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Corn+Soybean Digest

Managing the Big P

Higher fertilizer prices, along with lower commodity prices, are pushing growers to take a more critical look at how they might trim costs.

Phosphorus (P) is one nutrient that has seen its price rise dramatically. Yet experts warn against reducing the levels of phosphorus you apply to the soil. And they note that robust yields in areas of the Corn Belt, combined with increased corn-on-corn production, are removing a lot of P from the soils that must be replaced.

Knowing where you stand starts with a soil test. “It's the only way to know current soil nutrient levels,” says Jim Camberato, Purdue Extension soil fertility and plant nutrition specialist.”

If test results show a high P level, a grower can reduce or eliminate P application by calculating how long P can be taken from the soil. Phosphorus is removed from the soil by crops, and the amount removed varies depending on the soil type and productivity. Corn raised for grain removes 0.37 lb. of phosphorus oxide (P2O5)/bu., while soybeans remove 0.80 lb. of P2O5/bu.

PURDUE RECOMMENDS applying P to the soil for corn and soybeans only when soil test P levels are less than 40 parts per million (ppm). So producers with higher soil P levels can calculate how much P their crops will need for a growing season.

If your P levels are reduced too far the penalty is reduced productivity of your crops. “And often, the lost yields are greater than the fertilizer costs,” Camberato says.

Tony Vyn, Purdue University agronomist, says lowering P rates below crop removal rates can have longer-term detrimental impacts to the field from a productivity standpoint, especially if soil tests show that P is near the critical level.

Greg Schwab, Extension soil fertility and management specialist at the University of Kentucky, notes that it's important to remember a soil test also only tells you the average value of nutrients in the field, and can be impacted by how and where the samples are taken. “Just knowing the average doesn't necessarily tell you areas that might be 20% above average, or more importantly, areas that might be 20% below average,” he says.

Grid sampling may show an immediate payoff. Schwab says fields that are close to what their university considers the critical value (in Kentucky 60 lbs. of available P/acre or 30 ppm), are better candidates for grid sampling than fields that average very low or very high. “The penalty for not having enough fertilizer is much steeper than putting out too much,” Schwab says.

Livestock manure can offer a valuable source of P. “Even with extra costs associated with manure application, it's becoming a more economical alternative on cash crop farms because of rising P prices,” Vyn says. “I think that's a healthy trend, because it is good for the environment as well as the pocketbook.”

Yet producers not familiar with using manure need to be cautious, because manure offers its own management challenges, including variations in nutrient levels and application to fields.

“Analysis needs to be done to know what levels of nutrients you are putting on the field,” Camberato says. “And even within the same manure source, the levels can differ.”

UNIFORM APPLICATION OF manure can be a problem, which may create future challenges. “If the application is not uniform, you could create problems with future soil samples,” Camberato says.

“If manure is used, ensure that you do a really good job of spreading it out,” Camberato says.

“It all comes back to the soil test,” says Paul Fixen, director of research at the International Plant Nutrition Institute. “If soils have medium to high levels of P, then producers have a degree of flexibility. If levels are low, then skipping P will most likely be costly. At some point, you're going to get to a level where you must replace what you are removing or the system is going to fail.”

Fixen also advises that the bread-and-butter fields — those with the highest returns — should be closely monitored to ensure adequate P levels are maintained. “Highly productive soils are likely those that are also right on the threshold of maintaining adequate P levels,” he says. “Don't damage the performance of those star fields when allocating fertilizer resources.”


Deep banding of phosphorus (P) and potassium (K), vs. traditional broadcast application, has been a possible alternative to reducing overall application rates by concentrating nutrient levels in the root zone. However, questions still remain about rates, optimum depth and frequency, whether to change the band location in subsequent applications and even soil sampling techniques. Tony Vyn, Purdue agronomist, says inconsistent results and a lack of research dollars have hampered research into the subject.

Deep banding, Vyn says, will not improve corn and soybean productivity in all situations. However, it will likely be most beneficial when a number of the following conditions are met:

  • Soil test P and K levels are low to medium
  • Soil has a high P and/or K fixation capacity
  • Subsoil P and K levels are low due to nutrient stratification
  • Surface soil layers are dry due to below-normal rainfall, but the soil zone of deep-banding stays sufficiently moist for root growth and high nutrient uptake rates
  • Low soil temperatures restrict early root growth and nutrient uptake
  • The row crop being grown has been planted more than 10 in. away from previous corn rows or fertilizer bands
  • The cultivar planted exhibits good early season cold tolerance and extensive root growth even in dry conditions
  • Strip-tillage is integrated into the cropping system
  • Starter P and K fertilizers are not applied
  • Auto guidance is used for tillage, fertilization and planting operations.

“Not all of these conditions need to be present for a deep-banding system to improve crop productivity,” Vyn says. Furthermore, management strategies based on repeated rate reductions of 50% with deep banding vs. broadcast application will only result in faster declines of overall soil test P and K to below the critical level.

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