Author
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Kovar, John |
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Karlen, Douglas |
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Submitted to: North Central Extension Industry Soil Fertility Conference Proceedings
Publication Type: Proceedings Publication Acceptance Date: 11/15/2014 Publication Date: N/A Citation: N/A Interpretive Summary: Potassium (K) nutrition of crop plants has received renewed attention during the past few years due to interest in harvesting crop residues as feedstock for bioenergy production. The amounts of plant-available K deep in the soil has generally been ignored because the number of active plant roots in this subsoil zone was thought to be relatively small and because it would be very difficult to fertilize subsoil. However, recent research has shown that the root systems of newer, drought-tolerant corn hybrids tend to grow deeper into the soil when surface water becomes limiting. If a significant amount of the soil water being taken up is coming from the subsoil, it is possible that the plants are not getting sufficient K for optimum growth and development. With a field study, we evaluated the amount of plant-available K to a depth of 1.0 to 1.2 m in 1,116 samples collected during an eight-year period (2005-2012) from four research sites in central Iowa. We found that plant-available K in surface soil was at an optimum level for corn production at only one of the four locations. The other sites were rated as having low K availability. At all four locations, plant-available K tended to decrease with depth in the soil. These results suggest that roots growing into the subsoil will not encounter an abundant supply of plant-available K. We conclude that for successful crop production on central Iowa soils, more attention must be given to K management and that this essential nutrient should no longer be neglected. The results of this research will benefit commercial growers and both the fertilizer and ethanol industries by providing nutrient management guidelines that maximize crop utilization and biomass yields. Technical Abstract: Plant potassium (K) concentrations and soil fertility evaluations to predict available K have received renewed attention during the past few years due to increasing interest in harvesting crop residues as feedstock for production of bioenergy and other bio-products. Interest in K crop nutrition has also increased as more producers adopt reduced or no-tillage production practices. This latter situation is not new, because the potential for K deficiencies under reduced tillage corn (Zea mays L.) production systems is a well-documented soil fertility problem, attributed primarily to stratification of plant-available K in surface horizons. Generally, the amounts of plant-available K in subsurface horizons have been ignored because the amount of active plant roots in those zones was considered to be relatively small and because it would be very difficult to fertilize subsoil. However, recent research has shown that the root systems of newer, drought-tolerant corn hybrids tend to grow deeper into the profile when surface water becomes limiting. If a substantial portion of the soil water being taken up during that time is coming from the subsoil, it is possible that the plants are not getting sufficient K for optimum growth and development. In this study, we quantified the amount of exchangeable K to a depth of 1.0 to 1.2 m in 1116 samples of Des Moines Lobe soil collected during an eight-year period (2005-2012) from research sites being used for corn and soybean [Glycine max (L.) Merr.] studies at four locations in central Iowa. At each location, samples were collected in 0 to 15-, 15 to 30-, 30 to 60-, 60 to 90-, and 90 to 120-cm increments. Samples were air dried, crushed to pass a 2-mm sieve, and extracted with either 1.0 N ammonium acetate at pH 7.0 or Mehlich III extracting solution. We found that in surface (0-15 cm) soil, mean exchangeable K varied from 119 to 164 mg kg-1. Based on Iowa State University recommendations, only the site with the highest K level would be rated as having an optimum level (161-200 mg kg-1) for crop production. At deeper soil increments, exchangeable K concentrations tended to decrease even further, ranging from 86 to 129 mg kg-1 at the four locations. These results suggest that roots growing into the subsoil will not encounter an abundant supply of plant-available K. Furthermore, soil pH values below the 15-cm depth were generally above 7.0, and exchangeable calcium (Ca) and magnesium (Mg) concentrations were very high. These initial results support our hypothesis that K acquisition by plant roots, especially those in subsurface soil, may also be hindered by excessive soil solution concentrations of Ca and Mg relative to K. We conclude that for successful crop production on the Des Moines Lobe or similar soils, more attention must be given to K management and that this essential nutrient should no longer be neglected. |
