MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.      

Increasing worldwide demand for nitrogen, phosphorus and potassium has brought about significant increases in the costs of these materials for the 2008 growing season, especially potash. The supply of potassium may also be limited. Faced with this situation, farmers may have to make decisions on how to get the best return from the fertilizer they are able to purchase. Following are some suggestions.

1. Soil test. A soil test indicates the available nutrient status of the soil, the need for lime and how much nitrogen, phosphorus and potassium are needed for the crop to be grown. Test results from samples collected within the last two to three years can be used as a guide. For fields where no soil test information is available, collect soil samples as soon as possible and have them tested.

2. Apply lime where needed. Maintaining the soil pH between 6.0 and 6.8 improves nutrient availability and crop growth. In many situations, the return on invest is best with lime, both short and long term. When the soil pH is below 6.0, applied fertilizer nutrients or indigenous soil nutrients are not used as efficiently by crops.

3. Take credit for nutrients contained in manures, composts or other materials applied to crop fields. Nutrients contained in these materials are readily available and can offset the need for purchasing any additional fertilizer. Have manures and compost analyzed for the nutrient content.

4. Apply phosphorus only where needed. As the result of past buildup, 70 to 75 percent of Michigan farm fields contain adequate or above levels of phosphorus. Now is the time to use some of that phosphorus banked in the soil.

5. Focus of potassium. Only 20 to 25 percent of Michigan farm fields contain adequate levels of potassium. Increase the amount of potassium included in starter. At a 2 inch by 2 inch placement up to 100 pounds per of K2O per acre can be applied without concern for injury. Placement of potassium is especially important for corn planted no-till or into a lot of residue. By including some potassium in the band fertilizer, the total amount applied per acre may be reduced because of the increased effectiveness.

6. Band-apply nutrients. Band applied nutrients are used more effectively than broadcast nutrients. In high P testing soils, early growth from banded fertilizer comes more from nitrogen and potassium than phosphorus. Overall, reducing phosphorus in favor of nitrogen and potassium may prove beneficial.

7. Apply P and K first to fields most in need. The crop yield response to incremental additions of potassium or other nutrients can be characterized as the “curve of diminishing returns.” This means that the increase in yield from the first increment of potassium added (say 50 lb K2O per acre) is greater than the second. And return from the second increment is greater than from the third, and so forth. Eventually the cost of an additional increment of potassium is greater than the value of the yield increase. This being the case, it is better to apply or allocate potash first to fields with lower soil potassium values, where there will be a good yield response, than to those fields that have close to adequate levels.

8. Reduce amount of K applied uniformly across all fields. In fields that need K, applying slightly less, e.g. 25 pounds per acre, than is needed will have only a minimal, if any, effect on net income. Work from soil test results.

9. Apply preplant nitrogen close to planting time. This will minimize the length of time nitrogen is at risk of lost by leaching or denitrification before crop demand increases.

10. Use a presidedress soil nitrogen test for corn to determine need for more nitrogen. Some soils can provide significant amounts of available nitrogen, especially where cover crops or animal manures or a previous legume crop is incorporated prior to planting.