Lime application timing usually occurs in the fall, shortly after the previous crop has been harvested, which allows time for incorporation and to begin pH adjustment in the soil. Simultaneously, growers are also having fertilizer applied, specifically phosphorus shortly before or after the lime has been applied to the soil.

A frequently asked question this fall “will lime turn my phosphorus fertilizer into “calcium phosphate,” a.k.a. “rock phosphate?”

Calcium phosphate is a more insoluble form of phosphorus and this reaction usually occurs at pH’s of 7 and above.

One of my graduate school committee members at the University of Kentucky, Dr. Lloyd Murdock, Extension Soil Fertility Specialist always said “If fertilizer P is applied below a pH of 6.5, especially below 6.0, more of it will be tied up with aluminum and iron in the soil. So not applying lime would be bad also. If the pH is above 7 then more of the P will be tied up by calcium in the form of insoluble calcium phosphates (rock phosphate). So, you cannot win either way.

If you have a low pH, say in the high 5’s, and you apply lime and you add P also. You would have to get the soil pH where the lime is placed above 7 for much of this additional fixation to occur and it is going to be difficult for this to happen since you are starting at a low pH and lime is a rock and is only slowly soluble.

Bottom line is do not worry about P fixation and keep the soil pH about 6.5. Under the best of circumstances, you are only going to get about 30 percent of the P you apply in the crop. Very small differences in P fixation would happen with or without the lime."
 

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2020 Phosphorus and Potassium Management in fields in Prevented Planting

Adapted from Iowa State University “Integrated Crop Management” newsletter. 
By: Dr Antonio Mallarino, Professor and Extension Specialist in Soil Fertility and Nutrient Management
 

The P and K management for 2020 in fields or portions of fields where corn or soybean could not be planted or drowned out in 2019 present additional challenges. If P and K could not be applied last fall or spring, producers can apply the same rates they were planning to apply, and a new soil testing will not be essential for those fields or field areas. If P and K was applied, producers will not need to apply again for 2020 as long as volunteer weeds or seeded cover crops were not harvested for hay or silage and there was not severe erosion by rainfall or flooding. In Iowa soils, applied P and K from previous application (and no crop grown) will still be available for the 2020 crop.

If P and K was applied for the 2019 crop, and/or if biomass was removed or there was severe soil erosion, there is more uncertainty. In these cases, new soil sampling and testing may be the most rational and cost-effective solution. There is no reliable information about P and K removed by weeds or by several kinds of cover crops, and analysis of sampled biomass will provide quite variable and likely untrustworthy removal estimates. With severe soil erosion, much of the applied P and K may have been lost with surface soil runoff, especially in no-till fields.

Other options are to apply a conservative rate, perhaps average annual crop removal from the previous 3 or 4 years or use starter fertilizer.

There has been much talk of “flooded soil syndrome” for fields or sections of fields where no crop was planted and there were no weeds or crop growth since the flood. In these cases, soybean may require inoculation, and when P had not been applied, perhaps a higher P rate than planned or additional starter P may be justified for corn. Fact sheet “Flooded Soil Syndrome” provides further information.