Agrominerals (also called stone bread or petrol fertilizer) are naturally occurring minerals and rocks used in agriculture and horticulture to supply essential plant nutrients or improve soil physical conditions. Unlike soluble fertilizers, agrominerals are silicate, carbonate, phosphate and other geomaterials that release elements slowly through weathering and dissolution, placing them at the intersection of sustainable agriculture and enhanced-weathering research.
The term agromineral was coined in the 19th century. Early use was small-scale and horticultural, relying on perlite for aeration, pumice to limit evaporation, and vermiculite or zeolite to hold moisture. During the 20th century the practice expanded to commercial farms covering hundreds of acres, and the research focus shifted from soil physical amendment to the three major plant nutrients nitrogen, phosphorus and potassium. Two of these, potassium and phosphorus, are still obtained at scale mainly from potash, a single geomaterial whose supply is finite and whose price has risen as demand grows.
Using agrominerals begins with crushing rock into a rock powder and applying the powder to replenish soil nutrients. The process is called soil remineralization. While investigating alternative nutrient sources, researchers found that rock powders may also mitigate climate change, improve water retention and support soil management, because silicate weathering consumes atmospheric carbon dioxide and releases cations into the soil.
The systematic study of agrominerals is called agrogeology. Agrogeologists are concerned with restoring soil fertility in regions where agrominerals have been depleted by unsustainable farming. Modern practice often pursues high crop production at the cost of soil quality, and the loss of topsoil and nutrients outpaces natural replenishment. Nutrient depletion in turn drives increased chemical fertilizer use. Chemical fertilizers have been shown to produce runoff, contaminate groundwater and remain economically unaffordable for many farmers in developing countries.
One of the main raw materials for chemical fertilizers is potash ore. Potash supply is finite and becoming more expensive, and potassium, although the fourth most abundant element in the Earth's crust, has only this one major source at scale. Finding alternative sources for potassium and phosphorus therefore became the starting point of agromineral research, with the aim of remediating soil and raising productivity at low cost.
Early agromineral practice recreated soil conditions for exotic plants on a small scale, using perlite to enhance aeration, pumice to control evaporation, and vermiculite and zeolite to store moisture. This soil modification was the origin of the agromineral concept and evolved into a search for alternative sources of the three major nutrient elements. Remineralization was the term created for incorporating rock powders into soils as a nutrient source, and the practice has since been applied at larger scale with reported success in Brazil, Germany, Norway, South Africa, Sri Lanka and Uganda.
The original agromineral is potash, which remains the main source of potassium and phosphorus in chemical fertilizers. Because demand is high, potash ore is in low supply and its market value has increased, making alternative sources economically attractive. One popular alternative is to spread rock powders in fields so that minerals dissolve slowly and release nutrients.
A major research area compares feldspars and feldspathoids. One study showed that the feldspathoid nepheline had a much higher dissolution rate than potassium-rich granitic rocks, making it a more efficient potassium source under comparable conditions. Basalt, dacite, olivine and phosphate rock have also been evaluated, each with its own balance of nutrient content, dissolution rate and by-products.
Rock selection depends not only on nutrient content but also on mineralogy and bulk chemistry, because a rock may contain elements toxic to plants or to the people who eat them. Volcanic rocks were once considered a good source of rock powder, but some were later found to contain toxic elements. Phosphate rocks are nutrient-rich but may carry heavy metals and radioactive elements. Promotion of rock powders therefore requires thorough chemical characterization of the source rock.
The effect of rock powder also depends on the crop cycle. Basalt, for example, was effective for long-cycle crops but less effective than chemical fertilizer for short-cycle crops, and some rock powders take one to five years to show results. Once a suitable rock source is identified, the limiting factor becomes grinding. Grinding a mineral such as olivine to the one-micrometre particle size needed for effective action in soil consumes about 1.5 gigajoules per tonne of rock. Reducing comminution energy is therefore a key engineering problem for making rock powder a sustainable nutrient option.
Remineralization incorporates rock powder into soil to replenish nutrients removed by repeated cropping. Unlike soluble salt fertilizers, nutrients in rock powder are held in crystal lattices. Plants absorb only what they need, and unused nutrients remain in the mineral structure until conditions favor release, so they are less prone to leaching. Chemical fertilizers deliver nutrients as soluble salts, and the fraction not taken up by crops readily enters groundwater.
Rock powder has a cost advantage because rock is already a cheap by-product in many industries, creating the potential for large low-cost supply. Remineralization with rock powder can provide up to five years of nutrients in a soil. When combined with organic fertilizer, rock powder has been shown to be as effective as chemical fertilizer at much lower cost, and some long-term crops have produced higher yields. Plants grown with rock powder often appear healthier, and the powder helps maintain soil moisture. The effectiveness of remineralization depends on the mineralogy and chemistry of the powder as well as on soil texture, pH, organic matter and cation exchange capacity.
The central challenge is understanding dissolution rates. Rates depend on organic matter, pH and secondary clay precipitation, and dissolution kinetics are not fully understood. Laboratory and field conditions differ greatly, and laboratory rates are often two to five orders of magnitude higher than field rates, which makes direct extrapolation difficult. Building reliable field-scale dissolution models is a major research direction.
Remineralization with rock powder may also help mitigate global warming. When nutrients from certain silicate powders are taken up by plants, cations are released into the soil solution and react with carbon dioxide to form carbonate minerals, fixing carbon in a stable solid phase and creating a carbon sink in the carbon cycle. This mechanism is consistent with enhanced weathering, in which accelerated silicate weathering converts atmospheric carbon dioxide into dissolved inorganic carbon or carbonate.
Researchers are therefore assessing rock powder for combined agronomic and carbon-removal benefits, and attempting to quantify carbon sequestration under different rock types, grain sizes and application rates. Rock powder is thus seen as one of the few agricultural interventions that may simultaneously improve soil health, reduce fertilizer dependence and remove atmospheric carbon, although large-scale deployment still faces questions about dissolution rates, heavy-metal risk, grinding energy and long-term ecological effects.
Brazil was an early large-scale site of rock-powder remineralization. Basalt and gabbro powders from quarry by-products were applied to acidic tropical soils, with reported improvements in nutrient supply and crop yields. Germany and Norway have focused on the carbon-sequestration potential of silicate rock weathering, while South Africa, Sri Lanka and Uganda have used agrominerals for soil restoration and nutrient substitution in smallholder systems. These cases show that results depend strongly on the combination of source rock, soil and crop, and cannot be transferred directly between regions.
Adoption faces institutional and economic barriers, including fertilizer subsidies, mining and grinding costs, transport distance, application machinery and farmer awareness. Researchers therefore argue that agrominerals should be integrated into regional nutrient cycling and soil-health policy rather than treated as a single substitute for chemical fertilizer.
Agrominerals are intended to complement rather than completely replace chemical fertilizers. Fast-acting nutrients remain important during critical growth stages, whereas rock powders are better suited to building base fertility, creating long-term nutrient reserves and improving soil structure. Combining the two according to soil diagnosis can maintain yields while reducing nutrient runoff and groundwater contamination.
From a life-cycle perspective, the main environmental costs of rock powder come from mining, grinding and transport. Using quarry by-products or local rock and low-energy comminution can give a carbon footprint well below that of chemical fertilizer supplying the same nutrients. Conversely, grinding to excessively fine particles may consume enough energy to offset the environmental benefit. The sustainability of agrominerals therefore depends on whole-chain energy and logistics optimization, not only on field performance.
Current work focuses on four areas: dissolution-kinetics models that incorporate mineral composition, particle-size distribution, soil pH and organic-acid concentration, screening of source rocks with low heavy-metal and radionuclide contents and nutrient-release profiles matched to crop demand, low-energy comminution and classification technologies, and quantification of enhanced-weathering carbon removal with verifiable carbon-credit methodologies. As these problems are resolved, agrominerals may move from a regional soil-amendment practice to a component of global nutrient management and negative-emission technology.