Managing P and K in Flooded or Ponded Soils

Daniel Kaiser, Soil Fertility Specialist

The 2018 cropping season has been a challenging year for those in areas affected by heavy rainfall and saturated soils. Excessive water can result in the loss of soil nutrients with ponding resulting in a significant risk for the loss of nitrogen. The uptake and removal of nutrients depends on the yield. For areas with low or no yield, you likely won’t see substantial amounts of phosphorus or potassium removed. Here’s a look at how removal rate of P and K in 2018 will affect those nutrients ahead of the 2019 crop.

Unlike nitrogen, P and K a relatively immobile in the soil and are not lost unless erosion occurs, so water ponding won’t affect them in the same way. A soil test itself won’t be able to account for all of the P and K applied, as some of these nutrients will react with the soil and change to a form that may not be accounted for by a soil test prior to 2019.

Even though the soil test may not detect all P and K, it does not mean that it will not be available for the 2019 crop. Low yielding fields may warrant a reduction in P and K application rate. If you used prevent planting in a field and applied a full rate of P or K in 2018, it is possible that you won’t need to apply them for 2019.

Recent research funded by AFREC and the MN Soybean Research and Promotion council indicates that over a two year span in a corn-soybean rotation all the fertilizer needed for both crops can be applied ahead of either crop, resulting in maximum yield as long as the correct rate is applied. The only circumstance where additional P may be required is for growers in central and western Minnesota who have soils with greater than 10% calcium carbonate equivalency. In these cases an in-furrow starter option may be all you need for corn.

Potassium is not affected by soil carbonates but soil test K values can change over the growing season due to changes in soil moisture and crop uptake. Soil tests are still a good option to assess your options for a given field prior to fall fertilizer application. If soils test Very High in both P or K then the likelihood that either nutrient is in short supply is low.

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Support for this project was provided in part by the Agricultural Fertilizer Research & Education Council (AFREC).