Are we overfertilizing our fields? Researchers challenge conventional soil testing

What if agricultural soils could provide far more nutrients than scientists and farmers previously believed? A new study reveals that up to 74% of the potassium absorbed by crops can come from  mineral reserves that conventional soil tests largely overlook. The discovery challenges traditional fertilizer recommendations and could open the door to more accurate and sustainable farming practices that reduce unnecessary fertilizer use while protecting crop productivity and health.

A new study led by Dr. Shikma Zaarur of the Hebrew University of Jerusalem, in collaboration with Dr. Ran Erel of the Agricultural Research Organization–Volcani Institute, reveals that agricultural soils may contain a substantial un-accounted for reservoir of potassium, an essential nutrient for plant growth, that is largely overlooked by conventional soil testing methods. The findings could pave the way for more precise fertilizer recommendations, potentially reducing unnecessary fertilizer use while supporting sustainable agricultural production.

Published in the scientific journal Plant and Soil, the research challenges a long-standing assumption in agricultural management: that the potassium measured by standard soil tests accurately reflects the amount available to crops.

Potassium is one of the three major nutrients required for plant growth, alongside nitrogen and phosphorus. Farmers routinely apply potassium fertilizers to maintain crop productivity, with recommendations commonly based on measuring the soil's readily available, or "exchangeable," potassium. However, the new study demonstrates that this conventional approach may significantly underestimate the soil's natural capacity to supply this essential nutrient.

Soil mineral reserves: From geological to agricultural timescales

The research team conducted a 13-month greenhouse experiment using three agricultural soils characteristic of Israel's Mediterranean environment: clay-rich soil from the coastal plain, loess soil from the western Negev, and red sandy soil from the Rehovot area.

The researchers grew alfalfa, a crop with particularly high potassium requirements, and harvested it 16 times under intensive growing conditions. By carefully tracking potassium uptake and changes in soil chemistry, they discovered that plants absorbed between two and nearly four times more potassium than could be explained by conventional measurements of readily available potassium and natural inputs.

Remarkably, 50% to 74% of the potassium taken up by plants grown without additional potassium fertilizer originated from non-exchangeable soil reserves — potassium bound within minerals that have long been recognized as potential nutrient sources, but whose release was generally assumed to occur too slowly to contribute significantly within agricultural growing cycles.

"Our findings show that soil is not simply a passive reservoir of nutrients, but a dynamic system that continuously responds to plant demand," said Dr. Zaarur. "As plants uptake potassium, minerals in the soil can release additional supplies much faster than previously assumed. Understanding this natural process could help us develop more accurate fertilization strategies, reducing unnecessary inputs without compromising crop productivity."

Not all soils behave alike

The study also revealed important differences between soil types. Clay-rich soil showed little improvement in overall plant biomass following potassium fertilization, suggesting that its natural mineral reserves were largely able to sustain crop growth during the experiment. In contrast, potassium fertilization increased biomass by approximately 13.5% in the potassium-poor red sandy soil.,

Using a geochemical technique that examines the relationship between potassium and rubidium, the researchers identified clay minerals as the primary source of the additional potassium. These findings suggest that soil mineral composition, rather than conventional potassium measurements alone, plays a crucial role in determining fertilizer requirements.

Implications for sustainable agriculture

The findings are particularly relevant for Mediterranean agriculture, where alkaline soils are widespread and efficient fertilizer management is increasingly important.

By accounting for potassium released naturally from soil minerals, future soil testing approaches could help farmers better match fertilizer applications to actual crop needs, potentially lowering production costs and reducing unnecessary resource consumption.

However, the researchers emphasize that mineral potassium reserves are not unlimited. Their depletion over time must also be considered to maintain long-term soil fertility. Further research under field conditions will be needed before revised fertilization recommendations can be established.

The study, "Non-exchangeable K sustains plant K uptake in alkaline Mediterranean soils," was led by Dr. Shikma Zaarur of the Hebrew University's Faculty of Agriculture, Food and Environment, in collaboration with Dr. Ran Erel of the Agricultural Research Organization–Volcani Institute, and researchers from both institutions, the Geological Survey of Israel, and Ben-Gurion University of the Negev.