Fall fertilizer season is around the corner, and crop nutrition gives you much to consider. Making decisions about inputs for the 2027 growing season is particularly challenging, since fertilizer prices aren’t likely to back off anytime soon.
With all this, how should you approach soil sampling, fertilizer rates and budgets?
Depending on yields and what recent soil tests show, there may be opportunities to trim fertilizer applications in areas. The wide range of yield levels often found within fields makes it more challenging, but good yield data and soil tests will help fine-tune fertility programs.
There’s yield data from monitors, providing crop removal rates you can use, in addition to the most important tool available: soil tests. Most folks are probably on the same page as Iowa State University when it comes to soil testing, so we’ll just hit a few key points.
ISU recommends sampling soil every two to four years, or once in a crop rotation. Sampling every two years certainly provides a lot of information and has become pretty common practice.
However, some farmers are comfortable with three- or four-year intervals, especially when soil tests are in the high or very high range and crop yields align with expectations. Rather than go through the time and expense of resampling everything after two years, some farmers “spot-check” some areas in between sampling cycles.
For example, if during harvest you find some unusually higher- and lower-yielding areas, targeting these areas with new soil samples might provide more information to help maximize phosphorus and potassium dollars.
Now what?
You have good soil tests and yield maps; now what? Should you cut back on P and K application or stay put? Or do you have low soil test levels that could limit yield production?
PM-1688: A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, an Iowa State University Extension and Outreach publication, lays it out in detail. The percentage of P and K applications expected on average to produce a yield response within each soil test category is:
- 80% for very low-testing soils
- 55% for low-testing soils
- 25% for optimum-testing soils
- 5% for high-testing soils
- less than 1% for very high-testing soils
High- and very high-testing soils with low odds of a yield response are natural places to trim fertilizer dollars. With only a 25% chance of a yield response to P and K applications, skipping or cutting back on optimum testing acres is another solid opportunity to save fertilizer dollars.
If you have good confidence in the soil sample results, and they are well within the optimum ranges for P or K, trimming back on those acres for a year or two is a relatively low-risk proposition.
If test levels are consistently toward the lower end of optimum ranges, skipping or trimming for a year is probably still fine. Just keep a closer eye on those soils. Spot-checking or doing a full sampling of areas like this after next year’s crop will guard against drawing levels down too far.
There may be cases where soil test levels have been drawn into low or very low ranges — with more removal than what has been applied — and new soil tests cannot be taken. In such cases, strongly consider fertilizing for the 2027 crop, and test soil as soon as possible next fall.
With yield maps, you can factor in fertilizer removed by crops harvested. This information isn’t always necessary if you have recent soil tests, but it can be useful. If you overlay recent soil test data, yield data and fertilizer applied since the soil tests were taken, it can paint a clearer picture of how much carryover you may have for 2027.
Final thoughts
Work with your local dealer to set up variable-rate application equations that fit your needs.
The basics typically start with applying fertilizer to very low and low-testing soils, and skipping P or K application on high or very high-testing areas. Fine-tuning what you want done with optimum-testing soils and adjusting for the variability the yield maps show will hopefully be time well spent and offer some savings and better fertilizer efficiency.
Lastly, account for any other fertilizer sources, such as starter fertilizer or manure. Be sure to take the P and K credit from that source into account.