Editor’s note: The following was written by Emerson Nafziger, University of Illinois Extension crop production specialist, for the university’s crop management Bulletin newsletter.
The high number of prevented-planting fields in some areas, the late start to harvest and the inability to apply P and K fertilizer as planned last fall or this past spring combine to raise a number of questions about fall application of P, K and lime over the next few months.
Soils are currently dry enough to allow application of dry fertilizer materials over much of Illinois. The wettest part of the state is northwestern Illinois, where the crop still has to mature.
Harvest started slowly in Illinois, but with the warm weather this past week, it will accelerate quickly as long as it stays dry.
The development of wet conditions could slow both harvest and fertilizer application that follows harvest, but soils in the drier parts of Illinois can take in an inch or two of rainfall without turning muddy or forcing much delay.
Most people are anxious to start applying fertilizer after the delays and frustration in getting this done over the past year.
There has been a considerable amount of discussion about whether or not placing P fertilizer beneath the soil surface is a sound practice. The main reason for doing this is to keep the P in MAP or DAP, which is highly soluble, from dissolving and running down slopes and into streams in the event of heavy rain.
How much of this might occur is affected by slope, permeability of the surface soil, how dry the soil is, how much crop residue is present, and the intensity of rainfall. Soils following soybean harvest are generally more permeable than following corn harvest, but corn leaves more residue.
Tillage increases surface permeability, but also loosens soil to make it move more readily with runoff water. Drier soils can take in more water before runoff begins than can wet soils.
October and November are drier months, on average, than spring months. Crops growing into the fall extract a significant amount of water from the soil, thus leaving it drier, and high-intensity rainfall events are less likely in the fall.
So overall, chances of getting high-loss conditions are lower in the fall than in the spring, but they aren’t zero.
Surface-applied P will move into the soil under normal weather conditions and will end up safe from direct loss (it can still move if soil runs off the field) by December.
Most research has shown no yield benefit to subsurface P and K placement in the fall, and it is not clear that the added cost of subsurface placement will provide a positive return in most years and on most fields. In strip-till systems, however, where subsurface placement doesn’t add to the amount of surface soil disturbance, applying P and K beneath the strip while strip-tilling in the fall may be a cost-effective way to apply these nutrients.
Although we’ve found that the N in DAP tends to be available to the next year’s crop if DAP is applied after soils cool down to 50 degrees, applying MAP or DAP when soils are warm will allow much of the ammonium from these materials to convert to nitrate in the fall. Once it is nitrate it can move down with water into and through the soil, including to tile lines if there’s a lot of rainfall.
Even if the N doesn’t move too far down in the soil in the fall before the soil freezes, it will have a head start when water begins to move through the soil in the spring. There can also be direct movement of ammonium (along with P) in surface runoff during heavy rainfall before the MAP or DAP has had a chance to dissolve and move into the soil.
While it may not be practical to hold off on applying MAP or DAP until soil temperatures fall to below 50 degrees, we should recognize that even though the amount of N in these fertilizers is relatively small, it can add appreciably to the N that moves to surface waters through drainage tile.
One solution that has been suggested is to switch from using MAP/DAP as the P source to using triple-super-phosphate (TSP, 0-46-0) which contains no N. If TSP is available at about the same cost per pound of P as MAP or DAP, it would be a good source to use, especially for applications made before mid-October.
The “free” N that comes with MAP or DAP is more likely to reach tile lines than the roots of next year’s corn crop if it is applied when soils are warm in the fall. If it is applied after soil temperatures reach 50 degrees or applied next spring, the N in MAP or DAP does contribute to the N supply for next year’s crop.