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.