The Agricultural Fertilizer Research and Education Council (AFREC) of Minnesota
December 4, 2018
Version 2

Crop Fertility Findings:
Phosphorous Behavior in Soils

Fluctuating fertilizer prices can make decisions on the management of nutrients, especially phosphorus (P), challenging. Across Minnesota, soils can differ in their relative potential to retain or fix phosphorus. That’s an important variable to know, because it directly correlates to availability of the nutrient for the plant. In general, soils in the western part of the state tend to be high in calcium carbonate. This can be problematic since calcium can react with P, tying it up and making it unavailable for the crop to utilize it effectively.

Phosphorus (P) soil tests can be used to estimate the potential availability and response to P.  In Minnesota, the Bray-P1 and Olsen soil tests are recommended. However, these tests only measure a small fraction of the total P in the soil and can be affected by soil chemical properties (i.e., pH, organic matter, etc.).  Critical soil test values should be established so producers know how much additional P is required to support crop production in their region. The critical level is the level where there is only a very small chance (less than 5%) the crop will respond to additional fertilizer. Better understanding of the critical soil test value on each farm will help agronomists and growers make decisions that have a higher probability of increasing ROI.

As noted in an ongoing study by the University of Minnesota (On-farm Assessment of Critical Soil Test P Values in Minnesota, by Kaiser), where P is concerned, there is a high potential for a large increase in yield in low-testing corn and soybean fields, and a moderate chance of a slight increase in yield in the medium class (See Tables 14 and 15 in this article).

While AFREC advises growers to consistently practice all 4Rs of nutrient stewardship; the right source, right rate, right timing and right placement; to best manage phosphorous levels, rate and placement are by far the two most important factors to consider in making nutrient recommendations.

Approaches to Phosphorous Management

Phosphorus fertilizer management in Minnesota is based on one of two approaches:

  1. Build and Maintain (B&M), or
  2. Sufficiency

Both P fertilizer management philosophies use soil test phosphorus (STP) as their primary focus, but they use it differently:

  • The B&M approach recommends P fertilizer quantities needed to build the STP to or near a critical level (see above), or even beyond that level, over a period of years.
    • The number of years and amounts of P fertilizer required to reach the critical STP level will depend on growers’ finances and situation, difference between current and target STP, soil chemistry, and crop production level.
    • Once the target STP level is reached, annual applications of P fertilizer are required to maintain that STP. These applications are frequently based on P removal in the previous crop.
    • The B&M approach uses STP to monitor the perceived soil fertility status of the field.
    • The B&M approach does not directly account for the soil’s natural ability to supply P but supplies needed P through off-site sources such as commercial fertilizer or manure.
    • The B&M approach presumes to prevent any chance of P deficiency in order to maximize overall yield potential.
  • The Sufficiency approach uses STP to determine the likelihood that P fertilizer will increase crop yield and the rate of fertilizer required to optimize that yield.
    • The Sufficiency approach relies on the soil’s natural P supplying capability and supplements that capability with off-site P sources.
    • Under ideal conditions, the Sufficiency approach presumes to maximize economic return for each dollar of P fertilizer applied.

Field research in the 1970s and 1980s suggested the Sufficiency approach—in which less P fertilizer was applied than in the B&M approach—produced similar grain yields to B&M, indicating greater profitability with the Sufficiency approach. In recent years, however, it has been argued that higher fertilizer applications associated with the B&M approach are necessary to obtain and maintain greater production levels in today’s high-yield agricultural systems. The concern is that the Sufficiency approach will not maximize yield potential, because it is difficult for this approach to keep up with the advancing yields of today’s genetics, especially in soils that tend to tie up available phosphorus, as mentioned earlier in this article.

Yield Response to P and Critical Soil Test Values

Soil test P levels vary significantly across Minnesota. In some parts of the state, significant variability in soil pH can exist even within a grower’s fields. Some areas within a field may test low in P in spite of previous fertilizer application while other areas will have high soil test P. To address this challenge, variable rate application of fertilizers may be used to manage nutrients better within fields, after the fields have been tested.

Soil Testing Methods for P

There are numerous testing methods for P to account for the variability of soils.  The three most common are:

  1. The Bray-P1 test, which uses a strong acid to extract phosphorus. In soils with high levels of calcium carbonate, however, the carbonate can neutralize the acid in the Bray solution, which can cause an underestimation of the amount of P available to crops.
  2. The Olsen extraction, recommended in circumstances when the soil pH is greater than 7.4, since it is less susceptible to being affected by soil carbonates than the Bray-P1 test.
  3. The Mehlich-3 extraction is being used in the Corn Belt as a substitute for both tests and has an advantage that it is used to extract multiple elements such as P, K, and micronutrients. The Mehlich-3 test is buffered at a neutral pH, which is supposed to allow it to work better in high pH soils. Previous work has shown, however, that this test does not work well in soils in Western Minnesota.

 

On-farm Assessment of Critical Soil Test P Values in Minnesota

A two-year study conducted in Minnesota sought to compare grain yields with and without P to determine relative grain yield and relate that to the soil test value measured from the area encompassing the yes/no comparison. This comparison was used to determine the critical soil test P level, or the value at which crops do not respond to applications of fertilizer P. (Note: since routine phosphorus soil tests do not measure the total amount of nutrients within the depth sampled, values can vary between soil test methods.)

The probability of a significant yield response was calculated for both corn (Table 14) and soybeans (Table 15) and broken down into the current P classifications for the Bray-P1 and Olsen P tests. For corn, the probability of a significant yield response was 100% when soils tested Very Low, 72% when Low, 25% when Medium, 10% when High, and 5% when Very High. This data shows that as soil test decreases there is a high probability of a larger increase in yield.

Many farmers consider P and K applications every other year. However, modern high yield systems may warrant the consideration of annual applications on soybeans.

Source: AFREC and U of M “On-farm Assessment of Critical Soil Test P Values in Minnesota” research report.

This type of information can be critical when deciding where to apply fertilizer especially in cash-short situations. When in short-term rental situations if growers have to make a choice, the best option is to target Very Low or Low testing areas and potentially Medium testing areas due to the higher probabilities of response and the magnitude of the potential response. If money to cover fertilizer cost is not a problem, or you are farming owned ground, then building and maintaining in the High category around the critical level poses less of a risk, although there does not appear to be convincing evidence that maintaining a Very High soil test is needed or economical unless a grower is using an inexpensive source of P fertilizer.

Why You Might Build P Levels In Your Fields

There may be grower- and situation-specific reasons to decide to follow a B & M program beyond critical soil test levels. Some examples of those reasons are listed below:

  1. A grower may make a personal decision on owned fields to follow a B & M approach after following the strategy for several years with a history of yield increases.
  2. In good farm income years, a grower may consider this approach to manage tax strategy and also create some insurance for the future by building soil test levels.
  3. Reducing risk. Large acreage operations with rented land can sometimes have less room for error than others, and need to consistently produce the best crop possible. Ensuring there is no P deficiency can be a sound strategy to maintain maximum yields.
  4. Utilizing this approach in certain areas of the field they know are better producing than others (variable rate).
  5. To build land values in advance of a future sale.
  6. Belief that today’s higher yielding corn and soybean varieties are using P faster and in higher quantity than are accounted for in historical recommendations.

 

Soil Properties Affecting Phosphorous Capacity

In addition to determining rates based on test levels, understanding what soil properties affect P availability also is crucial to improving P management. Currently, two different mechanisms are used to study soil P:

  • soil P buffer capacity (PBC), or the capacity of the soil to resist pH change
  • sorption maximum (how much P can adsorb onto soil particles) and sorption strength (how strong the chemical bond is between the soil particles and the P)

Both mechanisms are extremely important in developing a good understanding of P dynamics in soils. Recent research* has sought to relate the soil PBC to sorption maximum and sorption strength, and also determine which soil property/properties controls these parameters.

Among the key findings of an ongoing University of Minnesota study were these take-aways:

  • The higher the PBC value, the higher the amount of P that has to be added to a soil to increase the available pool by 1 ppm.
  • By adding enough P to saturate the binding sites, researchers observed a completely new P behavior in the soil that questions the meaning of absorption studies and what the sorption strength and maximum really mean.
  • There is no real limit in how much P can really bind to a given soil.

While the work is still ongoing, this research suggests that our understanding of the impacts on P availability for plants continues to evolve.

Final Word

As a macronutrient (N, P and K), phosphorous is often considered to be well understood. Increasingly, however, researchers are discovering that its availability and yield impact are highly variable depending on soil types and their properties and field-specific situations. Determining the optimal rate and placement of P, therefore, requires ongoing field monitoring, soil testing, and the input and experience of a knowledgeable agronomic advisor.

* Source: AFREC and U of M “Phosphorus Availability and its Relationship to Sorption Maximum and Sorption Strength” research report

 

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