Are you backing down on phosphorus rates to save money? Do you know enough about phosphorus to make those cuts and still sleep well?
See how your phosphorus IQ stacks up. Jamie Bultemeier, supervisor at A&L Great Lakes Labs, outlines nine key things to know to manage phosphorus profitably:
1. Phosphorus levels in the soil. Phosphorus is naturally plentiful, in a range from 200 to 5,000 parts per million. However, phosphorus in soil solution is much less available, at 0.1 ppm. Plants contain only 0.3% P on a dry-matter basis, but it is critical for energy transport, DNA and RNA, fruit set, root development, and cell membrane integrity.
Many soils in the Corn Belt wound up high in phosphorus after decades of farmers applying 6-24-24 starter fertilizer, plus hog manure, typically nearest buildings. However, P soil test levels are trending downward today, Bultemeier says.
2. Phosphorus availability influenced by pH. “The first key to maximum availability of phosphorus is getting pH right,” Bultemeier says. Highest availability of P is around 6.5.
Below 5.5, iron ties up P. At or above 7.0, calcium fixates P. Aluminum can tie up P around 5.5, although many Corn Belt soils don’t contain much aluminum.
At low soil test values, most P is tied up with soil particles, absorbed by clay particles. As P levels increase, more P dissolves into solution.
3. How tillage choice impacts stratification. In 2012, Jim Camberato, Purdue Extension soil fertility specialist, confirmed that phosphorus levels increase near the surface in continuous no-till compared to moldboard-plowing. However, they also increase with chisel-plowing and ridge-tilling.
“I still prefer sampling zero to 8 inches for no-till fields,” Bultemeier says.
Bultemeier examined results from sampling at different depths in no-till. Using the Mehlich-3 test, at sampling depths of zero to 2, zero to 4, zero to 6 and zero to 8 inches, P levels in parts per million were 45, 34, 29 and 25, respectively.
4. Phosphorus movement. “Some phosphorus in soil solution can move,” Bultemeier says. That is why filter strips won’t prevent all phosphorus from entering streams. Filter strips settle out sediment, but not phosphorus molecules dissolved in water.
5. When applying phosphorus pays. “If soils test low at 10 to 20 ppm on the Mehlich-3 scale, there are higher odds for a yield increase,” Bultemeier says. He cites data showing that at 10 to 20 ppm Mehlich-3, fertilizing with P paid 34% of the time. At levels below 10 ppm, it paid two-thirds of the time. But at levels above 20 ppm, it only paid 12% to 14% of the time.
“If you apply some phosphate at levels of 20 to 40 ppm, you reduce your risk in case of a bad year,” he says. “But above 50 ppm, you will very rarely see a yield increase.”
Note: If your lab reports in Bray P1, the critical level according to the Tri-State Fertilizer Recommendations is 15 ppm. With Mehlich-3, it is 20 ppm for corn and soybeans. These numbers increase by 10 ppm for wheat.
6. Crop removal amounts. Corn grain removes about 0.38 pound of P2O5 per bushel, or 85.5 pounds per acre for a 225-bushel crop. Soybeans remove 0.84 pound, or 58.8 pounds per acre for a 70-bushel crop. Wheat removes 0.6 pound, or 60 pounds per acre per 100-bushel crop. It takes 18 to 22 pounds of P2O5 per acre to raise or lower P test levels by 1 ppm.
7. How banding affects P efficiency. University tests across the Corn Belt indicate that banding may show a yield increase for phosphorus, but not usually for potassium. Phosphorus requirements for plants do not change. Banding saturates binding sites but reduces total amount of soil near the fertilizer P application, slowing the tie-up of P by soil. Repeated banding can cause variability in soil test results, Bultemeier explains.
For banding P to show a yield response, it would most likely be due to low P soil test levels, pH extremes, restricted roots, cool and wet soils, or P being highly stratified.
8. Phosphate fertilizer differences. Two common sources are triple super phosphate, at 0-46-0, and single super phosphate, which is 16% to 20% P. Both have little impact on soil pH, and both have medium solubility.
MAP and DAP also contain nitrogen and sulfur, and both are highly water soluble. MAP temporarily reduces soil solution pH, which reduces risk of loss of ammonia. DAP temporarily increases soil pH, making it less suitable for high pH soils and increasing risk of ammonia loss. Polyphosphate also contains nitrogen but has no pH impact on most soils.
9. Reasons for purple plants. Purpling can indicate phosphorus deficiency, Bultemeier says, but other things cause purpling too, such as cold nights, leaf damage, soil compaction and lack of an ear.
“Phosphorus deficiency results in carbohydrate buildup, which causes purpling, but anything resulting in a buildup can cause purpling,” he says.