Volume 14, Issue 4 - June 2024

David Moseley, Conger, Stacia, Parvej, Md Rasel

Louisiana Crops Newsletter Plain Banner.

Tissue testing helps in determining hidden hunger potassium deficiency in soybean

Rasel Parvej, Assistant Professor & State Soil Fertility Specialist, LSU AgCenter, David Moseley, Assistant Professor & State Soybean Specialist LSU AgCenter

Core Ideas:

  • 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 K₂O 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.

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

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

Potassium deficiency symptoms during the reproductive growth stages of soybean.

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

Tissue sampling position, third node from the top, of a soybean plant and a trifoliolate leaf tissue sample of a soybean plant.

Figure 3. Tissue sampling position, 3rd node from the top, of a soybean plant (left) and a trifoliolate leaf tissue sample of a soybean plant (right).

Critical soybean leaflet potassium concentration from the R2 to R6 growth stages.

Figure 4. 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)

Atypical Weather Patterns Impact 2024 Crop Production

Stacia L. Davis Conger, Ph.D., State Irrigation Specialist, LSU AgCenter; Leah Reeves, 2024 LSU AgCenter ASPIRE Intern, Southern University; Charley Bieber, 2024 LSU AgCenter ASPIRE Intern, Louisiana Tech University

Louisiana’s weather conditions have exhibited extremes in recent years. The 2023 drought subsided over the winter months with a transition into frequent, significant, and sometimes intense rainfall occurring throughout the spring months. Normally the driest portion of the state, Northwest Louisiana received 80% of its annual rainfall by May 31, 2024. Comparatively, rainfall measured in Bossier City was 58% more than what has fallen in New Iberia over the same period. While both locations experienced many dry days, highlighting the sporadic nature of rainfall, consistently high soil moisture has delayed planting for many fields with heavy soil types across the state.

If “normal” weather patterns return this summer, late plantings may require irrigation to maintain adequate soil moisture through critical crop growth stages. Temperatures rise with lack of rainfall; the use of groundwater is advantageous in combatting extreme heat as it will provide a cooling effect that benefits the quality of the crop. However, it is highly recommended that irrigators take new water quality samples prior to initiating irrigation this year to ensure excessive groundwater pumping during last year’s drought did not change the characteristics of the water source.

LSU AgCenter Specialists

Specialty Crop Responsibilities Name Phone
Corn, cotton, grain sorghum Agronomic Trey Price
Soybeans Agronomic David Moseley 318-473-6520
Wheat Agronomic Boyd Padgett 318-614-4354
Pathology Cotton, grain sorghum, soybeans Boyd Padgett 318-614-4354
Pathology Corn, cotton, grain sorghum, soybeans, wheat Trey Price 318-235-9805
Entomology Corn, cotton, grain sorghum, soybeans, wheat James Villegas
Weed science Corn, cotton, grain sorghum, soybeans Daniel Stephenson 318-308-7225
Nematodes Agronomic Tristan Watson 225-578-1464
Irrigation Corn, cotton, grain sorghum, soybeans Stacia Davis Conger 904-891-1103
Ag economics Cotton, feed grains, soybeans Kurt Guidry 225-578-3282
Soil fertility
Corn, cotton, grain sorghum, soybeans Rasel Parvej

6/20/2024 3:49:47 PM
Rate This Article:

Have a question or comment about the information on this page?

Innovate . Educate . Improve Lives

The LSU AgCenter and the LSU College of Agriculture