Corn and soybean harvest continues to move along in Illinois, and as the 2013 crops come off, thoughts turn to fall fertilization. In this article we’ll discuss nutrients other than nitrogen. This will be followed soon by an article on nitrogen.
P and K
In areas with dry soils, we have in recent years had reports of lower than expected soil test K values. There might be some of this in 2013, but we’re also hearing some reports of soil test P and K levels higher than expected. Test levels lower or higher than previous test levels, additions, and removals would suggest are not uncommon, and reasons for this are not always clear. Unusually low soil test levels need not be taken as a sure indicator of deficiency; neither should high tests be taken as an indication that nutrients are not needed. As long as previous soil tests were not low, and nutrients removed in harvested crops are being replaced with fertilizer additions, crops should be getting the nutrients they need.
Some questions have been raised about whether the “book” values for P and K grain concentrations, used to calculate removal, are still accurate. Dr. Fabian Fernandez, now at the University of Minnesota, produced data from Illinois grain samples (taken mostly in 2009) showing average P and K concentrations in corn grain of only 0.27 and 0.19 lb per bushel, compared to the book values (from the Illinois Agronomy Handbook) of 0.43 lb P and 0.27 lb K per bushel. Soybean values were closer to book values, at 0.69 (new) compared to 0.83 (old) lb P and 1.17 (new) compared to 1.30 (old) lb K per bushel. Other states also have some data showing lower values than the ones commonly used.
It’s certainly possible that higher yields and different genetics over the decades since these book values were produced might have lowered these values. But until we have more data to confirm this, it seems prudent not to lower removal/replacement amounts by a lot. Let’s consider an example in which soybeans in 2012 yielded 45 bushels per acre and corn in 2013 yielded 180 bushels per acre. This would produce 2-year P removal totals of 115 lb P per acre under the old (book) values, and only 80 lb P per acre using the new values. Using the old values would calculate removal of 107 lb K per acre, and the new values would calculate to 87 lb K removed per acre.
A reasonable approach might be to split the difference, calculating replacement as the average of the old and new removal amounts. In this example, that would mean replacing (115 + 80)/2 = 98 lb P and (107 + 87)/2 = 97 lb K per acre. An exception to this might be where P and K test levels are likely to be on the low side, in which case we might want to base replacement on the old book values to minimize the chances of deficiency.
Materials and placement
Most P goes on as ammoniated phosphates MAP or DAP, but forms such as DAP manufactured to include other nutrients such as sulfur and zinc are also marketed. We have not seen much response to additions of S, Zn, or other micronutrients in Illinois field crops, but can’t rule out possible responses. Micronutrients such as manganese, iron, and zinc are required in small amounts and can be partially immobilized if they are applied long before crop uptake begins, so are often applied in the spring; applying these in the fall can probably work if rates are adequate. Sulfate can leach much like nitrate, so elemental S, which is gradually converted to sulfate in the soil, is a safer form of S for fall application than is sulfate.
Another practice that is growing some in Illinois is placement of P and K in subsurface bands, usually 4 or 5 inches deep. Some may call this strip-tillage, though much strip-tilling is done without fertilizer placement. Strip-tillage usually includes planting on top of tilled strips in the spring, a practice that is not always followed with nutrient banding. In some cases fields are tilled after banding, probably shallow enough not to disturb the band, and rows might or might not be on top of the bands. Banding P and K can also be coupled with NH3 application in the fall, with the NH3 knives usually running a few inches deeper than the dry fertilizer band. Equipment to do this was much in evidence at the Farm Progress Show this year.
The equipment expense, power requirement, and relatively slow speed of application compared to broadcast fertilizer make banding fertilizer more expensive than broadcast. This means that banding needs to increase yields or lower fertilizer costs in order to make it pay. There has been a fair amount of research done on this, and most has shown little effect of this practice on yield, compared to broadcast-applying the same fertilizer rate and form. When crops are planted over the band, roots of the crop reach the band of fertilizer quickly, and the crop can take up most of the P and K it needs from the band. But roots have to go out into the bulk soil to take up water, and there is no advantage to having the plant take up most of its P and K in only the small part of the root system – the roots in the band – rather than throughout the root system.
In soil with very low soil test levels, roots in the bulk soil might not encounter high enough P and K levels, in which case either band placement or broadcast fertilizer should help. Without tillage, broadcast fertilizer nutrients will take more time to get into the root zone than if nutrients are banded. Finally, in soils with a tendency to tie up nutrients, banding can increase availability compared to dispersing the nutrients throughout the soil. These factors are of limited importance in Illinois soils with medium or higher soil test levels of P and K – in other words, in soils where P and K are seldom limiting to yield.