 80 percent of the total phosphorous in the soil is immobile and plants may have a P deficiency early in the season because the volume of soil roots occupy and actively absorb nutrients is low. Phosphorus deficiency can persist if root growth is inhibited by factors such as low soil pH, soil temperature, or soil moisture. Nevertheless, most P fertilizers added to the soil react with iron, aluminum, and calcium to become insoluble and unavailable to the plant. Therefore, the amount of phosphorus available to plants is much lower than the total phosphorus in the soil.
Available phosphorus
Available phosphorus refers to inorganic P dissolved in a water/soil solution that is readily available for plant uptake. Inorganic P forms are primarily mixtures of aluminum (Al-P), iron (Fe-P), and calcium (Ca-P) phosphates; the relative percentages between these three forms are a function of soil pH, with higher percentages of Al-P and Fe-P occurring in acid soils, and a higher percentage as Ca-P in neutral to alkaline soils.
The methods to assess phosphorus in soil already harmonized by GLOSOLAN are the following:
- SOP on soil available P - Bray I method
- SOP on soil available P - Bray II method
- SOP on soil available P - Mehlich I method
- SOP on soil available P - Mehlich III method (available soon)
- SOP on soil available P - Olsen method