Low-grade silicate minerals as value-added natural potash fertilizer in deeply weathered tropical soil

Abstract

High price of chemical potassium (K) fertilizers together with low K use efficiency in weathered tropical soils necessitates the exploration of alternative K fertilizer sources. Effectiveness of low-grade silicate mineral powders (SMP) as a natural K fertilizer in a highly weathered tropical soil was evaluated in this study via laboratory and pot experiments by growing oat (Avena sativa L) and palmarosa (Cymbopogon martini var. motia). Five SMP materials (two mica, one feldspar and two greensand) were obtained from quarry mining sites across India. The total K content in the SMP followed the order: feldspar (12.3%) > waste mica (5.98–8.03%) > greensand (3.77–4.46%). The amount of K released from SMP by chemical extractants was in the following order: water < 0.01 M citric acid < 1 M ammonium acetate < 1 M nitric acid, which indicated that SMP was a slow-release K source. Of the five SMP materials, one waste mica, the feldspar and one greensand were included in the plant growth experiments based on their high K contents. Water and 0.01 M citric acid extractable K from SMP showed the best correlation with K uptake by plants. Compared to feldspar (19.7–22.5%) and greensand (5.4–7.5%), the K recovery from waste mica (39.3–42.5%) was notably higher in both palmarosa and oat. Soil K pool was significantly recharged with waste mica application as evident from higher exchangeable and non–exchangeable K in soil than chemical fertilizer treatment. Application of SMP, particularly waste mica and feldspar, also increased the soil pH and cation exchange capacity by releasing basic cations and improving their retention in the tropical soil. By using low-grade SMP in agriculture, it is thus possible to achieve green recycling of mining wastes as K fertilizer while improving crop production and amending poor quality soils.