Feasibility assessment of glacial rock flour as a suitable soil conditioner for agricultural applications

Nutrient depletion from cropland soils due to constant cropping leads to plant nutrient deficiency and increased abiotic stress susceptibility, resulting in reduced yields and profitability. Natural soil amendment alternatives to chemical fertilisers, including rock flours, offer a potentially cost-effective crop nutrient supplement. Glacial rock flour is naturally produced by the sheering force of glacial ice sheets milling the surface of bed rock ? producing a fine rock powder. For the first time, this study performs a comprehensive analysis of the physicochemical properties of four different rock flours: three glacial rock flours and one volcanic rock flour product. Results showed that 1% rock flour (RF) soil amendment reduced pH, and generally reduced nutrient availability. Next generation sequencing of 16S rRNA in RFs showed a low background level of bacteria. The RF effects on the growth of faba bean and soybean plants, associated interactions with symbiotic rhizobia, and effects on plant nutrient uptake were assessed. Compared to untreated controls, for faba bean plants the addition of 1% of each RF individually had a positive effect on shoot biomass (particularly for inoculated plants), however, significantly reduced (p<0.05) root biomass. For soybean, the addition of 1% RF significantly (p<0.05) increases shoot and root biomass by 10 to 90% above untreated controls. The similarity between uninoculated and inoculated plants indicates the RFs only had marginal favourable impacts on plant growth, however, didn?t negatively impact the plant-microbe interaction. Using inductively coupled plasma mass spectrometry, the plant elemental levels after treatment with the rock flours were assessed, showing the RFs altered plant nutrient uptake. Overall, the outcomes of this work suggest the RFs have mixed efficacy on cropped legumes and may not have the capacity to replace chemical fertiliser in agriculture but rather could offer a buffer capacity to enhance their efficiency.