The potential of alkaline rocks from the Fortaleza volcanic province (Brazil) as natural fertilizers

Abstract

This research evaluates the potential of three alkaline rock types to be used as soil remineralizers by measuring the availability of macronutrients and micronutrients in aqueous media and soils after a three-month incubation period. The lithotypes used were (1) phonolite; (2) nepheline syenite; and (3) volcaniclastic breccia, which are from the metropolitan area of Fortaleza, capital of Ceara´ state, northeast Brazil. All rock types are constituted of alkali feldspar, aegirine-augite, nepheline, kaersutite with minor apatite, and opaque minerals in different proportions. These minerals carry substantial amounts of K, Ca, Mg, P, Fe, Mn, Zn, and Cu, elements that are important for plant nutrition and metabolism. The concentration of the major and trace elements in the rocks was determined to quantify the efficiency of the solubilization relative to the element’s original content in the rock. The agronomic tests were conducted as a completely randomized design, which results were submitted to variance analysis. Several important elements were efficiently solubilized by the leaching experiment with citric acid diluted to 2%, indicating the potential of the studied materials to be effectively used as natural fertilizers. A typical surface soil from the region was incubated at room temperature with finely ground nepheline syenite and volcaniclastic breccia for three months. The amendment was applied at rates of 0, 2, 4, 6, 8, and 10 t ha− 1 to cover situations of moderate to intensive application rates. The incubation results with the treatment using the volcaniclastic breccia indicate a high potential for soil remineralization of P, Fe, and Mn, while the nepheline syenite yields potential for P, K, Mn, Zn, and Cu. The outcomes of this research in laboratory-scale have important implications for fertilizer and soil management and should be tested in the field, including rock blending and long-term experiments to evaluate the effects of soil’s improvement with time.