Negative potassium balances threaten global crop production

Technical Summary: Negative Potassium Balances Threaten Global Crop Production

Problem Statement
Potassium (K) is a critical plant nutrient essential for enzyme activation, water regulation, photosynthesis, and stress tolerance. Despite its importance, K management has been historically neglected compared to nitrogen (N) and phosphorus (P). This neglect stems from the perception that many soils possess sufficient native K reserves, the lack of visible deficiency symptoms ("hidden hunger"), and the lower direct environmental impact of K losses compared to N and P. Consequently, many agricultural systems have evolved toward negative K balances, where crop removal exceeds inputs, leading to progressive soil K depletion. The authors posit that current K inputs are insufficient to sustain existing yields or achieve attainable yields without further mining of soil K reserves, posing a significant threat to global food security, particularly as geopolitical trade disruptions affect potash availability.

Methodology
To evaluate the global extent and severity of K limitations, the authors compiled a comprehensive multi-indicator database spanning 31 major cropping systems that account for approximately two-thirds of global cereal, oilseed, and fiber production. The dataset includes:

• 22 million soil samples for exchangeable K concentration.
• 66,300 K-fertilization yield trials (comparing K-omission vs. K-fertilized treatments with adequate N and P).
• 117,000 field-level input-output K balances.

The study employed a tiered approach to estimate on-farm K balances, prioritizing field-level data and utilizing national/regional aggregates only where granular data were unavailable. K balances were calculated as the difference between inputs (inorganic/organic fertilizers, irrigation, seed) and outputs (harvested grain and residue removal/burning).

The analysis integrated:

1. Meta-analysis: To quantify yield responses to K omission and economic returns, weighting observations by replication and harvested area.
2. Spatial Analysis: To map K limitations across diverse biophysical and management contexts.
3. Process-based Crop Modeling: To estimate attainable yields (defined as 80% of yield potential) and project future K requirements under two scenarios: (a) achieving a neutral K balance for current yields, and (b) achieving yield intensification (attainable yields) while avoiding soil K depletion.

Key Results

• Soil K Insufficiency: In most cropping systems, indigenous soil K supply alone is insufficient to meet crop requirements at current yield levels. Omitting K fertilizer reduced yields by an average of 11% across all systems, with the largest deficits observed in South Asia, Southeast Asia, China, the U.S. Cotton Belt, and African rice systems.
• Economic Viability: K fertilizer application increased net profit in 78% of cropping systems, with an average additional profit of US$285 ha⁻¹ y⁻¹, representing a 3.7-fold return on fertilizer costs.
• Drivers of Limitation: Variation in K limitation was driven by soil properties (e.g., clay mineralogy, exchangeable K levels), cropping intensity, and residue management. Systems with low soil K, K-fixing clays, high cropping intensity, and extensive residue removal exhibited the most severe limitations.
• Regional K Balances:
  • Negative Balances: Persistent negative K balances were found in South Asia (–42 kg K ha⁻¹ y⁻¹), Southeast Asia (–23 kg K ha⁻¹ y⁻¹), and Africa (–32 kg K ha⁻¹ y⁻¹), driven by low fertilizer use and high residue removal.
  • Emerging Limitations: Systems historically buffered by high native K reserves, such as the Western U.S. Corn Belt (–33 kg K ha⁻¹ y⁻¹) and the Western Pampas (–44 kg K ha⁻¹ y⁻¹), are transitioning toward K limitation due to continuous nutrient mining.
  • Positive Balances: The Brazilian Cerrado (+46 kg K ha⁻¹ y⁻¹) and most cereal systems in China (+55 kg K ha⁻¹ y⁻¹) currently maintain positive balances due to high fertilizer inputs and/or residue retention.
• Future Requirements:
  • To merely avoid negative K balances and maintain current yields, global K inputs must increase by 30% (from ~15.0 to ~19.3 million metric tons annually).
  • To achieve attainable yields (80% of potential) without further soil K depletion, global K inputs must increase by nearly 70% (from 15.0 to 24.7 million metric tons annually).
  • The largest relative increases are required in Africa and South Asia (+100–103 kg K ha⁻¹ y⁻¹) and Southeast Asian rice systems (+81 kg K ha⁻¹ y⁻¹).

Significance and Claims
The paper claims that sustaining current yields and achieving yield intensification on existing cropland requires a substantial, immediate shift in K management strategies. The authors argue that relying on declining native soil K reserves is unsustainable. The study provides a "blueprint" for global nutrient management assessment, highlighting that:

1. Widespread Limitation: K limitation is not confined to tropical soils but is emerging in temperate, high-yield systems due to long-term mining.
2. Policy Implications: Current subsidy structures in many regions favor N and P over K, exacerbating imbalances. Furthermore, policies promoting crop residue removal for feed or biofuel without accounting for K export will accelerate soil depletion.
3. Management Needs: There is an urgent need to improve diagnostic approaches beyond simple soil testing, as soil K levels alone are poor predictors of K supply in diverse cropping systems. Effective management must integrate residue retention, organic recycling, and optimized K fertilization rates tailored to specific yield targets and residue management practices.

The authors conclude that meeting future food demand will require a coordinated effort to increase K fertilizer use, particularly in the Global South, alongside improved agronomic practices to enhance K recycling and soil health.