CHS Inc., Montana

Phosphorus Cycling, Testing and Fertilizer Recommendations

by Clain Jones and Jeff Jacobsen (excerpt)

Phosphorus Cycling

Phosphorus can exist in the soil as phosphate (HPO4-2 or H2PO4-), sorbed P, organic P, or in P minerals. Phosphate is the only form that plants can take up, yet in most agricultural soils there is less than 1 mg/L (1 ppm) of phosphate in solution, which represents much less than 1% of the total soil P. Organic P, which is P bound in organic matter, has been found to represent between about 25% and 65% of total P in surface soils, with mineral P (such as calcium phosphate minerals) and sorbed P, representing the remainder. Organic P decreases quickly with soil depth, paralleling decreases in organic matter. The processes that control the amount of available P in the soil are: plant uptake, sorption, desorption, mineralization, immobilization, precipitation, dissolution, and erosion.

Plant Uptake

Despite low concentrations of phosphate in soil solution, plants can take up substantial amounts of P due to P desorption and dissolution (discussed below), followed by P diffusion to the plant root. By taking up large amounts of P, a strong ‘diffusion gradient’ is created which moves P toward the plant root at much higher rates than water is moving via transpiration. P uptake for selected agricultural crops ranges from about 20 to 60 lb P2O5/ac per year, although these amounts are dependent on yield. Commercial P fertilizers have historically been expressed as the oxide form (P2O5) rather than the elemental form (P), therefore P values are still expressed as P2O5. Using the ratio of their molecular weights, %P2O5 can be converted to %P by multiplying by 0.44 (%P = %P2O5 x 0.44). To estimate P2O5 uptake for a specific field, one can divide actual yield by the yield in Table 2 and then multiply this result by the P2O5 uptake given. This is an estimate of P fertilizer that is being removed by the crop, and may help in determining P fertilization recommendations. A more exact method to estimate P removal is to have P analyzed in the plant tissue that will be removed from the field, and multiply the result by dry matter yield removed. The amount of P uptake is affected by the amount of available P in the soil; a quantity that is controlled by other processes involving P that are discussed below.

Sorption/Desorption

Sorption refers to the binding of P to soil particles. Because phosphate has a negative charge, it is attracted to, and binds strongly to positively charged minerals, such as aluminum (Al) and iron (Fe) hydroxides and oxides. Recall that rust is a type of iron hydroxide. Like other soil particles, these minerals become more positively charged at lower pH; therefore, more phosphate is sorbed at lower pH. Finer textured soils generally can sorb more P because they have more surface area. In the calcareous soils of Montana and Wyoming, sorption is likely not as important as precipitation at controlling available P levels. Sorption is decreased, and hence available P levels are increased, when the soil solution contains high levels of other anions such as bicarbonate, carbonate, silicate, sulfate, or molybdate that compete for sorption ‘sites.’ In addition, dissolved organic compounds associated with organic matter can increase P availability plant by competing for phosphate sorption sites or coating Fe/Al oxides. In addition, as more P fertilizer is added, P sorption sites can begin to become saturated, which increases the recovery, or P use efficiency, of added P. Although Fe/Al oxides do not represent a large fraction of Montana and Wyoming soils, they are some of the only minerals that carry a substantial amount of positive charge at pH levels typical in this region, and thus can sorb large amounts of dissolved P, which is negatively charged. Sorption of P generally increases with increased temperatures in P fertilized soils, but there is no correlation with temperature in soils that have not had P added. P desorption (the opposite of sorption) generally increases as solution P decreases, or under flooded conditions due to dissolution of Fe hydroxides and oxides, that release sorbed P.

Minerailzation/Immobilization

P mineralization is the process where organic P becomes converted to phosphate as organic P decomposes, and immobilization is the process where available P becomes tied up in microorganism cells. In the United States, annual amounts of P mineralization have been found to range from 4 to 22 lb P2O5/ac, representing a significant portion of crop P uptake. These values are averages from 30-80 year studies and would be expected to currently be less due to lower organic matter levels in most cultivated soils. Mineralization occurs most readily when the C:P ratio is less than 200:1, and immobilization occurs when that ratio is greater than 300:1. For comparison purposes, the C:P ratio for beef cattle manure is approximately 100:1, suggesting that decomposing manure should mineralize (release) P rather than immobilize P. Mineralization and immobilization of P are affected by temperature, moisture, aeration, and pH in similar ways as N mineralization and immobilization, because they involve the same microbial processes.

Precipitation/Dissolution

Available P concentrations are largely controlled by the solubility of P minerals that are dominated by calcium phosphates (Ca-P) in neutral to high pH soils typical of Montana and Wyoming soils, and by Al and Fe phosphates (Al-P and Fe-P) 100908070605040302010045678910 pHRelative phosphate soprtion % at pH levels below about 6.5. There are numerous forms of calcium phosphates in soil, ranging from the very soluble monocalcium phosphate (MCP) to the very insoluble fluorapatite. After fertilizing with P in a neutral or high pH soil, MCP will form first, followed by the other calcium phosphates in order from high to low solubility. The time for each mineral to form is highly dependent on temperature; for example, OCP formation has been found to be four-fold faster at 68o F compared to 50o F. If a soil has a mixture of the various Ca-P solids, the more soluble forms will dissolve more readily as phosphate levels in solution decrease during the growing season. Research has found that DCP will keep soluble P levels at approximately 1.5 mg/L or ppm (at pH 7.5), whereas TCP will maintain soluble P levels at about 0.03 mg/L. Some research has found that P limits wheat growth at soluble P levels below 0.03 mg/L; therefore, a soil could have high levels of TCP, HA, or FA, and still limit plant growth due to lack of available P. Accessing these more insoluble forms of P that are at fairly high levels in most MT and WY soils would be advantageous, although this would require either lowering Ca or pH levels, which is very difficult in calcareous soils. Decreasing pH would take large quantities of acid, or acid-producing substances such as elemental S, Fe+3, or manure. As may be expected, soils with higher levels of calcium carbonate (lime) will tie up more P due to precipitation of Ca-phosphates, and thus lower the soil test P. Note that the same large addition of P (114 lb P2O5/ac) increased the Olsen soil test result by about 13 ppm at 1% lime, but only 8 ppm at 12.5% lime. Therefore, higher amounts of P fertilizers are needed in soils with high amounts of lime to offset the effect of calcium phosphate precipitation. Lime concentrations may be especially high in some surface soils that have been eroded or leveled for irrigation purposes, exposing subsurface calcareous horizons. Al phosphates and Fe phosphates are the predominant P minerals in soils with pH levels below about 6.5. The solubility of these minerals decreases at lower pH, directly opposite of the solubility for calcium phosphates.

Erosion/Runoff

Soil erosion represents a loss of P from agricultural fields and can occur from water or wind because P is generally bound so tightly to the soil. Although low rainfall and relatively flat topography in agricultural valleys of Montana and Wyoming prevent high amounts of erosion from water, total annual erosion from most of Eastern Montana is quite high (8 to 16 tons/acre), largely due to wind. Assuming a typical total P concentration of 500 ppm, this equates to 18 lb P2O5/ac per year, a substantial loss in the overall P budget. Some eroded soil from upwind or upstream may be deposited to replace a portion of that lost, although rarely is the redistribution of eroded soil uniform as much of it is deposited at the edge of fields or in ditches. Some factors contributing to high amounts of soil erosion include: 1) long slopes in fields farmed without runoff diversions, 2) rows planted up and down moderate or steep slopes, 3) inadequate crop residue, 4) lack of buffer strips, 5) poor stands, 6) lack of windbreaks, 7) intensive tillage, and/or 8) overirrigation. Dissolved P in runoff can represent another loss of P from agricultural fields. However, the concentration of dissolved P in runoff is generally low due to the high amount of sorption and precipitation of P minerals. One exception to this general rule is animal manure stockpiles or manure application sites, where P is concentrated and sorption sites in the manure and surrounding soil may be approaching saturation. For example, soluble P has been found to approach 80 mg/L (ppm) in runoff from pastures fertilized with poultry manure. In another study, dairy manure applications of 100 lb P2O5/ac increased P in runoff by up to six-fold. With relatively new environmental regulations and guidelines involving Total Maximum Daily Loads (TMDLs) and Nutrient Management Plans (NMPs), it is important to realize that the general perception that P binds strongly to soil is true only to a certain soil P level or application amount. So, how do we know where this point is without collecting a water sample leaving a field? A large amount of research has correlated soil test P levels with dissolved P in soil solution, drainage water, or runoff. One of these studies found that P in solution began to significantly increase only when Olsen P levels were above 60 ppm. Therefore, maintaining Olsen P levels between the critical level and this threshold should optimize yield without losing substantial quantities of P in runoff. Preventing surface loss of P from erosion or runoff can be attained through no-till, conservation tillage, windbreaks, improved soil fertility, minimal residue burning, winter cover crops, runon/runoff control, and buffer strips, among others. Your local Natural Resource Conservation Service (NRCS) office can offer numerous solutions for decreasing soil erosion and runoff, and hence loss of P.

Leaching

Leaching was not shown on the P cycle because it does not occur very readily in most soils. One 20-plus year study in Wisconsin found that P from commercial fertilizers had not moved more than 5 cm below the plow layer, and P from manure applications had moved less than 20 cm below the plow layer. The higher movement of P associated with manure is likely due to the effects that organic materials have on sorption, as described previously. In sandy soils, commercial fertilizer applications have been found to increase available P concentrations up to 3 feet below the surface, but to have no effect on available P at 4.5 feet below the surface. A three-year study in Wyoming on a sandy clay loam, found that a very high irrigation rate (3 feet per season) leached P to approximately 4.5 feet. Therefore, P leaching is probably only a concern in Montana and Wyoming on coarse soils that are either frequently flood-irrigated or have had long-term, high rate manure applications.

P Budget

We have now looked at each of the inputs and each of the outputs to the available P pool. A summary of this available P budget is shown in Table 4. A total P budget can also be constructed for a specific field based on total P inputs and outputs. This exercise can be helpful in determining if soil P levels are increasing or decreasing. Unlike the N budget, where gains and losses from the atmosphere are virtually impossible to measure, a P budget is fairly easy to construct. The only major total P inputs are fertilizer and animal manure, and the only major total P output is crop removal, which can be estimated from Table 2 or calculated directly from tissue P concentration and dry matter yield. Cropland with initially low levels of available P will likely show increased total P levels as P fertilizer is applied to increase yields. Conversely, in soils with moderate to high levels of available P, the soil may be “mined” of P until yields are negatively impacted. Manure-applied fields will almost always show an increase in total P, due to high concentrations of P in manure. An accumulation of P may not be a problem if runoff and erosion from the area are minimal, although very high levels of P have been found to cause zinc deficiencies in some crops. By comparing annual P application rates with soil P test results (discussed below) for a certain field, the effect of fertilization amounts on P availability can be determined. The primary goal of soil testing for P is to determine P fertilizer requirements for a specific crop. Soil samples for P analysis are generally collected from the upper 6 inches because P from fertilizers will stay in this upper layer due to strong sorption and precipitation. In addition, P fertilizer recommendations are generally based on results from this upper 6 inches. Unlike N, simply measuring dissolved P will not give an adequate estimate of P availability. Therefore, extractions have been developed which are designed to remove the more readily available sorbed and mineral P fractions. The Bray-1 and Mehlich-3 tests were developed for acidic soils, and work by dissolving the more soluble Al-P and Fe-P minerals that control phosphate concentrations in acidic soils. The Olsen P test was developed for neutral to high pH soils, and relies on the bicarbonate extractant to dissolve the more soluble Ca-P minerals and release some sorbed P. Another promising soil test technique is using anion exchange resins, either encapsulated in a synthetic mesh or in a probe. The advantage of a resin over an extractant is that the resin better imitates a plant root by decreasing solution P, promoting P desorption and dissolution. Research in Montana on P-responsive calcareous soils has found that Resin-P is more responsive than Olsen-P to changes in P availability following P fertilization (Yang et al., 2002). The disadvantage is that there has been insufficient calibration of the resin P results against yield to enable a fertilizer recommendation to be made. A large amount of research has been conducted to determine relationships between P soil test results and relative yields. The general relationship between soil test P levels and yield is shown in Figure 5. Soil test levels are generally broken into low, medium, or high, and sometimes also into ‘very low’ or ‘very high’. At soil test P levels above the ‘critical level’ only minimal yield responses can be expected. The critical level will vary with crop and climate; for example, the critical level for spring wheat is approximately 16 ppm and for winter wheat is approximately 24 ppm. In addition, because there are so many factors that affect P availability and yield, fertilizing a soil that has a medium P test level increases the probability that there will a yield response, but does not guarantee a yield response. Conversely, there is still some chance that fertilizing a soil with a high soil test P level will produce a profitable yield increase. Specifically, in some calcareous Montana soils, significant growth responses to P fertilization are sometimes observed in small grains when the Olsen P level is above the critical level. Therefore, testing yield responses to P fertilization on specific fields is recommended to supplement and validate soil test results. In addition, starter P with or near the seed will usually provide some faster growth earlier in the spring growing season when soils are cool and wet. At P levels above the ‘environmental threshold’ there may be water quality degradation, although there is no agreement on what this threshold should be. The environmental threshold is primarily a regulatory criteria and varies from state to state. It is generally 2 to 4 times higher than the critical level. In Montana and Wyoming, there currently is no set threshold, although the federal NRCS Waste Utilization guidelines recommend no animal manure applications if the soil test P level is above 150 ppm and manure application rates cannot exceed crop P removal rates if the test level is above 100 ppm. Idaho uses an environmental threshold for Olsen-P of 50 ppm on sandy soils and 100 ppm for silt loam soils. One recognized problem with environmental thresholds is that they do not take into account the potential for erosion or runoff from the site, so are not a good predictor of P loss from a field. Due to this concern, the USDA has developed a ‘P index’ tool that considers erodibility, runoff, soil test P result, P application rate, and P application method.

P Fertilizer Recommendations

Soil analytical laboratories often provide a recommended P application amount based on soil P test results. However, producers may alter this amount depending on fertilizer costs, changes in yield potential, management intensity, or due to different philosophies. P fertilizer guidelines are useful for determining a P fertilizer requirement. Annual soil test levels, P fertilization amounts, and yield, one can begin to determine how P fertilization amounts affect both yield and soil test levels for a particular field. This may prove more fruitful than using published guidelines, because it is specific to your area, crop variety, and management practices. fertilizer P recommendations for grass are provided from the MSU Fertilizer Guidelines for Montana Crops, which uses a sufficiency approach. Note that if the soil test P level is 8 ppm, these guidelines would recommend applying 30 lb P2O5/ac. This value could be adjusted upward slightly for high yield goals and slightly downward for low yield goals. A typical grass yield in Montana is approximately 1.5 t/ac which would take up approximately 22 lb P2O5/ac, suggesting that the remaining 8 lb P2O5/ac would be bound to the soil. Table 21 of EB 161 can be used to estimate P2O5 removal amounts of major Montana crops for those wishing to use a build/maintenance approach for P fertilization. Specifically, the removal amount can be added to the sufficiency amount to determine the P fertilizer rate in order to build the soil test P level until it reaches the critical level.

Phosphorus Fertilizer Management

Proper management of P fertilizer applications is key to optimizing yield and protecting water quality. As mentioned previously, practices that increase P availability include banding, increasing organic matter through manure applications or conservation tillage practices, and applying fertilizers as close to peak crop uptake as possible. Some adaptations should also occur with annual versus perennial crops.

Fertilizer Type

The two most frequently used P commercial fertilizers in Montana are MAP (monoammonium phosphate) and DAP (diammonium phosphate). MAP may be temporarily more available in high pH soils, because it lowers pH, whereas DAP raises the pH. Specifically, in a water solution, dissolved MAP will result in a pH of 3.5, while DAP will cause a pH of 8.5. The acidic pH immediately surrounding a MAP granule should temporarily decrease the amount of P minerals that form, and may dissolve some P minerals already present in the soil. This effect is temporary, as neutralizing reactions will quickly return the pH to near a pre-fertilization level. Agronomically speaking, at the same rate of P there should be no difference in crop response between MAP and DAP for a crop that is limited by lack of available P. Nitrification of the ammonium in both MAP and DAP will also cause a slight decrease in pH; a potential benefit in high pH soils. Fertilizing with either MAP or DAP is preferable to fertilizing with triplesuperphosphate (0-45-0 or TSP) because there is evidence that NH4+ increases P uptake and both MAP and DAP are less expensive. However, when fertilizing legumes, such as alfalfa, with very high rates of P (150 lb P2O5/acre and higher), TSP is preferable because the legume does not need the N, and the additional N may favor grasses and weeds. Liquid ammonium polyphosphate (10-34-0) is quickly converted to phosphate, and thus will undergo similar sorption and precipitation reactions as other phosphate fertilizers. Rock phosphate, which is comprised of apatite and fluorapatite, is used only infrequently due to the low solubility of these two minerals, especially at high pH. Organic producers frequently use this form of raw, unprocessed P as a nutrient source, but at very high rates and pulverized into as small of pieces as possible, due to its low available P concentration and solubility.

Timing/Placement

P sorption and precipitation are relatively fast processes, happening in minutes to weeks after P fertilizers dissolve. Therefore, P fertilizer should be applied as close to the time of maximum P uptake as possible. In addition, P banding is recommended over broadcast applications because banding essentially saturates the soil with P in a small area, allowing easy access by plant roots. Conversely, if P is broadcast and incorporated, P will come in contact with much more soil surface area, leading to high levels of P sorption and low levels of available phosphate. Still, in established perennial forages, where surface broadcast application of P fertilizer is the only feasible application method, fertilization with 100 lb P2O5/ac resulted in a 35% yield increase in a Montana soil with a low available P level. Keep in mind that mineral P and sorbed P can become available if phosphate levels in solution become low enough to promote P dissolution or desorption; however, these can be slow processes.