Silicate rock powder: effect on selected chemical properties of a range of soils from Western Australia and on plant growth as assessed in a glasshouse experiment

Abstract

Soil samples were collected from 20 locations from the south western part of Western Australia and incubated at 25 °C for 60 days without or with finely ground granite powder at a rate of 20 g kg⁻¹ soil, equivalent to about 20 t ha⁻¹. Electrical conductivity and exchangeable Na, Ca and Mg were not significantly affected by granite application for most soils. Conversely, among the 20 soils studied, nine exhibited a significant increase in exchangeable K (atp<0.01) due to granite application. Six of them showed a consistent increase in soil pH as measured in a CaCl₂ extract, corresponding to less than 0.26 pH units. The concomitant increase in exchangeable K due to granite application ranged between 10 and 390%. However, in absolute value it amounted to less than 0.07 cmol K kg⁻¹ soil, suggesting that a maximum of 59 g kg⁻¹ of the applied granite dissolved during the course of this incubation experiment. One of the most granite responsive soils was used for a pot experiment conducted with wheat grown for 88 days in a glasshouse. In this experiment, the soil was either untreated (control) or mixed with either granite or diorite powders at six different rates of application. The wheat biomass and cation contents in plant tissue were not significantly affected by the application of diorite at any rate of application. Conversely, for the granite-treated soil a significant increase in wheat biomass was encountered for rates larger than 2.5 g kg⁻¹ soil. Since a significant increase in K content was obtained at the same rates of application it was concluded that the positive response of wheat growth to granite application was due to potassium supplied by granite dissolution. The use of granite powder as a potential K fertilizer thus needs further attention even though its efficiency as compared to a soluble fertilizer would almost certainly be poor.