Before you decide to skip fertilizer applications of phosphorus and potassium this year due to high prices of fertilizer, ask yourself a question. Do I know for sure that every area within each field has adequate levels of each nutrient?
While he's not trying to sell fertilizer, Randall Warden, Director of client services for A & L Great Lakes Labs in Ft. Wayne, Ind., says that just under half of the samples tested in their lab in '08 tested medium or lower for phosphorus. About one in five tested low or very low. A surprising number came back with single digits for parts per million of phosphorus, he notes.
Soil test values in Indiana have tended to run relatively high for phosphorus, after decades of many farmers applying 6-24-24 fertilizer back in the early days of commercial fertilizer. But that doesn't mean that there aren't a significant number of fields or areas within fields that could benefit from fertilizer applications.
Areas that test low or very low are thought by most agronomists to be good candidates for showing a yield boost to an application of phosphorus. The iffy area is in the medium range. It's those medium-testing areas, Warden says, that could prove troublesome in a weather-stressed year. If it's very dry at the wrong time, for example, plants may not be able to take up adequate amounts of phosphorus even if it's in the soil because there just isn't enough movement going on.
What could come into play this year, Warden believes, is that some farmers who haven't tested regularly or using precision testing strategies may decide to draw on the bank, meaning live off of nutrient buildup they think they have in the soil, only to find that the bank is already empty. If that happens, then yield reductions are possible next fall, he notes.
One factor some may not be allowing for is that extremely high yields from modern genetics and good growing seasons in some areas mean more nutrient withdrawal than some anticipate. For planning purposes, he assumes that each bushel of corn grain removes 0.3 pounds of potash equivalent per bushel. So on spots within the field that yielded 240 bushels per acre vs. those that yielded 140 bushels per acre, an extra 30 pounds per acre was removed. Conversely, those areas need an extra 30 pounds per acre of potash just to remain even with the areas that didn't produce as much corn.
For phosphate, the removal value he uses is about 0.4 pounds per bushel per acre. The Tri-State Fertilizer Recommendations, developed by Purdue University, Michigan State University and Ohio State University, use 0.37 pounds per acre. For planning purposes, the trend will be the same.
"Trends are what's important in soil testing," Warden says. "What you're trying to do is get a handle on how nutrient levels vary over time."