http://www.spectrumanalytic.com/support/library/ff/P_Basics.htm

I would paste the whole article here but there are many tables that would be lost:

Factors Affecting P Availability

At the risk of stating the obvious, higher soil P tests are typically the most significant factor controlling P availability to crops. However, P availability can be strongly affected by several other soil factors

Soil pH: As the soil pH increases above about pH 7.0, soil P is increasingly "fixed" into less soluble/available forms by excess calcium. As the soil pH decreases below about pH 6.0, soil P is increasingly "fixed" into less soluble/available forms by excess soluble aluminum.

Soil Compaction: Phosphorous moves very little in the soil. Because of this, plant roots must be healthy and actively explore new areas of the soil daily in order to obtain adequate P nutrition. Anything that inhibits aggressive root growth is likely to reduce P uptake, even in high P soils.

Soil Aeration: Inadequate soil aeration is often related to soil clay content, soil drainage, and soil compaction. Most cultivated plants require adequate oxygen (O₂) in the soil atmosphere. A lack of adequate soil O₂ can reduce P uptake by as much as 50%.

Soil Moisture: As moisture stress increases, P availability and uptake decrease. Higher levels of soil P result in higher P uptake at all moisture levels. However, as soil moisture begins to exceed field capacity, the excess water excludes the needed oxygen from the soil and P uptake begins to suffer due to the lack of O₂ in the soil.

Soil Temperature: Cold soil reduces P uptake, as well as most other chemical and biological activity in the soil. This is the reason that many fields respond to row-placed fertilizer. As the summer begins to warm up, uptake efficiency of both the plants and the soil improve. However, permanent yield losses can occur from early season P shortages.

Soil Texture: Generally, low CEC soils require higher soil P tests to supply equivalent amounts of P to a crop. Such soils typically hold less water at any point in time, which slows P diffusion to the roots. These soils also have less particle surface area and it appears that current soil testing procedures may extract a higher percent of the total P in lower CEC soils. This would lead to less capacity to quickly replenish the P in solution (buffering power) and require a proportionately higher soil P level for equivalent P supplying power. Some clay types have a high P fixation capacity. These types of clay are more common in tropical soils. In these cases, we would logically expect a higher CEC to require a proportionately higher soil P level for adequate soil fertility.

Soil Organic Matter: The organic matter (OM) in soil may account for anywhere from 3% to 75% of the total P in a soil (not necessarily the same as "available P"). Generally, increased OM results in greater fixation of Fe and Al, resulting in less P fixation by these elements, and more labile (available) P. Such reactions also tend to reduce the fixation of applied P as well. Typically, in soils developed in temperate climates, the contribution of P by OM is relatively small and the main source of P for plants is the inorganic forms.

Crop Residues: Incorporation of large amounts of crop residues can result in immobilization of available P by microbes. As they decompose the residue, they grow and reproduce, thus creating their own need for available P. During the decomposition of crop residue, soil microbes are effectively in competition with higher plants. Microbially immobilized P will gradually become available as decomposition is completed, the microbes die, and they are re-cycled. Factors such as temperature, moisture, soil pH, soil aeration, and the availability of other nutrients have a direct bearing on the level of microbial activity in the soil and the rates of immobilization and mineralization. While this cycle is present in all soils, it is not thought to be a cause for major adjustments in fertilizer recommendation programs.

Plant Root Systems: As mentioned earlier, soil P is essentially immobile, and the portion of soil P that is soluble and immediately available to plants is exceptionally small. Therefore, plant roots must constantly explore large volumes of soil to satisfy their need for P. There are significant differences between species in the relative size and effectiveness of their root systems. There can also be significant differences of this type between hybrids and varieties within the same species. This is one factor in explaining why some plants or crops require different soil P levels for a given level of performance.

Mycorrhizae: Mycorrhizae are soil fungi that form a symbiotic association with plant roots. The thread-like hyphae of the fungus connect with plant roots and extend into the soil. The hyphae act like extensions of the plants root system by absorbing nutrients and transporting them back to the plant roots. A major benefit in this respect is an increase in P uptake. In exchange, the mycorrhizae receive sugars manufactured by the plant.

While mycorrhizae can infect most plants, they typically are more of a benefit to trees than agricultural crops. It has been demonstrated with agricultural crops that the benefits of mycorrhizae decrease as the soil P level increases. Soil P levels adequate for good yields of most crops essentially eliminate the benefits of Mycorrhizae.

In flooded soils, mycorrhizae may die. If the soil has a low level of available P, the loss of the Mycorrhizae may greatly decrease the following crops ability to absorb enough P. This effect has been seen in wheat following flooded rice crops.

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.

Molybdenum: While the interaction between Mo and P has not been studied extensively, some rather convincing evidence indicates that P increases the short term uptake of Mo. In its available form, Mo is an anion. It is presumed that the reason for P increasing the uptake of Mo is similar to the same relationship of P with B and SO₄-S. In all of these cases, both elements react with Al and Fe oxides. A comprehensive report by Stout et al (1951), found that P applications without SO₄-S increased Mo uptake up to 10-fold. The beneficial effects of P on Mo uptake appeared to be stronger in acid soils. That study found that the same P applications plus SO₄-S reduced Mo uptake. Since SO₄-S is an anion, it can compete with Mo for the few anion-adsorption sites in the soil. Therefore, it appears that while P can increase the short-term uptake of Mo, this may be offset by any significant amount of associated SO₄-S that may be present.

Silicon: There is considerable evidence with rice and sugarcane that added Si increases water-soluble and easily extractable P. Other results suggest that Si does not increase P uptake, but increases the efficiency of P use within the crop. While there is little practical application for this information for most crops, Si has been shown to improve yields of rice and sugarcane. Silicon has been shown to be effective as a disease preventative measure in rice.

Balances and Ratios

While many people believe or suspect that there are desirable ratios of P and K in the soil or a fertilizer program, research has not demonstrated that such ratios exist. It has been shown that in a few crops, there may be a desirable ratio of P with certain other elements like Zn or Ca. However, these relationships rarely play a role in modifying a fertilizer program. For now, we find the best method to evaluate P is the sufficiency range approach to both soil and plant analysis.

Plant Deficiency Symptoms

The typical visual symptoms of P deficiency exhibited by most plants include

• Stunted growth
• Dull-green or blue-green color
• Possible purple coloration on some part of the plant
• Reduced flowering and/or seed production
• Delayed maturity

Managing Phosphorous

Soil Test P Buildup:Agronomists often discuss fertilizer recommendations in terms of crop removal with or without soil test buildup as if they were unrelated subjects. In fact, they are two aspects of the same subject. A soil with a weak P level and a medium-to-strong P fixation capacity will convert or "fix" much of the applied fertilizer P into unavailable forms and leave little for the crop. In many situations, the soil will fix about 65% of the applied fertilizer P. In extreme cases, as much as 90% of applied P may be fixed. In these cases, applying only the amount of P that is expected to be removed will likely lead to a P shortage for the crop. In such soils the P fixation capacity must be overcome by higher fertilizer P rates in order to provide adequate amounts for the plants. In other words, soils with less than optimum P levels must have some soil buildup P, to insure that there is enough for crop removal. The difficulty lies in determining, with some accuracy, the minimum amount of buildup P required to overcome a particular soils fixation capacity during the season.

Soil test buildup application rates are typically discussed in terms of the ratio of applied P₂O₅ to soil test P buildup amounts. Often you will hear that most soils require about 9 or 10 lb. of P₂O₅, in excess of crop needs, in order to increase the soil test P by 1 lb (a P₂O₅/soil P ratio of 9 or 10). While this may be the most commonly occurring relationship, it is not nearly that simple. The range of possibilities can be quite wide and will likely be highly dependent on the initial soil test and annual rates of application.

Research at the University of Kentucky, reported in 2002, illustrates how soil test P buildup occurs. In this study, they looked at 16 different soils with initial soil test P levels ranging from 6 to 240 lb P/acre (Mehlich 3 extraction). They applied 6 different rates of phosphorous ranging from 38 to 228 lb P₂O₅/acre. We developed a regression equation from the Kentucky results to produce the data in Table 1. These results indicate that the initial soil P test is a major factor in determining how much P₂O₅ is required to buildup the soil test P."

Ed