By: Emerson Nafziger, University of Illinois Department of Crop Sciences
With harvest almost complete after another year with high to very high yields, it’s time to review some basics of fall fertilization. Neither fertilizer nor grain prices are historically high, so there’s reason to be aware of costs while making sure to cover the nutrient basics.
P and K
Fall application of the dry fertilizer materials typically used to supply P and K to the next year’s (or next two years’) crops is normal practice, although there has been some moving of P and K applications to the spring. That’s not a problem with timing—even though P and K are relatively immobile in the soil, applying them as surface broadcast well in advance of crop emergence tends to work well. But fall soil conditions are often better for driving application equipment over fields, and many producers don’t want to add fertilizer application to the list of spring tasks. Most P and K fertilizers are broadcast, but some now apply these materials as bands placed into the soil, in some cases beneath where rows will be planted. Research has shown limited if any yield response to banding P and K compared to broadcasting, especially on productive soils with adequate P and K test levels already present. An advantage to placing P into the soil is that it is less prone to running off with rainfall. But this requires special equipment, and application of dry fertilizer in bands is substantially slower and more costly than broadcast application.
While most P and K fertilizer is applied to soybean stubble in preparation for corn the next year and then soybean the year after that, we have seen some claims recently that soybean “needs its own P and K” and that it shouldn’t have to “settle” for the P and K “left over” from the corn crop. In all but very low-testing soils, where crop roots can have trouble reaching enough P and K as they grow into the soil, research has failed to show a benefit to annual applications of P and K, at least in soils such as those in Illinois. We know for certain that it costs more to apply nutrients every year than only once in two years. There have also been claims that soils tie up P and K over time after they are applied, such that “freshly-applied” nutrients are more available to plants. But applying amounts of P and K that crops remove tends to keep soil test levels fairly constant, suggesting that any tieup of P and K is not a permanent “loss” of these nutrients; as long as soil test levels are adequate, both crops get enough even if their roots don’t encounter fertilizer granules as they grow.
A sound approach to determining rates for P and K is to add up the amount removed over the last two years (assuming a biennial application) and to apply that amount in preparation for the next two years. A year ago in a Bulletin article I reported the results from a recent NREC-funded grain nutrient sampling project in Illinois. We set grain removal levels as the values below which 75% of samples fell, so a little higher than the average amounts of nutrients we found in the grain samples. In some 2,100 grain samples of both corn and soybeans, we found removal levels of 0.37 lb. P2O5 and 0.24 lb. K2O per bushel of corn grain, and 0.75 lb. P2O5 and 1.17 lb. K2O per bushel of soybean grain. These are 10 to 15% lower than previous “book values” used in Illinois and many other states, and are in line with levels reported by Iowa State University scientists.
Even with slightly lower P and K removal levels than we have used in the past, high yields mean removal of a lot of nutrients from fields. In a field that produced 240 bushels of corn in 2017 and 75 bushels of soybean in 2018, we calculate that harvested grain over the last two years removed 0.37 x 240 + 0.75 x 75 = 145 pounds P2O5 and 0.28 x 240 + 1.17 x 75 = 155 pounds K2O per acre. At current estimated retail prices of $520 per ton for DAP and $370 per ton for potash, the fertilizer to replace these amounts would cost about $123 per acre, not including the application cost.
The still-sometimes-used “200-200” application (200 pounds DAP, or 92 pounds P2O5 and 200 pounds potash, or 120 pounds K2O) every other year was enough to keep soil test levels moving up when using such rates first became common. That’s because yield levels were much lower than in recent years; Illinois corn and soybean yields from 1961 through 1979 averaged 96 and 31 bushels per acre, respectively. Having applied rates exceed removal for decades in many fields is why soil test levels are as high as they are in such fields today. But using that amount of fertilizer at today’s yield levels will mean a steady drop in soil test values as more nutrients are removed than are replaced.
Low crop prices often have some people wondering if they might cut back some on P and K in order to save money, presumably until crop prices are higher (or fertilizer prices are lower) in a year or two. Despite imaginative claims of “hidden hunger” and some overwrought interpretations of tissue testing levels, P and K deficiency symptoms are very rare in Illinois; we tend to see such symptoms mainly when soils dry out after planting and roots have trouble growing into soils enough to take up adequate P and K, even when soil test levels are high. Such symptoms are more common in compacted soils and in no-till fields, but we hardly ever see such symptoms when spring rainfall is normal.
With adequate soil test levels of P and K in most fields and with crops that are good at extracting these nutrients, delaying the application of some or even all of the P or K for a year or even two years is likely to have little or no effect on the yield of the next crop(s). Still, nutrients removed by the most recent crops do need to be replaced, if not before the next crop or two then after that; higher soil test levels now provide more leeway. The real risk comes from allowing removal to exceed replacement over years, to the point where even good root systems can’t take up enough nutrients, and yields suffer. Reaching that point in most Illinois fields would take more than a year or two, but Illinois soils cannot generate enough P and K to meet the needs of high-yielding crops, so getting to that point is inevitable if the neglect continues. We can “kick the can (of nutrient replacement) down the road” for now, but that will mean having to replace ever-growing amounts of nutrients later, as grain, along with its nutrients, continues to come off the field every year.