Sixteen alfalfa harvests over 13 months revealed that Israeli soils supplied far more potassium than standard tests would have predicted. The finding points to a possible way to make fertilizer use more precise: measure not only what soil can release quickly, but also what its minerals can supply as crops grow.
Potassium is one of the three main nutrients plants need, alongside nitrogen and phosphorus. Farmers often base potassium fertilizer decisions on the amount of “exchangeable” potassium in soil—the portion that standard tests treat as readily available to plants. But research led by Dr. Shikma Zaarur of the Hebrew University of Jerusalem, with Dr. Ran Erel of the Agricultural Research Organization–Volcani Institute, suggests that this measure can miss a sizable natural reserve.
In a greenhouse, the team grew alfalfa, a crop with high potassium needs, in three soils from Israel’s Mediterranean environment: clay-rich soil from the coastal plain, loess from the western Negev and red sandy soil from the Rehovot area. The researchers tracked plant uptake and changes in soil chemistry through 16 harvests during the 13-month experiment.
Plants took up between two and nearly four times as much potassium as could be explained by conventional measurements of readily available potassium and natural inputs. In plants grown without added potassium fertilizer, between 50% and 74% of the potassium they absorbed came from non-exchangeable reserves—potassium held in soil minerals. These minerals have long been seen as a possible nutrient source, but their contribution during a crop-growing period was thought to be limited because they were expected to release potassium slowly.
The results varied by soil. Adding potassium fertilizer made little difference to overall plant biomass in the clay-rich soil, whose mineral reserves appeared able to support growth during the experiment. In the potassium-poor red sandy soil, fertilizer raised biomass by about 13.5%. A geochemical method that compared potassium with rubidium, another element found in soil, pointed to clay minerals as the main source of the extra potassium.
That difference matters for farmers and for efforts to use fertilizer wisely. If soil tests count only the readily available portion, they may lead to applications that some fields do not need. Testing that also accounts for mineral reserves could help match fertilizer to what each soil can actually provide, potentially lowering farm costs and avoiding unnecessary use of resources while keeping crops productive.
The researchers caution that these reserves are not endless. Their gradual depletion must be considered to protect soil fertility over time, and greenhouse results do not yet establish new advice for farmers. Further field research is needed before recommendations change. For now, the study, published in Plant and Soil, makes a clear case for taking a closer look at the minerals beneath the crop: they may be feeding plants on a timescale that standard tests have overlooked.