Starter P, K and zinc fertilizer: 3 takeaways from a long-term study in southwest Minnesota

By: Dan Kaiser, Extension nutrient management specialist

With low corn and soybean prices, I have been looking at some of our long-term data to figure out how growers can reduce fertilizer costs. One study I have not spoken much about is a long-term trial that was started in 2011 at the Southwest Research and Outreach Center in Lamberton, Minnesota. Data from this study show some interesting insights on starter fertilizer containing phosphorus (P), potassium (K), and zinc.

About the study

Initially, the study was established to look at fertilizer additives used in combination with starter fertilizer, and one thing that is unique about this site is that the soil pH is very acid, testing below 5.0.

I have maintained the study over time to look at reduced rates of P and K with and without a P and K starter fertilizer in a three-year corn-corn-soybean rotation. The broadcast treatments for the first six years were similar, focused on application of removal rates of P and K for the corn crop, which resulted in a buildup of soil P from 20 to 30 ppm according to the Bray-P1 test and an increase from 150 to 200 ppm in extractable soil test K.

In year seven, the broadcast P and K rates changed, with one treatment continuing to get no P and K, one treatment receiving 30 lbs. P₂O₅ and 30 lbs. K₂O ahead of each corn crop, and one treatment getting 60 lbs. P₂O₅ and 60 lbs. K₂O ahead of corn. Basically, this was to compare what I would consider about half versus a full P and K application in the rotation to cover the removal of P and K by Corn over the three years.

In-furrow starters have been used in addition to the P and K application all years, with five gallons per acre of 3-18-18 applied to corn. One added wrinkle is that, at the same time that the P and K rates change, I have also been looking at one quart per acre of a 9% chelated zinc. Around year nine, I also started to apply a low rate of 3-18-18 to the soybeans in the rotation (2 gallons per acre) with and without the one quart of chelated zinc. This was the third soybean crop grown as part of the study.

Three takeaways from the study

There is a lot to unpack here but there are three main questions we can address with this study to answer some pretty common questions I get from growers and consultants.

1. Can we get away with reduced P and K application rates?

The change in P and K application started in 2017, at the start of the third rotation. Since the start of the study, the data has always shown a clear benefit to the P and K broadcast at this site. Averaged across the two corn years, corn yield has technically not been significantly different between the low and high P and K rates for rotations three through five. However, I am not sure that trend with hold out for corn over time, especially with the widening gap in soil test K between the two treatments. The low rates of P have been able to maintain soil test P over 20 ppm but I am concerned about whether the K rate is too low to maintain yield over time. (The K rate will likely be increased by 50% when applied for the 2026 corn crop.) Soybean yield has never varied between the low and high rates regardless of year. The 2025 soybean crop is still in the field, so the jury is out as to whether there will be yield differences moving forward.

So, what does this all mean? I think reducing P rates can be more viable in the long-term in some fields than reducing K rates, especially if you are routinely soil testing. The P soil test is very good at predicting the potential risk of yield loss if P fertilizer is not applied. I have more concerns about soil test K on loamy soils in the 150 ppm range which is where the K soil test is currently in this study.

2. Can a low rate of starter fertilizer be incorporated to help reduce the risk of yield loss?

A lot of my early career research when I was in graduate school focused on the claims that a low rate of an in-furrow starter can replace a much higher rate of broadcast P. For a high or very high testing soil, that might be the case, but 20+ years of research has taught me that we need to decouple what we see visually in the starter effect on corn and focus more on what the crop needs for nutrients, as that is more important to the bottom line.

Early plant growth responds to the placement of P but yield is impacted by a shortage of P in the soil. I chose 3-18-18 as it supplies around 10 lbs. of P₂O₅ and K₂O with the 5-gallon rate. I summarized the data across the last six corn crops and there was a small advantage to starter for both P and K fertilizer treatments. One might say that my previous statement then is incorrect as to the impact of early plant growth, but I think a lot of this response is coming from the K in the starter just by factoring in the drop in soil test K over the past six growing seasons in the broadcast treatments.

For me, a good target for crop removal of K is between 180 to 220 ppm K in the soils at Lamberton, where the soil test values have been dropping below 180 ppm for several years. Could starter fertilizer help? In this case, yes, but the question that I cannot answer is: if I had applied 50 and 100 lbs. K₂O, would the data be different? Nutrients in liquid starter fertilizers tend to cost more on a per pound basis than dry fertilizer. A cheaper option in the long-term may have been to broadcast more K and apply less as starter. Soybean data showed no positive or negative impact of starter and I would not suggest any grower start putting any liquid N-P-K starter in-furrow ahead of soybean.

3. What about chelated zinc, is there any value?

This has been the most surprising finding of this work. Initially, when I looked at this data on a year-by-year basis, I did not see any benefit to zinc added in the starter. However, when I averaged the two corn years together for each rotation, the data for rotations four and five (2020 onward) started to show a significant yield increase from the 1 quart of 9% chelated zinc, averaging four bushels per acre for rotation four and eight bushels per acre for rotation five. The soybean crop never yielded more with zinc, so, again, I do not recommend applying starter for soybean.

There have been several studies in the past looking at zinc in starter and none have found consistent results. Normally, I would not expect a response to zinc unless the DTPA zinc soil test was below 0.5 ppm. However, this site is testing close to 1.0 ppm and there is very little difference in the soil test values between where we have and have not applied zinc. I am unclear right now why things changed but this data indicates that a low rate of chelated zinc in a starter program may have value for some growers. If you have a soil test well over 1 ppm, I still doubt there would be an advantage, but around 1 ppm or less it may be something to consider for those utilizing starter.

Conclusion

To sum things up, with P and K, soil testing is still the best way to decide where these nutrients are needed or not. With higher priced phosphate fertilizers, I would highly recommend looking at your soil tests and reducing application rates, especially if soil tests are in the Very High soil test category. By default, I would not recommend favoring phosphate over potassium when making fertilizer decisions. In the long term, based on some of my data, I am more concerned about the under-application of potassium.

The data is not as clear on zinc. I still am a firm believer that many acres in Minnesota are sufficiently high in zinc and that variable rate application of zinc to raise the soil test to a sufficient level gives a better long-term benefit than using chelated zinc with starter. However, the zinc data from this study at Lamberton surprises me and made me think that we should take another look at zinc for corn in the near future.

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