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Phosphorus is a crucial nutrient for plant growth. It is one of the three primary nutrients (along with nitrogen and potassium) because natural levels of phosphorus in soil often limit plant growth.

Phosphorus binds tightly to soil and loss from agricultural fields is rarely an economic problem. Unused phosphorus fertilizer from one year can be used the next.

When phosphorus is lost from fields, it is mainly by erosion. Relatively small amounts of phosphorus that are lost this way can have negative impacts on water bodies, where the phosphorus can greatly increase growth of aquatic plants.

Yield response to phosphorus

A network of experiments across Missouri has shown that there is little or no yield response to P fertilizer when previous fertilizer management has maintained soil test levels at the target levels recommended by the University of Missouri.

A network of experiments measuring soybean, corn, and wheat response to phosphorus was conducted all across Missouri between 2000 and 2002. These were conducted in cooperation with the University of Missouri variety trials, mainly on farmer fields. Cooperating farmers had maintained soil test P at levels consistent with University of Missouri recommendations. Most experimental fields had soil test levels between 30 and 60 lb Bray1 P/acre; the University’s target level is 45 lb/acre.

Average yield response to P in these experiments was very nearly zero (see table). This is expected and intended in the University of Missouri’s recommendation system. At the target soil test level, the soil should be able to supply the crop’s entire phosphorus need. Fertilization is then to replenish the soil supply as it is used up. Timing of P fertilization is not critical if soil test values have been maintained at or near the target level.


Average yield response to P

Number of experiments










See the complete reports: Soybean (PDF), Corn (PDF), Wheat (PDF).

Field-scale response to phosphorus

Measuring field-scale response to P, K, or both is fairly easy to accomplish using yield monitor data. Yields can be compared from side-by-side strips with and without P/K to see how large the response is, where it is occurring in the field, and to monitor and understand patterns of P and K response on a given landscape or farm.

Soil tests are designed to predict the probability of a yield response to P and K. On-farm measurement tells you much more accurately whether you got a response and how large it was. Soil tests are interpreted based on experiments done under different conditions and on different soils than are likely to exist on your farm.

Yield responses to P may depend on how much P is being supplied by your subsoil.

The University of Missouri is starting a small project to assist producers with this type of on-farm P/K response test. Please email Peter Scharf ([email protected]) if you are interested in participating.


Phosphorus in Missouri subsoils

Soil samples to a 3-foot depth show that the subsoil is a greater reservoir of P than the topsoil over most of Missouri.

Current University of Missouri fertilizer recommendations have a target level of 45 lb P/acre (22 ppm P) measured with the Bray 1 test. Measurements of Bray 1 P in 180 deep soil samples across Missouri shows that the subsoil often contains more than three times this much P in eastern Missouri, with levels declining toward the west. See the Subsoil Bray-1 Phosphorus Map (PDF)

Variable-rate phosphorus: economic analysis tool

Variable-rate P and K increase costs to producers, both in soil sampling and in application. Where is this cost recouped? It has to be either by increasing crop yield, or by reducing P and K use enough to more than cover the cost of using this type of management.

A statewide network of on-farm small-plot experiments showed that under prevailing P and K management conditions, response to the current year’s P and K applications is near zero. This suggests that producers are managing P and K in such a way that the soil can supply all that the crop needs, and that it is not possible to increase yield by managing P and K differently.

Although variable-rate P and K management may sometimes reduce the total amount of P and K applied, I think there is potential to enhance these savings using the variable-rate approach.

In the network of experiments mentioned above, there was evidence of a small yield response to P in corn and soybean when soil test levels were below 30 lb P/acre. So why is the target level for soil test P set at 45 lb P/acre by the University of Missouri? I don’t know the history, but it makes sense to me that there should be a ‘cushion’ between the known ‘critical value’ and the recommended ‘target value’. This is because of the spatial variability of P in the field. See an example for P variability (PDF).

Having a ‘cushion’ built into the target value for soil test P ensures that even the lowest-testing parts of the field are at full yield potential. Variable-rate management may offer a cheaper way to ensure that the whole field is at full yield potential.

By taking spatially intensive soil samples, the low-testing areas of the field can be located and fertilized to achieve their full yield potential. The ‘cushion’ is no longer needed. This means that a lower target level for soil test P can be used, and savings on fertilizer will be increased to where they should easily exceed the cost of variable-rate management.

Ray Massey and Peter Scharf have developed an economic analysis tool for variable-rate P and K (PDF). Note: PDF capability and Flash Player required. The user can change the cost of P, the cost of K, and P and K target levels for both uniform and variable-rate management. The program will then calculate the 10-year Net Present Value (amount of money made/lost over 10 years including the interest cost of money spent) for any of five fields that were grid sampled with no previous variable-rate management. We suggest that soil test thresholds of 45 lb P/acre and 280 lb K/acre will produce full yields with uniform management, and allow the flexibility to miss a year’s application without losing yield. With variable-rate application, thresholds of 30 lb P/acre and 200 lb K/acre will also produce full yields and allow the flexibility to miss a year’s application without losing yield. If the producer is committed to applying P and K every year, these numbers can be reduced farther (but we don’t know how far).

P and K prices are lower now than when we developed this tool in January 2009. This means that the potential to save money with variable-rate P and K is now less than it was then.

Phosphorus placement: economic analysis tool

P and K placement is a hot (and hopeful) topic. Many producers hope that they can save big money by going with placement.

What can placement actually do for you? Multiple studies in Missouri have failed to find that placement of P and/or K can increase yields. That leaves the possibility that placement will allow you to lower your rates. This is where the typical producer considering a placement machine is pinning his or her hopes.

Placement can improve the efficiency with which this year’s fertilizer is delivered to the crop. Remember that for producers who follow soil test recommendations, this year’s fertilizer is not actually necessary to attain full yield. Fertilization only serves to replenish the soil’s ability to supply all of the crop’s needs. This means that increasing the efficiency of this year’s fertilizer is a moot point.

The only way to take advantage of greater efficiency is to lower rates and allow soil test levels to drop. Then your repayment on the time and money that you spend on placement is the interest on the money that you saved on P and K. The kicker that many producers have overlooked is that you can’t put on a half-rate of P and K and let soil test drop forever. If you’re 100% efficient with delivering half of what the crop needs, where is the other half coming from? The soil. And the soil’s supply is being depleted. Eventually you will get to a point where you are cutting into your yields unless you go back to applying full removal rates of P and K with your placement machine. At that point, your savings have ended but your obligation to band P and K every year is permanent.

Ray Massey and Peter Scharf have developed an economic analysis tool for P and K placement (PDF) based on the principles discussed above. Note: PDF capability and Flash Player required. The user can change the acres on which the placement machine is used, the cost of P, the cost of K, and P and K target levels for both broadcast and variable-rate management. The program will then calculate the 10-year Net Present Value (amount of money made/lost over 10 years including the interest cost of money spent).

A network of small-plot P and K response experiments all over Missouri showed zero yield response to P and K when soil test P was 30 lb P/acre or higher and soil test K was 200 lb/acre or higher, but small yield responses below those values. We suggest that soil test thresholds of 30 lb P/acre and 200 lb K/acre will produce full yields with broadcast management, and allow the flexibility to miss a year’s application without losing yield. (These values ignore spatial variability issues with soil nutrient supplies.) If the producer is committed to broadcasting P and K every year, this level can drop lower without losing yield, but we’re not sure how far. My guess is 20 lb P/acre and 150 lb K/acre would be safe. Getting value from P and K placement requires banding every year and allows the producer to let soil test values drop even farther.

Our economic analysis tool suggests that if the placement machine is purchased new and used on 2000 acres, you can let soil tests drop to zero and still not come out ahead with 2010 prices for P and K. Even with astronomical P and K prices in 2009, you have to let soil test values drop ridiculously low with banding to come out ahead.

Producers also often mention banding P and K as a solution for low-testing rental ground with unstable land tenure. They don’t want to invest in building the P and K levels of this land. This makes sense, but investing in a placement machine may not put them ahead. Our economic analysis tool suggests that on 500 acres of low-testing rental ground you can’t come out ahead by using placement.

Grain phosphorus removal

Grain nutrient removal per bushel of yield appears to vary from year to year and region to region based on samples collected at a range of sites around Missouri from 2006 to 2008 for corn (PDF), soybean (PDF), and wheat (PDF).

The variability seen means that calculated grain removal values for N, P, and K will not always be accurate. Most fertilizer recommendations use these removal values as part of the basis for rate decisions. Current National Research Council feed grain values for P as P205, which are currently the basis for University of Missouri nutrient removal calculations, are shown in golden brown.

Applying nutrients at rates to replace those removed by the crop is a sound approach, but putting too much faith in the ‘book values’ for removal could lead to problems. Soil testing should be used as another source of information about the nutrient status of soils.