Powdered granite is not an effective fertilizer for clover and wheat in sandy soils from Western Australia

Granite (silicate) rock dust, a by-product of quarry operations, is being a dvocated and used as a fertilizer in the wheatbelt of south-western Austral ia (WA). The dust is insoluble and based on its nutrient element content (1 .9% K and 0.3%P and negligible N) it is not expected to be a useful fertili zer. Previous laboratory studies and glasshouse experiments in WA suggest t he dust is a slow release K fertilizer. This paper extends the previous stu dies to consider the dust as an NP or K fertilizer in the year of applicati on in a field experiment on a soil deficient in N, P and K. In addition, th e effectiveness of the dust as a K fertilizer was compared with the effecti veness of KCl (muriate of potash), the K fertilizer used in WA at present, in glasshouse experiments using K deficient soils. In the field experiment, compared with NP fertilizer or NPK fertilizer (urea, supplying N; superpho sphate, providing P, S, Ca, Cu, Zn and Mo; KCl providing K), the dust had n o effect on grain yield of wheat ( Triticum aestivum); in fact dust applied at 20 t ha(-1), for unknown reasons, reduced yields by about 65% compared to the nil (no fertilizer, no dust) treatment. Relative to the nil treatmen t, applying NPK fertilizer increased yields about threefold, from 0.54 to 1 .79 t ha. The glasshouse experiments showed that, relative to KCl, the dust was from about 0.02 to 14% as effective in K deficient grey sandy soils fo r producing dried tops of 30-day old wheat plants or 42-day old clover (Tri folium subterraneum) plants. In soils with adequate K (yellow sands, sandy loams or clays, loamy clays, clay loams and clays), neither KCl nor the dus t affected yields of 30 to 42-day old wheat or clover plants grown in the g lasshouse. In the glasshouse experiments, no yield depressions were measure d for the dust applied up to 17 g dust per kg soil (equivalent to 17 t dust ha(-1) mixed into the top 10 cm of soil in the field). It is concluded tha t the dust has no value as a fertilizer.