Phosphorus

Phosphorous is essential to many plant functions and structures. It plays a role in

·   Photosynthesis

·   Respiration

·   Seed and fruit production

·   Energy production, storage, and transfer

·   Cell division and enlargement

Adequate supplies of P promote or enhance

·   Early root formation and growth

·   Greater flowering and seed production

·   Fruit, vegetable, and grain quality

·   Better growth in cold temperatures

·   Water use efficiency

·   Early maturation of fruit and grain

The primary functions of P in plants are

·   Structural component of proteins, enzymes, nucleic acids, and DNA

·   Photosynthesis (production of sugars and starches)

·   Respiration (producing energy by oxidizing sugars and starches)

Interactions of P with Other Elements

Nitrogen: Many observations have found that P uptake is enhanced when in combination with ammonium N (NH4-N). In most cases, NH4-N has been shown to be superior to other forms of N at enhancing P uptake. This benefit typically requires that the N and P be applied in either a chemically combined form or as a concentrated mixture, such as a banded fertilizer blend. The exact mechanism for this reaction is not clearly understood. However, it is thought that as the NH4-N undergoes nitrification, P uptake is increased. It is also well known that increased N uptake stimulates the uptake of many other elements, and this may play a role in the effect.
Potassium: Potassium has been shown to co-precipitate with P when soluble phosphoric fertilizers are applied to soils. This effect is more pronounced in soils with high exchangeable K levels or with easily decomposed K-bearing minerals. However, this reaction has rarely been demonstrated to have a significant effect on plant growth. There is little or no evidence to show an interaction between P and K within the plant.
Calcium: As mentioned in the section on pH, calcium will combine with P to make insoluble compounds that are unavailable to plants in the short term. The general trend in the reaction is that as the soil Ca content and pH increase more P will combine with Ca to form compounds with ever-decreasing solubility. In these situations, it is typical to find that crops will require a correspondingly higher soil P test for equal growth. Alternatively, growers have seen that banding P fertilizers, especially when the band can be made acidic, improves crop growth in these conditions.
Magnesium: Phosphorus and Mg are often highly reactive in fertilizer manufacturing processes. The result of the reaction being the formation of highly insoluble compounds that coat or clog equipment. However, this effect has not been demonstrated to be a concern in the soil. In fact, much work has shown that Mg fertilization can enhance P uptake by plants. Within plants, Mg is an activator of certain enzymes that are critical to P transfer and as such, proper Mg nutrition would be essential to the uptake and utilization of P within the plant.
Sulfate:There has been some work that suggests that sulfates (SO4-S) may compete with soluble phosphates (H2PO4-) for the limited amount of anion retention sites in soil. These retention sites appear to primarily be Al and Fe hydroxides. The effect of such a relationship would be that high applications of either element should displace the other. In theory, this would cause a short-term increase in the amount of the displaced element in the soil solution, possibly followed by increased leaching of that element. The long term effect could be a depletion of the displaced element. While it does not seem likely that high rates of applied SO4-S would have a significant effect on P movement in the soil, the reverse seems possible in some sandy soil.
Zinc: Phosphorus interactions have been studied and widely publicized for many years. The results have shown that high levels of either element can depress the uptake of the other. While we know that the interaction can occur, we do not know enough to accurately predict when problem will occur. However, when soil P tests are above about 100 to 150 lb. /acre by either the Bray-P1 or Mehlich 3 procedures, the possibility of depressed Zn uptake should be a concern. The problem may be more severe, or occur at a lower soil P test in soils with a pH significantly higher than 7.0. It is rare in everyday situations for Zn applications to reduce P uptake. However, it can occur under the right conditions.
Copper: High soil P levels can depress Cu uptake, especially when other Cu limiting conditions are present, such as high soil pH and high soil organic matter. As early as the 1940's it was found that high P applications alleviated Cu toxicity by reducing the availability of soil Cu in Florida citrus groves. Other work with citrus confirmed the original findings, but little work has been done with other crops. However, at Spectrum Analytic we see evidence of this effect in plant analysis samples each year. While our observations are not research, it is common to receive plant samples of various species where elevated P uptake occurs with low plant Cu levels. These situations often occur on soils where a Cu shortage would not otherwise be predicted.
Boron:There has been little research into the possible interactions between B and P. However, boron is an anion in the available form. As such, it reacts with Al and Fe oxides. Since this process is similar to that of soluble P, it seems reasonable that it may interact with P in much the same way as SO4-S. In the late 50's and early 60's, researchers in California reported that applications of Ca(H2PO4)2 resulted in lower availability of B, especially in acid soils.