How would you manage in-season potassium deficiency in soybean?
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 and the severity of K deficiency increases, 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 middle 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 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.
Figure 1. Potassium deficiency symptoms during the early vegetative growth stages of soybean.
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 sidedressing of high rate of liquid potassium.
How would you manage in-season potassium deficiency in soybean?
Potassium Enhances Crop Production Of Maize
Maize (Zea mays L.) is an imperative food and feed crop of the world and is often stated as “the king of grain crops”. After wheat and rice maize ranks third cereal crop in the world production and it is also a high value cereal crop of Pakistan.
Maize grains contain protein (10%), starch (72%), fiber (5.8%), oil (4.8%), sugar (3.0%), and ash (1.7%). It contains vitamin B-complex such as B1 (thiamine), B2 (niacin), B3, B5 and B6. That’s why it commendable for skin, hair, digestion, heart and brain.
Along with vitamin B-complex it also contains vitamin A, C and K together with large amount of beta carotene and small amount of selenium that play main role in suitable working of the immune system and helps to improve thyroid gland.
Maize has higher contents of fat and protein as compared to other cereal crops. The oil present in maize embryo is used for cooking and manufacture of soaps.
In current days the excessive amount of maize has been utilized in the production of varnishes, paints, ammunition, shortening the compounds and related several products.
Maize is a multipurpose crop which provides feed for the animals, food and fuel for the human being. The by-product seed cake of maize is used for livestock feed.
Pollen and seeds of maize are the nutritious and edible parts for human consumption. Raw seeds are consumed and cooked form that works as good source of carbohydrates. It forms the main nutritional parts in the form of cake, porridge and bread of the many people in several regions of the world Africa, America and Asia.
Maize is able to take better advantage of sunlight than the most other major cereals crops and grows more rapidly because of the size and distribution of its foliage.
Due to high yielding cereal crop the maize could be the better choice for dealing with the problem of food shortage because it provides forage and food for animals and human beings.
Globally, maize crop is grown on an area of 159 million hectares with the total yield of 796.48 million tonnes.
In Pakistan maize crop is cultivated on 1.08 million hectares of land which has overall yield of 4631 thousand tonnes and average grain yield is 4.26 tonnes ha-1. Punjab shares 39% of the entire zone and 30% of overall production of maize; KP shares 56% of the entire area and 63% of the overall yield production while Baluchistan and Sindh contributes 5% of the entire area and 3% of the total yield production in Pakistan.
In Pakistan this level of yield is lower than the other countries which produce the maize like the USA (9840 kg ha-1), France (9474 kg ha-1), Italy (9668 kg ha-1), Canada (9193 kg ha-1) and Egypt (8173 kg ha-1). Pakistan is ranked, 24th among the maize producing countries.
In Pakistan average production of maize is very low due to abnormal plant density, insufficient fertilizer usage, scarce water supply, weeds problems, selection of unsuitable varieties and insect pest attack under a particular set of environmental conditions.
The maize production should be improved to sustain the quickly increasing demands for human food, livestock feed and biofuels at the world level.
Potential yield of maize can be attained by the proper use of agricultural inputs and advanced agronomic operations. For the achievement of high yield and good growth the maize crop must be provided with sufficient nutrients particularly phosphorus, nitrogen and potassium.
Maize crop can be cultivated in tropical, subtropical and temperate regions of the world therefore this crop has worldwide adaptability. In various maize cultivating areas of the world the hybrid maize production has increased maize yield manifolds.
Several management practices requires for hybrid maize production especially of its fertilizer requirement which is higher than compared to that of ordinary maize varieties. Efficient maize hybrids exhibit the good fertilizer use efficiency.
These plant hybrids conserve the environment and reduce the input cost. These hybrids contain the high plasticity and higher grain yield because of genetic yield improvement. Maize hybrids have also improved morphology of the plants. Hybrids also increase the dry matter partitioning to the ear.
Modern maize hybrids respond differently to the potassium function application due to variation in their uptake, accumulation, utilization, translocation and growth habits.
Most of the farmers in Pakistan prefer to grow hybrid seeds of the maize which are high yielding. The maize crop differs with agri-management techniques, specifically management of fertilizers and the planting geometry.
In nutrient management the balanced nutrition is an imperative feature which shows a major function in increasing the quality and production of crop.
The presence of nutrients like sulfur, magnesium, phosphorus, nitrogen and potassium in well-adjusted forms is necessary for the main functions of plant growth, development and final yield production. The global research shows that application of fertilizers increase more than 50% production of crops.
Potassium is considered as one of the most essential macronutrients needed by the plants in growth, development and sustainable yield. It is an important fertilizer for increasing the grains yield of maize. It is considered that potassium function with the most imperative essential macronutrient of plants due to its main function in biochemistry and physiology of the plants.
Maize consumed 5.2 kg P2O5 ha-1 per day during the peak flowering period. After sowing during 38 to 52 days the maize plants require the total potassium up to 38% for the whole growing season.
Potassium function has also been deliberated as the most important quality and yield enhancing element in maize hybrids production. The growth and final production constituents of maize crop like the leaf area, crop growth rate, plant height, cob length, number of grains per cob, 1000-grains weight, biological and grain yield are considerably improved by the application of potassium fertilizer.
The fertilizer based on Potassium function improves the overall yield of maize due to increase in the kernel weight. All the growth parameters of maize increased by the application of potassium, viz. plant height, stem diameter and production of biomass.
Application of fertilizer based on potassium function also improves the effectiveness of the photosynthetic apparatus of the plants (leaf area and number of leaves). Irrespective of the sources the effects of higher application rate of potassium function was found meaningfully greater to the inferior application rate.
Due to calcareous nature Pakistani soils under the normal conditions have poor capacity to provide adequate potassium to crop plants.
Potassium function nutrient helps the plants to regulate the movement of stomata. Accumulation of potassium in the roots of plants creates osmotic pressure gradient that pulls up the water from soil into the roots of plants. If potassium supply is insufficient then the response of stomata become slow and the water vapors become lost.
Deficiency of potassium function nutrients resulted the largely disposed of crop plants to water scarcity. Potassium plays major function in the activation of enzymes, photosynthesis, protein synthesis, regulation of osmotic pressure, movement of the stomata, phloem transport, transfer of the energy, cation-anion balance in soil and in plants and resistance against the stress.
From all the mineral nutrients, Potassium function mainly in the metabolism and growth of plant life. It helps in survival of plant against the several biotic and abiotic stresses. Potassium is found one of the most ample nutrients in the plant tissues.
It is made up of about 1–10 percent total dry matter of the plants. It is an important nutrient of plants and is also considered as the largely ample cation found in the plant tissues. In the cytoplasm the potassium ions (K+) concentration has been observed between 100 and 200 mM.
The plants which contain higher potassium amount shows less pest damage due to lack of pests preference under the adequate concentration of the nutrients. In the depth of soil the potassium function in association with the other inorganic nutrients, such as phosphorus and nitrogen produces the deeper rooting of plants.
Adverse effects of water logging on plants could be decreased by exogenous application of potassium. Application of potassium function underwater logging condition not only improved the photosynthetic capacity, photosynthetic pigments and plant growth, but also enhanced the nutrients’ uptake by the plant roots as an effect of higher accumulation of Mn2+, Ca2+, Fe2+, K+ and N.
Potassium Enhances Crop Production Of Maize
Potassium-enriched salt substitution and heart disease
Nature Food volume 1, page254(2020)
Salt has been on the public health agenda for decades, with great investments globally in salt awareness campaigns, front-of-package labelling, industry reformulation of foods — and even activism. Now, Matti Marklund of Tufts University and colleagues have modelled the impact of potassium-enriched salt substitution, via effects on systolic blood pressure, on cardiovascular morbidity and mortality nationally in China.
The study reports that 45% of the adult population in China has hypertension. One in seven deaths from cardiovascular disease and about one in three fatal strokes in those aged under 70 can be attributed to high sodium intake. Discretionary addition of salt to food in the home accounts for 70% of sodium intake in China.
The model mainly utilizes data from the Global Burden of Disease Study and interim data from the Salt Substitute and Stroke Study (SASS). SASS is a cluster randomized trial of potassium-enriched salt substitutes (composed of 70±10% sodium chloride and 30±10% potassium chloride, supplied at 20 grams per individual per day) ongoing in 600 villages across five provinces in China.
The researchers estimate the effects of potassium-enriched salt substitution on systolic blood pressure would prevent 461,000 deaths per year from cardiovascular disease in China, including 208,000 from stroke. It would avert 743,000 non-fatal cardiovascular events and 7.9 million disability-adjusted life years related to cardiovascular disease. The incidence of chronic kidney disease would be reduced by 120,000 cases annually, 32,000 deaths from chronic kidney disease would be prevented, but the increased intake in potassium could potentially result in an additional 11,000 deaths from cardiovascular disease, half of which due to hyperkalaemia in cases of advanced chronic kidney disease.
The modelling is presented in a streamlined manner, and the study represents the kind of persuasive snapshot that may appeal to decision-makers. While the China Health and Nutrition Study of 2015 indicates that the majority of sodium consumed is added in the home, surveys from the US and elsewhere indicate that the domestic salt-shaker contributes relatively less compared with processed foods — and in such regions, industry approaches to salt reduction may be favoured for public health.
Potassium-enriched salt substitution and heart disease
Crop stress responses
It was by no means inevitable that the unprecedented levels of rainfall over the autumn and winter would end with a drought, however I’m sure many could see it coming. It has been stressed many times about the importance of potassium in crops suffering from drought stress, and each spring appears to provide a reminder of this.
Photosynthesis is the process by which plants capture light energy to convert carbon dioxide and water into carbohydrates, such as sugars, required for growth and building yield. The rate of photosynthesis therefore dictates the efficiency of the production of sugars and ultimately crop yield and quality. Plants subjected to stresses, such as moisture stress, considerably reduce photosynthetic rates, reducing the efficiency of carbohydrate production resulting in lower yield. Photosynthetic rates are maintained in potassium sufficient plants through efficient stomatal activity. Potassium is required by plants to regulate the opening and closing of the stomata (tiny apertures on the underside of leaves) surrounded by guard cells.
The stomata are important for allowing the movement of carbon dioxide into the plant, whilst releasing oxygen. The plant regulates the opening and closing of the stomata through movement of potassium into or out of the guard cells. When potassium moves into these cells, they accumulate water as a result of their higher salt concentrations (through osmosis) and therefore swell, opening the pores. If potassium levels within the plant are low, the stomata become slow to respond and do not close as quickly, resulting in the loss of water vapour and a reduced photosynthetic rate.
By-products of photosynthesis are Reactive Oxygen Species (ROS), such as hydrogen peroxide, that are highly toxic and cause damage to the plant through chlorophyll degradation, leaf chlorosis and necrosis. These ROS are usually removed from plant cells by antioxidants and antioxidative enzymes, however, their production can be particularly high when plants are exposed to environmental stresses such as drought. Potassium deficient plants suffering from drought stress are therefore more susceptible to ROS damage.
High light intensity combined with drought stress can further exacerbate the problem causing very rapid leaf chlorosis and necrosis. Plants suffering from potassium deficiency are therefore also extremely sensitive to increased light intensity.
The graph below shows the net photosynthesis of wheat leaves subjected to varied drought stress and potassium supply (Gupta et al., 1989).
Crop stress responses
The currently spreading COVID-19 pandemic has been found to present in various ways, often with respiratory symptoms, but also with significant gut symptoms, skin symptoms, and in critical illness, multi-organ dysfunction may occur. Now a new study by an Italian team of researchers published in June 2020 on the preprint server medRxiv* reports that potassium levels are often low in COVID-19 disease, mostly due to the urinary loss of potassium.
The incidence of severe or critical COVID-19 is known to be higher among older patients and those with underlying medical conditions, including diabetes, cardiovascular disease, and obesity. These patients often need to remain in hospital for supportive medical care for weeks, with various needs ranging from mechanical ventilation to cytokine inhibitors to reduce hyperactive inflammation.
Study: Hypokalemia in Patients with COVID-19. Image Credit: Stephen Barnes / Shutterstock
Hypokalemia is Common in COVID-19
Both the protean manifestations of sickness, as well as the side effects of the medications used can cause electrolyte imbalance. Accordingly, the current study shows that hypokalemia, or low potassium levels in the blood, occurs quite commonly. This is a concerning issue since, below a certain threshold, low potassium levels can lead to abnormalities of the heart rhythm, sometimes fatal.
The exact reasons for the development of hypokalemia are not clear, but several have been proposed. These include activation of the renin-angiotensin system, loss of potassium through the gut, loss of appetite and poor diet due to the infection, and kidney damage, perhaps due to direct viral cytotoxicity on tubular cells.
The current study focused on describing the incidence, impact, and mechanism of causation of hypokalemia on hospitalized COVID-19 patients. There were 1,671 blood samples, collected from 290 patients. Among these, 171 had normal potassium levels and were selected to be a control group.
Low potassium was present in 119 patients, accounting for 41% of the sample. The serum potassium levels ranged from 2.4 mEq/l to 3.5 mEq/l with a mean of 3.1 mEq/l. Most patients in this group had mild hypokalemia (about 91%), occurring along with hypocalcemia, and lower average magnesium levels.
Clinical Characteristics of Hypokalemic Patients
This group of patients also required a more extended period of follow-up, probably because the disease took a more severe form. In less than 40%, the urine potassium-to-creatinine ratio was measured and found to be over 1.5 mEq/mmol in over 95% of them. This indicates a high potassium excretion in urine.
Half of these 45 patients were on diuretics, and a quarter on steroids when potassium in serum was measured. Of the remaining quarter, 90% had low sodium excretion, while all of them had normal serum magnesium levels. Most of them were in metabolic alkalosis, with the remaining 30% equally distributed among respiratory alkalosis, respiratory acidosis, and normal acid-base balance.
Treatment of Hypokalemia
Hypokalemia was corrected using potassium salts orally in a quarter of patients and intravenously in 6%, with both being used in one patient. Magnesium deficiency was corrected with intravenous magnesium sulfate. Notably, frusemide was in use among 38% of patients at the time of hypokalemia, perhaps because of hypertension, cardiovascular disease, and/or poor renal function.
Risk Factors for Hypokalemia
The three factors most closely related to the occurrence of hypokalemia were the female sex, the use of diuretics, and corticosteroid therapy. Being female increased the risk by 244%, while diuretics were correlated with a 194% increase. Females are known to have less interchangeable potassium stores in their bodies, especially as they age, which puts them at higher risk for hypokalemia following diuretic use.
Explanations and Implications of Hypokalemia
However, hypokalemia was not associated with either severe disease or death during hospitalization. This may be explained by the mostly mild level of hypokalemia, induced by factors such as reduced intake of food, medication-induced diarrhea, viral injury to the gut, and the use of diuretics and corticosteroids.
Despite the very low incidence of moderate or severe hypokalemia, which can cause cardiac rhythm abnormalities, paralysis, and rhabdomyolysis, the study indicates the need to monitor this group of patients for arrhythmias, especially when drugs like hydroxychloroquine and azithromycin are used off-label to treat COVID-19. These medications are known to reduce the rate of cardiac conduction.
Does COVID-19 by itself cause urinary potassium loss? The viral has been found in urine samples from patients with severe COVID-19 and may possibly cause acute kidney injury, causing abnormal potassium handling. Another explanation is that the renin-angiotensin system is abnormally activated by the loss of ACE2 function in disrupted cells. However, more data on the levels of renin and aldosterone are required to support this hypothesis, despite the findings of metabolic alkalosis and low sodium excretion.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
Low potassium levels in COVID-19 disease