Phosphorus is considered as an essential nutrient for all forms of life without which living organisms fail to complete their life cycle in both aquatic and terrestrial ecosystems. Moreover, most ecosystems both terrestrial and fresh water also exhibited low net primary productivities and phosphorus has been implicated as number one cause next only to nitrogen. But unlike nitrogen, which has very reactive atmospheric gaseous component, most phosphorus exists as solid minerals that are sparingly soluble in the soil system. Chemical interactions of these phosphorus bearing primary and secondary minerals in the soil system dictate the amount of bioavailable portion of the total Phosphorus that is necessary for sustainable growth of most ecosystems.
The major sources of phosphorus containing minerals in soils are apatites which are calcium bearing phosphorus minerals which are further classified as chlor, flor and hydroxyl apatites depending upon presence of chlorine, fluorine and hydroxyl in the structure of the mineral. Most of these are sedimentary in origin and very little soluble in the soil solution. During the course of weathering, the phosphorus released from these minerals might be utilized by living organisms besides also undergo its reactions with soil constituents. The interactions include adsorption on to the soil surfaces, and precipitation into secondary phosphorus bearing minerals that might be stable under changing soil environment. The bioavailable portion of phosphorus refers to that portion of the total phosphorus that becomes available to growing plants during the growing season.
Many factors affect its release to plant and the pH of the soil is one of them which greatly influence the bioavailable phosphorus. The soils with pH less than 6.0 and more than 7.5 begin to show deficiencies of phosphorus. The main reasons for this behavior are that under acidic conditions phosphorus released during weathering of apatities might react with soluble iron, aluminum and manganese and results in secondary minerals which are very stable in acid environments with very low solubility. Similarly, alkaline calcareous soils are not conducive environment for rapid weathering of apatities. Whatever phosphorus released might be adsorbed on to the layer silicate clay and calcite minerals as well as also resulted in the formation of secondary calcium and magnesium bearing phosphorus minerals of low solubility. Aforementioned reactions and low solubility of phosphorus bearing minerals offer dilemma and restrict the growth of most ecosystems.
In managed or agricultural ecosystems, the phosphorus deficiency could partially be offset by the application of phosphorus sources like phosphorus fertilizers, manure and other agricultural by-products and in most cases increased crop production could successfully be attained worldwide but these routes for increased production might not be available for natural ecosystems such as forests or not at least feasible economically at present. The soils of tropics which are regarded as highly weathered under moist and warm conditions show greatest challenge for increased growth of most plants under natural ecosystems. Most of apatities disappeared due to acidic soil environment and new secondary phosphorus bearing minerals of iron and aluminium only would likely offer very little solution phosphorus that might be taken up by plants. Similarly, prevalence of large proportion of aluminium and iron oxides as well as hydroxides minerals also offer large surface area for irreversible adsorption of phosphorus onto their surfaces referred to as occluded phosphorus. Therefore, only mineralization of organic phosphorus might contribute some phosphorus to the forest plants in these soils.
