Key Points
- Objective: This is a 10-year study (2021-2030) to look at fertilizer performance, cost of fertilizer, and overall soil test levels. We are evaluating varying rates of dry fertilizer to evaluate agronomic yield, soil impact, and economic optimums.
- A Turning Point?: 2023 is the third year of this long-term study and proved to be the first year where fertilizer rate reductions did not offer higher economic returns.
- Economic Impact: In the first two years of the study, cutting fertilizer costs offered higher economic returns. However, in 2023, cutting fertilizer to 75% rates resulted in losses of 13.6 Bu/A and continued at a loss as the rates decreased. The losses ranged from $39.98/A to $71.72/A.
- Soil Test Levels: Two years of rate reductions and savings of fertilizer worked agronomically and financially, especially with higher actual fertilizer prices. Is it possible that by year 3, we drew soil test levels low enough to drive yield lower?
Evaluating Dry Fertilizer Rates Over Time
2023 was the third year of this 10-year study, where we are evaluating varying rates of dry fertilizer to track agronomic yield, soil impact, and economic optimums.
Soil tests were pulled in the Fall of 2022 and recommendations were generated by a local soil test lab using University of Illinois fertility standards. Based on these results, a 100% rate of fertilizer was established for 250 Bu/A corn in a corn/soybean crop rotation. 100% rates of DAP (18-46-0) and Potash (0-0-60) were then evaluated compared to lower rates of 75%, 50%, 25%, and 0%. These rates were applied in both a traditional broadcast, as well as a banded application, and averaged over both scenarios.
In 2023, we saw the highest yield at the 100% rate, while all the lower rates suffered a yield loss. Cutting fertilizer to 75% rates resulted in losses of 13.6 Bu/A and continued at a loss as the rate decreased, with economic losses ranging from $39.98/A to $71.72/A. This confirmed the 100% fertilizer rate as the economic optimum for our 2023 data.
Since soil samples are taken annually, it gives us the ability to monitor soil test levels at each fertilizer rate over time. In the charts below, you’ll see summaries of soil test P and K levels from our first soil test in Year 1 (2021), compared to the latest soil test in Year 3 (2023).
You’ll see that P levels have held within a loss of only 4% over the three-year time period at the 75% rate. However, we saw a reduction of 25-29% in the 25-50% rates. 0% rates resulted in the highest losses at a 46% deficit.
We saw much lower deficit reductions in our soil test K levels in comparison to P levels. 0% rates resulted in the highest deficit at 21%, while 25-75% rates showed an 11-16% reduction of soil test K.
These reductions in soil test levels most likely are the reason for lower yields in 2023. In 2021-2022, fertilizer rate reductions offered higher net returns than the 100% recommended rates of fertilizer. However, as soil test levels have been diminished or “mined,” yield deficit is beginning to occur. Two years of rate reductions and savings of fertilizer worked agronomically and financially, especially with higher actual fertilizer prices. However, by year 3, have we drawn soil test levels low enough to drive yield lower?
Since this is a 10-year study, we’ll continue to keep an eye on this study over the next 7 years to discover what this all means!
Study Details
Study data collected 2021-2023 at the PTI Farm in Pontiac, IL.
