By Robert Mullen

What should you be considering when making a phosphorus application?
Soil testing is obviously the place to begin when it comes to making a phosphorus fertilizer decision. A soil test tells us a couple of things: 1. whether additional phosphorus is required, and 2. it helps us determine how much phosphorus should be applied to decrease the risk of yield loss due to phosphorus deficiency.

Let's run a couple of scenarios.

• Scenario 1. Assume you are interested in applying enough phosphorus for two crops (corn and soybean). You obviously cannot completely rely upon a planter application of phosphorus to satisfy the phosphorus demand of both crops (unless your soil test level is high — greater than 40 ppm). Therefore, you will be making a broadcast application (likely in the fall) with or without an application of some planter phosphorus with the corn crop.

• Scenario 2. Assume you are interested in applying enough phosphorus for just the upcoming crop (corn). Depending upon the rate recommended, you can apply that phosphorus in the fall (as in the previous scenario), or, if the rate recommended is low enough, you can apply all your phosphorus as a starter.

Essentially, we have just run through a scenario of applying the 4Rs to your phosphorus decision. Soil testing allowed you to make a rate determination, and based upon rate decision and desire to apply enough for one crop or two, you have made a timing and placement decision (fall broadcast with/without planter applied or just planter applied).

What else should you be considering?
Now that you have determined how much phosphorus you need and how/when you are going to apply it, let's think about how to minimize loss of the phosphorus you apply. We obviously want to minimize phosphorus loss to the environment, but what fractions of phosphorus cause environmental concern?

Water quality issues in Lake Erie, and monitoring of waterbodies that feed it, have revealed that while total P loading (sum of particulate-P and dissolved-P) has declined since the 1970s, dissolved-P has experienced sizable annual loads since the mid-1990s. Why are we so concerned about dissolved-P? 

Particulate-P is the P fraction that is attached to soil particles, so as soil particles are eroded from a field they can be transported to surface water bodies. Once particulate-P is in a waterbody, a portion of that P is desorbed and becomes bioavailable and usable by aquatic plants. Generally, we assume 30% bioavailability from particulate-P.

Dissolved-P is the fraction of P that is in solution (not bound). Its bioavailability is 100% once it finds its way to a waterbody. Thus, this fraction has a greater ecological impact than particulate-P.

From a management perspective, our desire should be to manage both fractions of P to minimize loss of both.

Conservation tillage practices can effectively manage particulate-P, but, depending upon how fertilizer P is managed, can increase dissolved-P transport. Intensive tillage practices can decrease dissolved-P effectively, but they can increase particulate-P transport. Ideally, we would prefer to maintain surface residue to decrease soil erosion, but apply P in such a way that P is placed/incorporated below the soil surface to decrease dissolved-P losses.

So, back to Scenario 1. Managing the fall application (assuming soil test level is low enough to require a higher rate of P) by incorporating/injecting the P below the soil surface would be our preferred method of application. If tillage is already planned, minimize the length of time between fertilizer application and the tillage event.

In Scenario 2, assuming you only need a low amount of P, a starter P application is the best way to minimize P loss.

Managing P is not a trivial decision, nor are any of the other decisions you make as a farmer. While this is not an exhaustive discussion on this topic, keep in mind the issues discussed here can help you make better decisions — for your bottom line, and for the quality of our water.

Mullen, Ph.D., CCA-Director of Agronomy-Nutrien.