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Field-by-Field Phosphorus

October 26, 2013
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
Since yield potential is determined in the first six weeks of growth, phosphorus (P) needs to be available early, especially with cold soil, continuous corn and P-fixation issues.  

The soil environment tells you when and where to apply this essential nutrient

Life isn’t fair—not with phosphate fertilizer, anyway. Timing and placement of phosphorus (P) applications—a critical part of crop production—is easier for a central Illinois farmer in a corn/soybean rotation than for his neighbor who grows continuous corn, or for a North Dakota farmer who has a shorter growing season and colder soils.

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Timing and placing P applications might even be simpler in one part of a field than another, if a field contains different soil types. That’s because P management must be based on the soil environment—moisture, temperature, crop residue, calcium level and pH.

"Wherever you farm, whatever your soil types, your goal is to have phosphorus available throughout the vegetative life of the plant, especially the first six weeks when yield potential is determined," says Farm Journal Field Agronomist Ken Ferrie. "Phosphorus is critical for cell development and growth."

The availability of soil P is controlled by soil microorganisms, and the soil environment affects microbial populations. "By decomposing crop residue, microbes convert phosphorus from the unavailable organic form to the inorganic form that plants can use," Ferrie says. "It’s part of what we call the phosphorus cycle. The health­ier the microbial population, the easier it is to release phosphorus."

Extreme moisture conditions can reduce decomposition. Microbial activity declines during periods of drought. In saturated soil, there’s no oxygen for the microorganisms to breathe.

Soil temperature. When soil temperature is below 60°F, P availability is limited because microbes are not very active. "Because microbes use phosphorus for energy, as well as releasing it from crop residue, a positive balance of available phosphorus doesn’t begin to show up until the soil temperature reaches about 65°F," Ferrie says. "Until that time, there isn’t much phosphorus available to plants, even if the soil test reveals optimum levels, because the phosphorus remains unavailable in the fixed or organic form."

Because corn seed can germinate at 50°F, that can create a problem. "The young plants may spend lots of their early growth in soil with temperatures from 45°F to 60°F," Ferrie says.

"Depending on where you farm, soil moisture and temperature conditions have a big effect on early growth. In Minnesota, farmers plant as early as possible so their crop will mature before frost. They can’t wait for 60°F soil temperatures; they have to plant at 50°F. Not much P will be released from the soil until the soil temperature reaches 65°F. In Louisiana, that won’t be a problem because farmers plant into 60°F to 65°F soil temperatures.

"As a result, farmers in colder climates are more likely to see a yield response to phosphorus applied in starter fertilizer," Ferrie continues. "When we band phosphorus fertilizer next to the plant, we apply a water-soluble form, which does not depend on temperature or microbes to

become available. It supplies the crop with P until the microbes become active and start releasing phosphorus."

Paying the carbon penalty. Crop residue, which is determined by crop rotation, affects P availability by creating what Ferrie calls the carbon penalty. Think of residue as a load of carbon being applied to soil, which microbes must decompose. (The same kind of carbon penalty exists for nitrogen and sulfur, which also must be converted from organic to inorganic forms by soil microbes.)

"Both plants and microbes need phosphorus as a source of energy," Ferrie says. "As soil warms up, microbes become active and start to release phosphorus by decomposing crop residue. This is called mineralization. But they also consume phosphorus for energy to drive the process, making it temporarily unavailable to plants. This is called immobilization. Whether we have net mineralization or net immobilization depends on the amount of residue to be decomposed and its carbon to phosphorus (C/P) ratio."

If residue has a low C/P ratio, cycling P from residue into the soil nutrient pool is fairly easy and fast. If it has a high C/P ratio, cycling of P is much more complicated. "The more complicated the process, the more phosphorus is immobilized," Ferrie says. "So your crop rotation plays into timing of phosphorus fertilizer.

"With continuous corn, which has a higher C/P ratio, more immobilization occurs in the spring. So continuous corn growers face more availability issues than growers in a corn/soybean rotation. That’s why we see a good response to starter fertilizer in continuous corn.

"If the carbon/phosphorus ratio is less than 200:1, there will be a net gain of available phosphate in the soil because of mineralization," Ferrie summarizes. "If the ratio is between 200:1 and 300:1, you break even. If the ratio is greater than 300:1, there will be a net loss of available phosphorus through immobilization."

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FEATURED IN: Farm Journal - November 2013

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