Louisiana Soybeans: In-Season Potassium Deficiency Management

    Potassium deficiency in soybeans. Photo: Michigan State University

    Potassium deficiency symptoms in soybean first appear as irregular yellowing on the edges of K deficient leaves and can occur as early as at the V3 vegetative stage (three trifoliolate leaves) mainly on the lower older leaves (Figure 1). But symptoms often occur on the upper younger leaves during the reproductive stages especially in severe K deficient soils (Figure 2).

    Soybean fields with K deficiency symptoms early in the growing season are very easy to diagnose and manage. However, most of the soybean fields often suffer from K deficiency and exhibit yield losses without showing any visible deficiency symptoms at all or at least until the later reproductive growth stages (beginning seed, R5 to full-seed, R6).

    This type of phenomenon is called hidden hunger and its most common in soybean fields that are low to medium in soil-test K level (80 to 120 ppm or 160 to 240 lb Mehlich-3 K for 0- to 6-inch soil depth), have not received K fertilization, have coarse-textured soils with high leaching potentials due to low cation exchange capacity (CEC) and excessive rainfall, or undergo severe drought conditions.

    Tissue sampling during the growing season is the best and perhaps the only tool to diagnose hidden K deficiency in soybean. Tissue sampling is predominantly conducted at the full-bloom (R2) stage; but can be done at the later reproductive (early pod, R3 to beginning seed, R5) stages.

    However, diagnosis at the early growth stages would be more effective and economical in correcting K deficiency and rescuing yield losses than diagnosis at the later growth stages.

    For proper tissue sampling, 15 to 20 recently mature trifoliolate leaves excluding petioles from the 3rd node from the top of the soybean plant should be collected, the date and soybean growth stage should be recorded, and the sample should be sent immediately to the plant diagnostic lab for K concentration.

    The critical K concentration at the R2 stage ranges from 1.46 to 1.90% and any K concentration below the critical level would be deficient and above the critical level would be sufficient (Figure 3).

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    From the R2 stage, critical tissue K concentration declines linearly with the advancement of growth stage due to K translocation from vegetative to reproductive plant parts (pods and eventually seeds). Therefore, the growth stage at the time of tissue sampling should be recorded for properly interpretation of tissue K concentration.

    Soybean K deficiency can easily be corrected by top-dressing or flying 60 pounds K2O per acre (100 lb Muriate of Potash per acre; 0-0-60) until the R5 stage or about 5-weeks past the R2 stage.

    Foliar application of liquid K would not be effective and economic to correct severe K deficiency since foliar product contains very small amount of K. Also, foliar product requires several applications since K has a high salt index that can burn soybean foliage if applied in high concentrations.

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    Figure 1. Potassium deficiency symptoms during the early vegetative growth stages of soybean.

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    Figure 2. Potassium deficiency symptoms during the reproductive growth stages of soybean.

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    Figure 3. Critical soybean leaflet K concentration from the R2 to R6 stages. (Source: Parvej, M.R., N.A. Slaton, L.C. Purcell, and T.L. Roberts. 2016. Critical trifoliolate leaf and petiole potassium concentrations during the reproductive stages of soybean. Agronomy Journal 108:2502-2518. doi:10.2134/agronj2016.04.0234; Y-axis is changed to English unit) Click Image to Enlarge

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