Soil tests may miss potassium reserves in Mediterranean soils

Conventional soil potassium testing may miss a large part of the nutrient that crops actually take up, according to a greenhouse study led by Shikma Zaarur of the Hebrew University of Jerusalem and published in Plant and Soil.

Potassium is one of the three major plant nutrients, alongside nitrogen and phosphorus. Farmers apply potassium fertilizers to maintain yields, and rates are usually based on soil test results. Fertilizer recommendations for potassium usually rest on the soil’s readily available, or “exchangeable,” potassium. The study tests that assumption. The research team, working with Ran Erel of the Agricultural Research Organization’s Volcani Institute, ran a 13-month greenhouse experiment on three soils typical of Israel’s Mediterranean climate: clay-rich soil from the coastal plain, loess soil from the western Negev and red sandy soil from the Rehovot area.

What the study measured

The team grew alfalfa, a crop with high potassium needs, and harvested it 16 times. Plants took up between two and nearly four times more potassium than conventional measurements of readily available potassium and natural inputs could explain.

On plots that received no added potassium fertilizer, 50% to 74% of the potassium plants absorbed came from non-exchangeable reserves, which are bound within soil minerals.

Where the extra potassium came from

Using a geochemical method that compares potassium with rubidium, the researchers identified clay minerals as the main source of the additional potassium. The effect differed by soil. Clay-rich soil showed little gain in overall plant biomass after potassium fertilization, which suggests its mineral reserves were already sustaining growth. In the potassium-poor red sandy soil, potassium fertilization raised biomass by about 13.5%.

Zaarur said soil “is not simply a passive reservoir of nutrients, but a dynamic system that continuously responds to plant demand.” She added that minerals can release potassium “much faster than previously assumed.”

What this could mean for fertilizer rates

If soil minerals supply more potassium than current tests indicate, recommendations based only on exchangeable potassium could overstate the need for fertilizer in some fields. The authors say future testing could account for potassium released from soil minerals, which would help farmers match applications to what the crop needs. The findings are especially relevant for Mediterranean agriculture, where alkaline soils are common.

The team’s framing matters here. The result is not a claim that farmers over-apply potassium everywhere. It is a claim that the test value alone may understate what the soil can supply during a growing season.

The study has clear limits. It was a greenhouse trial, not a field program, and it covered three soils and one crop. Alfalfa has high potassium demand, so the results may not carry over to cereals or to other soils without field testing. The work does not show that potassium use is wasteful in general, and the findings should not be read as a verdict on all fertilizer use.

For fertilizer makers and agronomists, the practical question is whether soil test calibrations in other regions miss the same mineral reserves. The paper points to that gap, but the next step is field trials that compare test-based rates with crop response over several seasons.

Other work on acidic soil has looked at how biochar and potassium-solubilizing bacteria can raise the potassium plants are able to reach, another sign that soil supply is more than a single test number.

Source: Phys.org