Be Wise When Cutting Back on Broadcast Fertilizer

When working with a reduced fertilizer budget, some approaches are wiser than others. When evaluating the approach to take in reducing broadcast fertilizer rates this fall, be sure to ask some questions.

• How will this plan impact the higher yielding areas of the field?
• Is this field already have low fertility and will this reduction have a negative impact?
• How long will/can I follow this plan?
• Should the same approach be used with phosphorus and potassium?
• What is the producer’s tolerance to financial and yield risk?

Before answering these questions, let’s look at the basics of fertilizer recommendations. While there are a wide range of fertilizer recommendations structures, they all have some common fundamentals.

Soil Test Levels – Every fertilizer recommendation structure has a target soil test value that internally will lead to the following categories.

• Below this target value there is a strong probability that crop yield will increase with the application of fertilizer. In these areas, applying a minimum of crop removal of P and K is advisable. This means applying an equivalent amount of P and K to replace the nutrients removed from the field in grain, or forage. Without the financial restrictions, this is the area of the field would possibly receive more fertilizer than crop removal to increase soil test levels. The goal is to prevent soil test levels from declining further and gain any possible yield benefit from the application.
• Above the target soil test level, but below an optimum soil test level, the probability of yield increase from the application of fertilizer is diminished and not expected. If the nutrient is not applied the soil test level could decline below the target level over time. The long-term goal for this category is to apply enough fertilizer to keep the soil test level in this range such as crop removal or slightly less.
• Above the optimum soil test level, there is no benefit to increasing soil test levels any higher. This range of soil test levels receive a fertilizer rate less than crop removal, some fertilizer recommendation structures take this category to a zero application rate. The goal for this soil test range is to not build up or maintain, rather utilize the inventory of nutrients in the soil.
• There is a maximum soil test value where applications need to stop entirely. In most cases the soil reaches a point where it cannot retain all of the applied nutrients, and a notable portion of the nutrients will be subject to environmental loss. For financial and environmental benefit, do not apply nutrients to soils at these levels.

Secondly, we need to look at the behavior of yield in each field. One of two scenarios will develop. When fertility is the limiting factor to yield, soil test levels will correlate closely to yield. There will be low fertility in the low yielding area of the field. When fertility, or the soil nutrient in question, is not the limiting factor, soil test levels will have an inverse correlation to yield, it will result in higher soil test levels in the areas of low yield.

Finally, we need to look at the behavior of the nutrients themselves. While the values can vary, it is accepted on average that for every 18-22 pounds/acre of P₂0₅ applied to the soil above crop removal, phosphorus soil test (Bray or M3) should increase by 1 ppm. Research is beginning to show that it takes that amount, or more, P₂0₅ removal to lower the soil test 1 ppm. A very good yielding crop may reduce phosphorus soil test levels 1-3 ppm/year or less.  Conversely it requires an application of 6 to 10 pounds/ac of K₂0 (depending on CEC of the soil) to raise the soil test 1 ppm (ammonium acetate or M3). Recent research indicates that though crop removal, soil test potassium levels can reduce soil test levels by 1 ppm with less than 6 to 10 pounds of K₂0 removal per acre. A good yielding crop can reduce soil test potassium levels by as much as 20-30 ppm/year.

Knowing these factors and how they interact helps begin to prioritize ways to reduce fertilizer application rates and the associated risk from doing so. Some suggestions may include:

• Reduce/stop fertilizer application on soil testing above max and possibly those above optimum.
• For soils testing below the target, consider reducing or removing any fertilizer rates above crop removal.
• Phosphorus rates can be reduced or eliminated for a short period of time with little impact on phosphorus soil test levels. Once the levels do start to decline, it will take a significant application to build back up.
• Potassium levels can drop quickly in high yielding grain or forage fields, application rates below crop removal should be kept to a minimum. More frequent soil testing is advisable to monitor soil test potassium levels during aggressive potassium application rate reductions.
• When soil fertility is limiting yield, be sure to focus on applying at least crop removal on soils below the target level. Using a yield map to better define crop removal rates is advisable.
• When soil fertility is not a yield limiting factor, shift the fertilizer to the higher yielding areas of the field to maintain or increase yield in those areas.
• Consider applying a percentage of crop removal, maybe apply 70-80% of phosphorus above target level and full crop removal below the target.

Evaluating how a cut in fertilizer application rates may impact the situation will lead to wiser budget cutting. The worst thing you can do is simply reduce standard recommendations by a flat percentage across the board. This will usually place the greatest yield production risk on the highest yielding areas. Your ALGL regional agronomist can help evaluate the various options based on your situation.