Louisiana: Fall Phosphorus, Potassium Application Considerations

    Phosphorus fertilizer. Photo: University of Minnesota

    Louisiana producers mostly use triple superphosphate (TSP; 0-46-0) for Phosphorus (P) fertilization and muriate of potash (MoP; 0-0-60) for potassium (K) fertilization and apply both fertilizers mostly in the Fall rather than in the Spring. The main reasons for fall applications are due to wet soil conditions or limited application time in the Spring.

    A misconception about Spring applications of both P and K fertilizers is that they require a long time to dissolve and become available for plant uptake. Practically, both fertilizers are highly water soluble and can rapidly release nutrients, regardless of application time, when dissolved with adequate soil moisture and/or rainfall/irrigation water.

    Many studies including research conducted in Louisiana showed that spring applications of both TSP and MoP fertilizers is either equal to or better than fall applications in increasing crop yield especially in soils that are deficient and highly prone to nutrient losses via leaching, runoff, and/or erosion.

    However, both fertilizers when applied in the Spring need to be incorporated by rehipping the bed (for 30-40-inch row spacing) to bring most of the fertilizers, broadcasted in the furrow, back to the bed, to reduce fertilizer-P and K losses from furrow with rainfall and irrigation water over time, to increase fertilizer-P and K availability in the bed near plant roots, and to improve P and K uptake and crop yield.

    The following factors need to be considered in making decisions regarding P and K fertilizer application time.

    • The rapidity of P and K fixation to unavailable forms usually increases with the decrease of soil-test P and K concentrations. For example, soils with deficient P and K will fix applied P and K more rapidly than soils with sufficient P and K. Therefore, fertilizers should be applied in the Spring at or near planting in P and K deficient soils to ensure maximum nutrient availabilities during the time of rapid plant uptake.
    • Soil P availability is maximum between soil pH 6.0 and 7.5. Fertilizer-P is fixed to unavailable forms as aluminum phosphate when soil pH falls below 5.5 and as calcium phosphate when soil pH exceeds 7.5. Therefore, fertilizer-P should be applied in the spring as close to planting as possible for fields with low (pH <5.5) or high (pH >7.5) soil pH to ensure maximum fertilizer-P availability for plant uptake.
    • Spring application of fertilizer-P and K should be considered for coarse-textured soils with very low cation exchange capacity (CEC <10) such as loamy sand to sandy loam (sometimes silty loam) soils where nutrient leaching and soil erosion are common, and nutrient deficiencies are often observed. However, applying P and K in fall in fine-textured soils with high CEC (>20) may not be a problem.
    • For soils that are very prone to waterlogged/flooded conditions, fertilizer-P and K should be applied in the spring at or near planting since the alternating flooding (anaerobic) and non-flooding (aerobic) conditions decreases soil nutrient availability and increases losses.
    • Fall application of P and K should be considered for soils with nutrient concentration within (medium) or above (sufficient) the critical level, where fertilizers are mainly applied to replace soil nutrients that are removed by harvested grains to maintain the same level of soil nutrient level. In addition, fertilizer-K should be applied in the Fall in fields that have long history of chloride (Cl) toxicity problems and are poorly drained. Since K fertilizer (MoP) mainly consists of KCI, fall application will allow enough time to decrease Cl toxicity by reducing Cl accumulation from fertilizer KCl through winter and early spring rainfall.

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