Potassium in Calcareous Soils and the Chemical Extractants Used to Evaluate It: A Critical Narrative Review of Availability Mechanisms and Soil-Test Performance

Abstract

Calcareous soils occupy a substantial share of the cultivated land of arid and semi-arid regions, and their carbonate-buffered, calcium-dominated chemistry alters almost every process that governs potassium supply to crops. Soil-test procedures used to advise potassium fertilisation in these soils were largely inherited from temperate, carbonate-free systems, and their transfer has never been fully reconciled with the mineralogical and chemical peculiarities of carbonate-rich media. This review examines what the accessible literature establishes about potassium behaviour in calcareous soils and how reliably the principal chemical extractants represent plant-available potassium in that setting. Evidence was assembled from open scholarly indexes and appraised for design adequacy, calibration quality and interpretive transparency rather than for citation frequency alone. Four themes emerge. Mineral inheritance, in particular the abundance and particle size of micaceous minerals, exerts stronger control over potassium supply than does exchangeable potassium alone, and interlayer release is kinetically slow yet agronomically substantial. Neutral ammonium acetate, acidic multi-nutrient reagents and dilute salt solutions frequently correlate closely with one another in calcareous soils, and this mutual agreement has repeatedly been mistaken for agronomic validity. Extractants that engage the non-exchangeable pool, including boiling nitric acid, sodium tetraphenylboron, cation-exchange resins and organic acid mixtures, generally track plant uptake more closely in soils with appreciable interlayer reserves, although the advantage is inconsistent and depends on clay mineralogy. Carbonate dissolution, sample drying, extraction ratio and analytical finish introduce method-dependent bias that is rarely quantified or reported. Confidence in current critical limits is limited by the scarcity of multi-season field calibration in genuinely calcareous soils, by the predominance of short glasshouse studies, and by narrow geographical representation. Priorities include mineralogically stratified field calibration, standardised reporting of extraction conditions, and evaluation of kinetic and diffusion-based indices against measured crop uptake rather than against other extractants.