Camryn Price, Parvej, Md Rasel
Dr. Rasel Parvej, LSU AgCenter Soil Fertility Specialist; Dr. David Moseley, LSU AgCenter Soybean Specialist; Dr. Josh Copes, LSU AgCenter Agronomist; and Dr. Syam Dodla, LSU AgCenter Soil Scientist
Potassium (K) is the second most yield limiting nutrient in soybean. Even though nitrogen (N) is the most limiting nutrient, soybean plant meets its own N requirement through biological N-fixation. Therefore, soybean is mainly fertilized with K and phosphorus (P) fertilizers in soils that are tested very low to medium K and P levels. Soybean is more responsive to K than P fertilizer and requires a large amount of K to maintain optimum water balance in plants, increase photosynthesis and assimilate translocation from source to sink, reduce transpiration losses of water, and improve uptake of other nutrients. A 55-bushel soybean requires about 160 pounds K2O (potassium oxide) per acre, approximately 2.9 pounds K2O per bushel grain harvested.
Potassium deficiency can decrease soybean yield more than 50% across soil types that range from sandy loam to clay loam. In addition, K deficiency decreases P uptake by soybean plants and reduces soybean seed quality by decreasing seed oil and protein content and increasing purple seed stain. Potassium deficiency can occur in any soybean field that is very low to low in soil-test K level and is not fertilized with K. Potassium deficiency, however, often occurs in coarse-textured soils with low cation exchange capacity (CEC <10) such as loamy sand to silt loam soils. Coarse-textured soils are highly prone to K leaching below the root zone. Sometimes, fall application of K fertilizer in coarse-textured soils results in late-season K deficiency due to K leaching from excessive rainfall during winter and/or spring. Coarse-textured soils are also poor in water holding capacity and drought in these soils often causes K deficiency by decreasing K uptake by plant roots.
Soybean K deficiency symptoms first appear as irregular yellowing on the edges of K deficient leaves. As growing season progress, the entire leaf edges turn brown and eventually the whole leaf dies. Potassium deficiency symptoms 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 under severe K deficiency conditions (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, have not received K fertilization, have high leaching potentials due to low CEC and excessive rainfall, or undergo severe drought conditions. Soybean grown in low pH (<6.0) soils also suffer from hidden K hunger effects because low pH decreases soil K availability even after fertilization.
Diagnosing hidden K deficiency early in the soybean growing season is very difficult and requires thorough scouting along with additional information such as fertilization history, soil texture, soil pH, soil-test K level, crop rotation, rainfall amount and distribution after fertilization and during the growing season, drought period, etc. 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.
After tissue sampling, tissue K concentration at a particular growth stage is interpreted to diagnose K deficiency. Many current tissue K interpretations, used by most of the plant diagnostic labs, only allow interpretation of K concentration for soybean leaflet (without petiole) collected at or around the R2 stage. Recently at the University of Arkansas, Parvej et al. (2016) developed critical trifoliolate leaflet and petiole K concentrations from the R2 to R6 reproductive stages (Figure 3). These critical K concentrations would allow soybean producers, agronomists, and crop consultants to sample either leaflet or petiole or both to diagnose K deficiency across the reproductive growth stages of soybean.
For proper tissue sampling, 15 to 20 recently mature trifoliolate leaves including petioles from the 3rd node from the top of the soybean plant should be collected and the date and soybean growth stage should be recorded (Figure 4). Then the leaflet of each trifoliolate leaf should be separated from the petiole and both the leaflet and the petiole or the leaflet only should be sent immediately to the plant diagnostic lab for K concentration. After receiving the results, tissue K concentrations for both the leaflet and the petiole at the specific growth stage can be interpreted using Figure 3. For example, the critical K concentration at the R2 stage ranges from 1.46 to 1.90% for leaflet and 3.01 to 3.83% for petiole and any K concentration below the critical level would be deficient and above the critical level would be sufficient. 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 to properly interpret the tissue K concentration.
For maximum soybean growth and yield, tissue K concentration should be above the critical level across the growth stages. If the tissue K concentration falls below the critical level, especially during the early reproductive stages, soybean should be fertilized with K to make sure K is not yield liming. Soybean K deficiency can easily be corrected by applying K fertilizer during the growing season. However, the effectiveness and economics of applying K fertilizer to rescue yield loss depends on soybean growth stage and the severity of K deficiency. The earlier the growth stage for K application the more effective and economic it would be in recovering yield loss. Recently, research conducted at the University of Arkansas suggests that soybean K deficiency can be effectively and economically corrected by applying 60 pounds K2O per acre until the R5 stage or about 5-weeks past the R2 stage. This is because soybean uptakes more than 70% of the total K after blooming and maximizes (100%) K uptake near the R6 stage. Therefore, diagnosis of K deficiency followed by an immediate K application early in the growing season would allow soybean plant enough time to actively uptake K from soils or through leaves and recover significant yield losses. However, pre-plant K application is the best way to maximize soybean yield.
Both dry and liquid fertilizers can be used in correcting soybean K deficiency during the growing season. However, dry fertilizer would be more effective and economical for correcting severe K deficiency since a high amount of K would be required. Foliar application of liquid K may be effective for small amount of K requirement since K fertilizer has a high salt index that can burn soybean foliage if applied in high concentrations (Figure 5). Therefore, foliar method requires several applications to correct a severe K deficiency that would increase application cost. Also, foliar K fertilizer is more expensive than dry K fertilizer. The most effective and economical method is either by top-dressing or flying 100 pounds Muriate of Potash (0-0-60; 60 pounds K2O) per acre. An in-season K deficiency management experiment
for soybean has been established at the Macon Ridge Research Station, Winnsboro, LA this year and the results will be shared during the growers’ extension meeting in the winter.
Figure 1. Potassium deficiency symptoms during the early vegetative growth stages of soybean. The leaves are yellowing, starting from the outside working its way in.
Figure 2. Potassium deficiency symptoms during the reproductive growth stages of soybean.
Figure 3. Critical soybean leaflet and petiole 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)
Figure 4. Steps of soybean tissue sampling during the R2 reproductive stage. Pencil in the picture indicates 3rd node from the top of the plant.
Figure 5. Soybean foliage damage due to the application of high rate of liquid potassium.