Planning for Phosphorus
Two weeks ago, we started covering some of our main nutrients for crop production with a discussion on Potassium. This week we are going to dive into another big hitter: Phosphorus. Phosphorus is used in the plant for cell division and growth and development of tissues. It is involved in energy creation and transformation as well as protein synthesis. Phosphorus deficiencies can result in limited yield, second only to nitrogen in the frequency in which this occurs. It is with these facts in mind that we spend time and money each year to assess P levels in the soil and make a plan for our fertility. In this post, we will cover some refreshers on where P hangs out in the soil, and how that impacts how the plants absorb it and how we apply fertilizer.
Forms in the Soil
There are three main groups in the soil that Phosphorus will fall into. Fixed, active, and solution. These are also called stable, labile, and solution P.
This is the largest source of phosphorus. Plants however are unable to draw from this source. Most of this phosphorus is bound very tightly to the soil on adsorption sites. It can move into the active group over time, however, this happens very slowly and in small quantities. As plants draw down the active and solution portions of phosphorus content, the fixed/stable pool can slowly resupply them. Phosphorus can remain in this group for years with no major effect on soil fertility.
This group is phosphorus in a solid state than can be easily moved into the soil solution. As plants remove P from the soil solution, this will recharge that system. This is the main source of phosphorus that is eventually used for crop uptake. An article from the University of Nebraska states “It has been estimated that the phosphorus in the soil solution must be replenished on an average of about twice every day for normal crop growth. This is the basic phosphorus problem—to adequately re-supply the soil solution as the crop roots remove available phosphorus from the soil solution.”
The soil solution is the phosphorus reserve where all direct uptake happens. It contains the smallest amount of phosphorus of the three pools. The levels in solution are frequently depleted by plant uptake and then resupplied multiple times a day by the active phosphorus. The solubility of phosphorus is somewhat a unique attribute. Phosphorus is technically an anion like NO3, which means they do not bond readily to the soil. Anions do however readily bond in water making them water and soil mobile. However, despite being an anion, phosphorus has a low water solubility meaning it will move to the soil solution but not be washed through the soil profile or lost via runoff readily.
Soil Factors Affecting Availability of P
The ideal range for phosphorus availability is between 6.0-7.0. At higher pH levels, phosphorus binds with calcium to become unavailable in compounds. At low pH levels, the phosphorus binds with iron and aluminum. Liming soils to correct to a pH of 6-7 can have the greatest impact on availability of phosphorus. Conversely, while this is not an issue we deal often with around Brown County, lowering the pH of very basic soils would also help make phosphorus more available.
As organic matter decomposes, organic phosphorus becomes available to the soil system. While this must be converted to inorganic phosphorus for plant uptake, it is still a valuable source of phosphorus in the system. Contributions by organic matter can be as high as 50% of total available phosphorus. Amount of moisture and temperature are influential factors in available P. Both of these factors affect weathering of OM, and subsequently, P levels. More phosphorus will be released in warm, wet climates than in cool dry climates.
Sandier soils will have less available phosphorus, both due to less binding sites for the active soil phosphorus as well as lower organic matter and consequently lower organic phosphorus. Soils higher in clay have more surface area, resulting in more binding sites for phosphorus particles.
The presence and quantity of other anions will affect how much phosphorus is available in the soil. Anions like sulfate, carbonate, and silicate will compete for binding sites on soil particles resulting in less binding of phosphorus.
Uptake by Plants
Overall, there is a small supply of phosphorus available to plants at any given moment. Plants can only pull from the solution pool of phosphorus, and they can only do this through root contact.
Uptake of phosphorus takes place through contact with the root system. Unfortunately, the extent of a plants root system is not very broad. It has been estimated that roots only come in contact with 1% of the soil. This makes uptake of phosphorus a challenge. When it comes to uptake by the root itself, up to 98% is moved into the plant by diffusion. The rest is through mass flow, carried in by water. As phosphorus diffuses into the plant, the soil water solution is completely depleted of phosphorus and must be recharged by the active/labile sources of phosphorus. This is part of the reason the active phosphorus supply is so critical.
A study by Purdue University indicated that phosphorus uptake is a component of the following: the size and type of root system, amount of phosphorus in the soil, how fast water can be absorbed by the root system, and the ability of the fixed and active supplies of phosphorus to resupply P to the soil solution.
Mycorrhizae, a symbiotic fungus that grows on roots, acts as an extension of the root system. This essentially increases the surface area that the root system and soil can come in contact through and allows for an increased amount of phosphorus available to the plant.
Fertilizer Amounts, Timing and Sources
The type, frequency, application method and timing for best phosphorus application are all dependent on the unique field situation you are working with. Interestingly, we express the content of phosphorus in the soil as the phosphorus content, but when we discuss our fertilizers, we talk in terms of phosphate P2O5. This form is not even actually in our fertilizers but is used as an industry standard.
Forms and amount
We get the majority of our phosphorus fertilizers from rock phosphate. This however is not very soluble in soils and must be treated with acid to make is soluble in neutral and high pH soils. This graphic from Pioneer explains the different fertilizer outcomes when treated with different fertilizers.
Generally, the method for amount applied falls into the sufficiency approach or the build and maintain approach. The sufficiency approach is applying the amount of phosphorus needed to produce a single crop. If soil test levels are higher, it will allow the test levels to decline as long as they are still considered above the critical level. Some publications consider this the most economic way to fertilize for P, while some suggest that yield may only reach 90% with this approach. This method is generally recommended when you may not have long term possession of the land, are in tight economic conditions, or the cost of fertilizer compared to crop price is lopsided.
The build and maintain approach will not be the most economic option in a single year. However, it is intended to provide a more consistent economic return over many years by slowly getting phosphorus test values to a level where they would not be considered a limiting factor anymore. In addition to what the crop will need for a growing season, build and maintain will apply an amount above that to raise test levels that are critically low. This method is preferred when crop prices are high, or fertilizer prices are lower, when you have stability in the land you are farming, and when you are aiming to increase crop yields over the long term. Remember, because phosphorus is considered stable in the soil, your “extra” application this year will be available for years to come for future crops. This is not a lost investment but should be considered a long term asset that will be to your economic benefit for years to come.
There is no real difference between fall verses spring applications of phosphorus in general. Fall applications are often times the most convenient for operators. Do keep in mind that if you are using a phosphorus source that also has nitrogen in it, the nitrogen will be much less stable over the winter than the phosphorus. Winter applications are acceptable as long as they are not applied on top of snow. Essentially, we want the phosphorus to come in contact with the soil to bind to soil particles and move into the active pool of P. Attachment of P to soil particles can also be reduced with the top soil is frozen. We want to reduce the risk that the fertilizer is washed away with runoff.
Spring applications will provide phosphorus closer in time to plant uptake. However, spring rains can be detrimental to application. While P is not readily mobile and is not frequently lost in by binding to water like Nitrogen, P bound to soil particles or still as granules can be washed away through erosion in heavy rain events. Apply far enough ahead of a rain to avoid this problem.
Phosphorus can even be applied at planting. In low phosphorus soils, starter has been shown to help with early plant establishment. A University of Nebraska study indicated that this was not always well correlated with increased yields. Don’t get cosmetic improvements confused with an increase in yield. Set up some on-farm test strips with and without starter to better understand how this fertilizer source works for you.
Both Pioneer and Purdue University note that applying biannually is an acceptable practice. Be sure to account for the removal rate for two crops and give yourself a buffer for higher yields than anticipated. Additionally be sure to apply at an ideal time. If application can be completed when conditions are favorable, the amount of P that can be lost is reduced to just one instance in two years instead of each year.
Broadcast verses banding is a major discussion point of application. Overall, research has shown that banding is more effective than broadcasting. By moving the nutrient source directly to the root zone, the root system is better able to come in contact with the nutrients and utilize them. This can be particularly true in heavy clay or wet soils. Banding additionally will reduce fixation by decreasing the surface area spread that will bind fertilizer and move it to the fixed pool of P.
Broadcast fertilization still has its place. Studies have shown that in no-till systems, the bio activity and root system proliferation near the surface makes this method just as effective as banding. In general, due to soil structure of no till systems, more roots can penetrate the top several inches of soil leading to adequate uptake of phosphorus.
Keep in mind that no matter what method is used, the majority of applied fertilizer will be bound and not immediately available to the crop. In fact, a portion of the applied fertilizer will become part of the fixed or stable pool and over time slowly become available again to the plant. Other portions of the applied fertilizer will become part of the active pool, some will stay in the soil solution to be used by plants immediately and some will be immobilized by microorganisms. This fixation of applied phosphorus is why placement of fertilizers is so important. When we band phosphorus, we are attempting to reduce the fertilizer-soil contact while still keeping the source within easy reach of the root systems.
It is estimated that in the year the phosphorus was applied, the crop grown that year will use less than 10 percent of the amount applied. This is true for corn, soybeans and sorghum. Wheat has been known to utilize up to 30% of the amount applied.
Many factors can result in reduced efficiencies of phosphorus. The amount of phosphorus fertilizer applied at one point, the pH of the soil, organic matter content, and soil texture can all impact the availability of phosphorus. Additionally, the method of application and type of plant root structure can also impact what remains available to the plant.
While not water soluble, phosphorus can be lost via soil erosion. Take careful consideration when applying fertilizer to apply the right amount at the right time and the right place to maximize uptake and crop yields while reducing losses. Losses of phosphorus can build up in lakes and cause harm to wildlife and human health.