The total phosphorus content of most surface soils is low, averaging only 0. This compares to an average soil content of 0. The phosphorus content of soils is quite variable, ranging from less than 0. Many factors influence the content of soil phosphorus:. Type of parent material from which the soil is derived. Degree of weathering and erosion. Climatic conditions. Crop removal and fertilization. Soil phosphorus is classified into two broad groups, organic and inorganic.
Organic phosphorus is found in plant residues, manures and microbial tissues. Inorganic forms of soil phosphorus consist of apatite the original source of all phosphorus , complexes of iron and aluminum phosphates, and phosphorus absorbed onto clay particles.
The solubility of these phosphorus compounds as well as organic phosphorus is extremely low, and only very small amounts of soil phosphorus are in solution at any one time.
Most soils contain less than a pound per acre of soluble phosphorus, with some soils containing considerably less. Soluble phosphorus, either from fertilizer or natural weathering, reacts with clay, iron and aluminum compounds in the soil, and is converted readily to less available forms by the process of phosphorus fixation.
Because of these fixation processes, phosphorus moves very little in most soils less than an inch , stays close to its place of origin, and crops seldom absorb more than 20 percent of fertilizer phosphorus during the first cropping season after application. As a result, little soil phosphorus is lost by leaching.
This fixed, residual phosphorus remains in the rooting zone and will be slowly available to succeeding crops. Soil erosion and crop removal are the significant ways soil phosphorus is lost. Precipitation of phosphorus as slightly soluble calcium phosphates occurs in calcareous soils with pH values around 8. Under acid conditions, phosphorus is precipitated as Fe or Al phosphates of low solubility.
Maximum availability of phosphorus generally occurs in a pH range of 6. This is one of the beneficial effects of liming acid soils. Adequate supplies of other plant nutrients tend to increase the absorption of phosphorus from the soil. Application of ammonium forms of nitrogen with phosphorus increases phosphorus uptake from a fertilizer as compared to applying the phosphorus fertilizer alone or applying the nitrogen and phosphorus fertilizers separately.
Applications of sulfur often increase the availability of soil phosphorus on neutral or basic soils, where the soil phosphorus is present as calcium phosphates. Soils high in organic matter contain considerable amounts of organic phosphorus that are mineralized similar to organic nitrogen , and provide available phosphorus for plant growth. Older leaves will have a dark green color and purple pigmentation. Since phosphorus is mobile in the plant, the bottom leaves are always affected first and show the earliest signs of deficiency.
Linkedin Youtube. Plant nutrients. It is obvious that an adequate P nutrition is essential for the proper photosynthetic activity of plants. Phosphorus has been found to be a key mineral nutrient responsible for an effective biological N2 fixation in leguminous plants. Under low P supply, nodulation and nodule functioning are impaired, and amount of N fixed is reduced.. Nodules represent a significant sink for P. It has been shown that P concentration of nodules of various legume plants is 3-fold higher compared to other parts of the plants Schulze and Drevon, , J.
Qin et al showed that P deficiency significantly impairs nodulation in soybean plants and causes marked decreases nodule number and nodule size Figure 4. When legume plants suffer from P deficiency, increasing amount of P is allocated in the nodules. Figure 3 : Changes in ATP formation and activities of ATP synthase enzyme in chloroplasts of barley plants grown under low and adequate P supply and also in P-deficient plants after re-supply of P For the details see Carstensen et al.
There is often a close relationship between nodule formation and nodule P concentration. High dependency of N2 fixation to sufficient P nutrition is related to the fact that that N fixation process is a highly energy-demanding process and requires at least 16 moles ATP to reduce one mole N2. Hence, the activity of nitrogenase enzyme is also positively influenced by improving P nutritional status of plants.
In whole nodules of soybean plants, the concentration of ATP and energy charge are significantly reduced under low P supply compared to the P sufficient conditions Sa and Israel, ; Plant Physiol. These findings highlight importance of P in optimal nodule performance and N fixation process.
Figure 4: Nodulation and nodule growth performance in soybean roots with low and adequate P application Qin et al.
One of the main structural changes in response to low P availability in growth medium is the process of tiller development. Tillering and number of tillers are known to be major plant factors affecting yield capacity of cereals. Under low P supply, severe reduction or suppression of tillering is very common, depressing biomass and yield formation Figure 5; Hammond and White, , J Exp Bot 93—;.
Similar to tillering, also leaf expansion is highly sensitive to P deficiency, and impairment in leaf expansion as a response to low P supply has been suggested to be the earliest change in P-deficient plants Radin and Eidenbock, , Plant Physiol —; Lynch et al. As expected, a decrease in leaf area will diminish light interception and biomass production.
Phosphorus is also associated with complex energy transformations in the plant. Adding phosphorus to soil low in available phosphorus promotes root growth and winter hardiness, stimulates tillering, and often hastens maturity. Plants deficient in phosphorus are stunted in growth and often have an abnormal dark-green color.
Sugars can accumulate and cause anthocyanin pigments to develop, producing a reddish-purple color. This can sometimes be seen in early spring on low phosphorus sites.
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