Tissue Testing Helps In Determining Hidden Hunger Potassium Deficiency In Soybean

DR. RASEL PARVEJ AND  DR. DAVID MOSELEY

BATON ROUGE, LOUISIANA

  • Hidden potassium (K) deficiency often occurs in soybean fields with low to medium soil-test K levels.
  • This deficiency can be diagnosed by sampling trifoliolate leaves at the R2 (full-bloom) stage and analyzing their K concentration.
  • The critical leaf K concentration at the R2 stage is 1.50%, and K deficiency can be corrected by applying 60 pounds of KO per acre (100 pounds of Muriate of Potash per acre).

Potassium deficiency symptoms in soybean first appear as irregular yellowing on the edges of K-deficient leaves, typically manifesting as early as the V3 vegetative stage (three trifoliolate leaves), primarily on the lower older leaves (Figure 1). In severe K-deficient soils, symptoms often appear on the upper younger leaves during the reproductive stages (Figure 2). Early-season K deficiency symptoms are relatively easy to diagnose and manage. However, many soybean fields experience K deficiency and consequent yield losses without displaying visible symptoms until the later reproductive growth stages (beginning seed, R5 to full-seed, R6). This phenomenon, known as hidden hunger, is common in soybean fields with low to medium soil-test K levels (80 to 120 ppm or 160 to 240 lb Mehlich-3 K for 0- to 6-inch soil depth). Fields most susceptible to hidden hunger are those that have not received K fertilization, have coarse-textured soils with high leaching potential due to low cation exchange capacity (CEC) and excessive rainfall, or experience severe drought conditions.

Tissue sampling during the growing season is the best and perhaps the only reliable tool for diagnosing hidden K deficiency in soybeans. Although tissue sampling is predominantly conducted at the full-bloom (R2) stage, it can also be done during later reproductive stages (early pod, R3, to beginning seed, R5). However, diagnosing K deficiency at the early growth stages is more effective and economical for correcting the deficiency and preventing yield losses than at later stages.

For proper tissue sampling, collect 18 to 20 recently mature trifoliolate leaves, excluding petioles, from the 3rd node from the top of the soybean plant (Figure 3). Record the date and soybean growth stage, and send the sample immediately to a plant diagnostic lab for K concentration analysis. The critical K concentration at the R2 stage ranges from 1.50 to 1.90%. Any concentration below this range is considered deficient, while concentrations above are sufficient (Figure 4). From the R2 stage onwards, the critical tissue K concentration declines linearly as K translocates from vegetative to reproductive plant parts (pods and eventually seeds). Therefore, recording the growth stage at the time of tissue sampling is essential for accurately interpreting tissue K concentrations.

Soybean K deficiency can be effectively corrected by top-dressing or aerially applying 60 pounds of K2O per acre (100 lb Muriate of Potash per acre; 0-0-60) up until the R4 stage, or approximately 4 weeks past the R2 stage. Foliar application of liquid K is not recommended for correcting severe K deficiency, as these products contain very small amounts of K and require multiple applications. Additionally, foliar products have a high salt index, which can burn soybean foliage if applied in high concentrations.  ∆

DR. RASEL PARVEJ AND  DR. DAVID MOSELEY: LSU AgCenter

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