AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Winter 2002, No. 5
If the plan calls for maintenance applications, then changes needed may be minor. Recalculate needed application rates based upon the harvested yields. If build up nutrient rates are part of the ongoing plan, and yields were lower than expected, the extra nutrients left from the 2002 crop will help speed up the progress to the build goal. Or 2003 applications can be adjusted downward slightly. If 2002 yields were above expectations, the 2003 nutrient applications may need to be ratcheted upward to stay on course.
Very likely, the changes from the plan will be small. But for some fields that yielded 100 bushels per acre or more below expected levels, there may be reason to make adjustments in fertilizer application plans. “Left over” nutrients can be estimated from the change in expected crop yield multiplied by the crop removal per unit of yield. As a rough estimate, a 10 bushel change from expected corn yield equates to about 3 pounds of K2O and 4.4 pounds of P2O5 per acre. Similarly, a 10 bushel change in soybean yield would equate to about 14 pounds of K2O and 8 pounds of P2O5 per acre.
For an average Midwest soil, it takes 4 pounds of K2O to change the soil test potassium level by 1 pound, and 9 pounds of P2O5 to change the soil test phosphorus level by 1 pound (or 8 pounds of K2O and 18 pounds of P2O5 to change the potassium and phosphorus tests by 1 part per million, respectively). It takes a large yield change to cause an unanticipated change in soil tests.
Every field’s nutrient plan should be reviewed before final decisions are made on fertilizer application. Go back to the most recent soil tests, then review crop removals and fertilizer and/or manure applications since the samples were collected. Calculate a net nutrient balance, and the expected impact on soil tests using the relations above. Make adjustments to the nutrient plan for fields where the calculated progress is not meeting expectations. Collect new soil samples if none have been done in the last three or four years. It may be helpful to have tests taken more frequently during a major buildup period.
A new Excel spreadsheet tool, PKalc, can be downloaded from the PPI website, www.ppi-far.org, under the listing, “Toolbox Resources”. PKalc provides a simple worksheet to compute estimated phosphorus and potassium nutrient balances over a period of up to 10 years. While this “checkbook” approach is a crude estimate of nutrient balance, it offers a useful guide to help keep nutrient management on target.