Increasing Agricultural Production and the Importance of Potassium Nutrition

Increasing agricultural production while improving crop quality remains a major objective, particularly in developing countries. The substantial rise in food prices and the growing global economic crises have had profound effects on the nutritional status of human populations, exacerbating worldwide malnutrition. These developments have made nutrient security and food security critical global challenges.

Among the many factors influencing agricultural productivity, adequate and balanced mineral nutrition is fundamental to achieving high crop yields and superior product quality in practical agriculture. Among mineral nutrients, imbalanced potassium (K) nutrition has become a major constraint to crop production, especially in developing countries. The extremely low application of potassium fertilizers in the cultivation of various field and horticultural crops in developing countries, including Iran, has resulted in the rapid depletion of soil potassium reserves. The depletion of plant-available potassium leads to several adverse consequences, including inefficient utilization of nitrogen and phosphorus fertilizers, reduced farmers' income, diminished sustainability of cropping systems, and ultimately threats to future food and nutrient security. Therefore, farmers and agricultural advisors should adopt advanced potassium fertilization strategies and updated potassium recommendations based on the latest scientific findings and sound nutrient management practices.

What Is Potassium and Why Is It One of the Most Important Plant Nutrients?

Potassium is classified as one of the essential macronutrients and is the most abundant cation in plant cells. It is also the second most abundant nutrient in plant leaves after nitrogen, making it more abundant than phosphorus.

Newly developed soils generally contain relatively high phosphorus but low nitrogen levels. As soils age, nitrogen gradually accumulates, whereas phosphorus becomes increasingly depleted. Potassium is more susceptible to leaching than either nitrogen or phosphorus. However, natural ecosystems such as forests and shrublands possess highly efficient biological mechanisms for conserving potassium through processes such as biological pumping and potassium uptake by vegetation. Consequently, potassium deficiency is rarely observed in these ecosystems.

Forms of Potassium in Soil

Potassium in soil exists in three major pools that contribute to plant uptake through the root system.

The first pool consists of potassium dissolved in the soil solution. The second is potassium adsorbed onto clay particles and organic matter, while the third is potassium trapped within the crystalline structure of soil minerals.

Organic matter contains only negligible amounts of potassium because potassium is not a structural component of biological molecules. Consequently, due to its high solubility, potassium is readily and rapidly leached from plant tissues.

However, the fraction of soil potassium that is directly available to plants (i.e., potassium present in the soil solution) generally represents only a very small proportion (0.1–0.2%) of the total soil potassium. The combined water-soluble and exchangeable potassium fractions account for approximately 1–2% of total soil potassium, whereas 96–99% exists in unavailable mineral forms.

The plant-available potassium pool is easily lost through runoff and leaching. Therefore, the concentration of plant-available potassium in the soil is often lower than that of available nitrogen and phosphorus, despite the fact that the total potassium content of most soils is generally greater than that of nitrogen and particularly phosphorus.

The Role of Potassium in Plants

The absorbable form of potassium for plants is the potassium ion (K⁺). Potassium plays an active role in numerous metabolic processes within plant cells and serves as an activator for a wide range of enzymes. These enzymes function primarily as catalysts in the synthesis of compounds that directly determine both the quantitative yield and qualitative characteristics of agricultural products.

Major Functions of Potassium in Plants

The most important physiological functions of potassium in plants include:

• Promoting root growth and improving plant tolerance to salinity, drought, and low-temperature stress.
• Maintaining cell turgor, thereby reducing water loss and preventing wilting.
• Regulating stomatal conductance and enhancing photosynthesis through its involvement in electron transport and carbon dioxide fixation.
• Controlling stomatal opening under stress conditions, reducing respiration, and preventing unnecessary energy loss.
• Enhancing the translocation of sugars and starch from leaves and other photosynthetically active tissues to sink organs such as fruits, roots, flowers, buds, and storage tissues.
• Increasing plant protein content by supporting the orderly production of a wide range of plant metabolites.
• Participating in cellulose synthesis and cell wall formation, thereby reducing lodging in crops.
• Improving plant resistance to biotic stresses by increasing tolerance to plant diseases.

Potassium in Stress Tolerance and Crop Quality

A growing body of scientific evidence demonstrates that potassium performs diverse functions ranging from the molecular level to whole-field crop performance. Its involvement in stress signaling has become an increasingly important research topic that deserves greater attention from molecular biologists, plant physiologists, and plant nutrition specialists.

One of potassium's most significant functions is its ability to alleviate environmental stress, particularly under drought, salinity, and pathogen infection. Adequate potassium nutrition is also essential for determining the composition of harvested products and improving the nutritional quality of human diets.

Furthermore, nutrient management disorders and imbalances between nutrient inputs and outputs—especially those involving potassium—can seriously reduce soil fertility over time.

Considering the broad range of physiological functions performed by potassium in plant nutrition, it becomes evident that potassium fertilization should not be limited solely to fruit, grain, or tuber filling stages. Instead, sufficient potassium supply is required throughout the growing season, particularly under drought and saline conditions, where crop demand for potassium increases substantially.

Factors Affecting Potassium Availability in Agricultural Soils

A range of soil and environmental factors influence potassium availability in agricultural systems. These factors primarily affect the solubility of potassium in the soil solution as well as the release of potassium ions from clay minerals and organic matter. Understanding these factors is essential for improving potassium management and maximizing fertilizer efficiency.

Key Factors Influencing Potassium Uptake

Soil pH

Soil pH generally has little effect on potassium uptake. Potassium absorption remains relatively stable within a pH range of 6 to 10, and potassium deficiency is rarely attributed solely to soil acidity or alkalinity.

Soil Moisture

Adequate soil moisture increases the availability of potassium ions, particularly in clay-rich soils where potassium movement depends largely on diffusion. Consequently, greater potassium application may be required under limited irrigation conditions or in sandy soils where potassium is more susceptible to leaching.

Soil Aeration (Tillage System)

Excessive soil moisture combined with high clay content can reduce root aeration, thereby limiting potassium uptake. In such soils, appropriate tillage and soil management practices are necessary to improve aeration, enhance root activity, and promote efficient nutrient absorption.

Soil Temperature

An increase in soil temperature generally enhances potassium uptake by stimulating root growth and increasing nutrient mobility within the soil.

Organic Matter

Higher levels of soil organic matter improve potassium availability by enhancing soil structure, increasing cation exchange capacity, and promoting nutrient retention.

Soil and Water Salinity

Salinity, along with elevated concentrations of competing cations such as sodium (Na⁺), calcium (Ca²⁺), and magnesium (Mg²⁺), reduces potassium uptake by plant roots. Therefore, crops grown under saline conditions require higher potassium inputs to maintain adequate nutrition.

Potassium Depletion in Agricultural Soils

Unfortunately, agricultural soils are increasingly experiencing gradual potassium depletion. This process is accelerated by potassium fixation in heavy-textured and saline soils, as well as by the conversion of potassium into unavailable mineral forms.

At the same time, the increasing cost of potassium fertilizers and other agricultural inputs often prevents farmers from supplying sufficient potassium to meet crop requirements. As a result, crops continuously draw upon the native potassium reserves stored in the soil. Over several years, this practice progressively depletes soil potassium, reduces soil fertility, and ultimately limits the ability of agricultural fields and orchards to sustain high yields and long-term productivity.

Potassium Deficiency Symptoms in Field Crops and Fruit Trees

The average potassium concentration in the leaves of field crops and fruit trees ranges from 1.5% to 4.5% of dry weight, while the sufficiency range is generally considered to be 2–3%. Potassium concentrations below 1.5% indicate deficiency, whereas concentrations above 4.5% may result in toxicity symptoms. However, potassium toxicity is rarely observed in field crops and fruit trees.

Potassium deficiency symptoms typically first appear on the older, lower leaves before gradually progressing toward the younger leaves. One of the most common visual symptoms is marginal chlorosis or leaf scorch along the edges of the leaves. These symptoms become particularly pronounced during hot weather in crops with high potassium requirements, such as potato, tomato, pistachio, and sugar beet.

In cereal crops, potassium deficiency during early growth reduces root development and produces weak stems, making plants more susceptible to lodging and wind damage.

In tuber crops, potassium deficiency restricts the transport of nitrate from the roots to the shoots, leading to the accumulation of amino acids within the root system. Excessive amino acid accumulation may promote nitrate efflux, increase the susceptibility of root crops to diseases, and reduce nitrogen reabsorption, ultimately restricting plant growth. Consequently, potassium deficiency significantly decreases nitrogen uptake efficiency and nitrogen use efficiency.

Potassium deficiency also interferes with crop maturation and reduces the yield of tuber-producing crops. Plants suffering from potassium deficiency experience reduced cell turgor, making them more susceptible to water stress. In addition, potassium deficiency disrupts numerous physiological processes, including photosynthesis, metabolite transport, and enzyme activity. At the same time, respiration rates increase, resulting in slower plant growth and a decline in overall crop quality.

Conclusion

In general, although total potassium reserves in most soils are relatively abundant, the majority of this potassium exists in forms that are unavailable for plant uptake. Therefore, the potassium requirements of field crops and fruit trees must be supplied through appropriate potassium fertilization. The demand for potassium fertilizers is expected to increase substantially, particularly in developing regions of the world.

Under potassium-deficient conditions, the transport of photosynthates to sink organs is impaired, causing sugars to accumulate in the leaves. This not only reduces crop yield but also negatively affects quality attributes, including the quality of potato, tomato, and grape production.

Because potassium also contributes to human health, its concentration in harvested agricultural products is itself an important quality parameter. Owing to its essential roles in maintaining cell turgor, supporting primary metabolism, and facilitating long-distance transport of assimilates within plants, potassium plays a critical role in improving crop tolerance to drought, salinity, excessive light, low temperatures, pests, and plant pathogens.

Despite the indispensable functions of potassium in agricultural production, current soil potassium testing methods remain inadequate for many soils, particularly those found in Iran, increasing the risk of imbalanced fertilizer recommendations. To combat the ongoing decline in soil fertility, improve food security, and enhance potassium fertilizer use efficiency, further research and the development of innovative potassium-based technologies and products are urgently needed.

source:

Römheld, V., & Kirkby, E. A. (2010). Research on potassium in agriculture: needs and prospects. Plant and soil, 335(1), 155-180.

Zörb, C., Senbayram, M., & Peiter, E. (2014). Potassium in agriculture–status and perspectives. Journal of plant physiology, 171(9), 656-669.

Sardans, J., & Peñuelas, J. (2021). Potassium control of plant functions: Ecological and agricultural implications. Plants, 10(2), 419.