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5 Tips for Cutting Phosphorus, Potassium Fertilizer Costs

By Dan Kaiser

Low commodity prices prompt ever-increasing questions about what crop inputs are necessary.

Fertilizer represents a major cost in cropping systems, and over-application of nutrients can lead to decreased profitability.

In 2018, the 20% most profitable farms made $24 per acre more than the 20% least profitable farms on rented land, and $21 more on own-ground, according to FINBIN data.

While the use of commercial fertilizers has been vital for increasing and maintaining high productivity, there are a few things to consider that can help to trim fertilizer costs and maintain profitability.

1. Soil tests give you the likelihood that a crop will respond to a nutrient. Soil test results for phosphorus and potassium are indexes of nutrient availability for crops. This index represents the probability that a nutrient is needed. The soil test classification indicates how likely the soil can supply all nutrients the crop needs. A high soil test value suggests that it is unlikely that fertilizer application is needed.

2. Short-term reductions in the application rates of P and K do not result in rapid depletion of nutrients. Many farmers worry that soil test levels will plummet if fertilizer is not applied. While a reduction in soil test levels over time is expected as nutrients are removed from the field, the rate of depletion is slow even with no fertilizer application. Our research has consistently found that applying two-thirds to one half of the expected removal of P or K can maintain soil test values in the medium to high classifications without risking a reduction in yield.

3. Use variable rate technology to its full advantage. Optimal use of variable rate fertilizer application can help to more precisely apply nutrients where needed. Increasing profitability using variable rate applications depends on 1. better addressing under-fertilized areas of the field, potentially increasing yield, and 2. reducing costs in areas of the field where the probability of a yield response to fertilizer is very low. If possible, it is best to target a soil test value that you can build up or draw down to where there is an agronomic advantage to applying fertilizer.

4. Consider application timing. Fertilizer applications made at one time for multiple cropping seasons can be enough to maximize yield. However, keep in mind application timing. A 10-year study in Minnesota studying phosphorus using a two-year corn-soybean rotation found a four-bushel per acre yield advantage for corn when all or a portion of the P fertilizer was applied ahead of the corn. Yet, soybean yield was not affected by P application timing.

For potassium, corn grain yield was not impacted by time of application. However, soybean grain yield was one to two bushels per acre greater when the K was applied ahead of corn. We can conclude from this research that application of P and K ahead of corn is a good practice and, assuming the proper rate is applied, will result in maximum productivity.

5. It is always a good time to consider taking soil samples to assess nutrient availability. Remember that the likelihood of a crop response to P is very low when the Bray-P1 soil test is 20 parts per million or the Olsen soil test is 15 ppm or greater. For potassium, the likelihood a crop will need K is very low when the soil test is 200 ppm or greater. Understanding these levels can help you decide where fertilizer is needed and where costs can be reduced.

If there is concern about yield reductions in high or very high soil test levels, small rates of starter fertilizer can be effective at reducing the chance of a yield reduction. Only a small amount of nutrients is needed when the soil can supply a large portion of what a crop demands.