Episode 15: Your pH Is a Rainfall History. Why the Number Sits Where It Sits Part 2

Send us Fan Mail Last episode we unpacked what pH actually is. This one asks the better question: why is *your* number the number it is? Drive east out of Perth and the pH climbs as the rainfall drops. That is not a coincidence. Leonie walks through the four drivers of soil pH, parent material, rainfall, time, and management, and why only the last one is in your hands. Ammonium fertiliser and every truckload of produce leaving the farm are both acid programs, whether you meant them to be or not. Then the useful half. Buffering is why a clay shrugs off the same fertiliser program that moves a pale sand a full unit. The nutrient chart is shaped like an hourglass because phosphorus gets locked by iron and aluminium on the acid side and by calcium on the alkaline side. A water pH and a calcium chloride pH are not the same number;  they can differ by nearly a full unit. And an acid result and an alkaline result point you at completely different problems. If the soil is acid: test deeper, look for aluminium, expect potassium to be walking out. If it is alkaline: check traces, check sodicity, check salt, and stop buying lime. References are in the episode notes. Thank you to Williams Woolshed in Williams and Naked Bean Coffee Roasters in Albany. **References** Brennan, R. F., Bolland, M. D. A., & Ramm, R. D. (2013). Changes in chemical properties of sandy duplex soils in 11 paddocks over 21 years in the low rainfall cropping zone of southwestern Australia. *Communications in Soil Science and Plant Analysis, 44*(12), 1885–1908. Davies, S., Fisher, J., Roper, M., & Murphy, D. (2022). *Soil quality: 7. Soil water repellence*. SoilsWest / Department of Primary Industries and Regional Development. Gazey, C., Azam, G., Clausen, J., & Rengel, Z. (2019). *Soil quality: 4. Soil acidity*. SoilsWest. https://library.dpird.wa.gov.au/sq\_ebooks/4/ Hoyle, F. C., & Murphy, D. V. (Eds.). (2017). *Soil quality: 1. Constraints to plant production*. SoilsWest. McArthur, W. M. (Comp.). (1991). *Reference soils of south-western Australia*. Australian Society of Soil Science Inc. (WA Branch). McKenzie, N., Jacquier, D., Isbell, R., & Brown, K. (2004). *Australian soils and landscapes: An illustrated compendium*. CSIRO Publishing. Murphy, B. W. (2015). Impact of soil organic matter on soil properties: A review with emphasis on Australian soils. *Soil Research, 53*(6), 605–635. Norton, R. (2020). Potassium and sulphur: Emerging deficiencies in the southern region. GRDC Update Papers. Stuart-Street, A., Short, N., Galloway, P., & Schoknecht, N. (2025). *A simple guide for describing soils* (2nd ed.). Department of Primary Industries and Regional Development. Tang, C., Rengel, Z., Diatloff, E., & Gazey, C. (2003). Responses of wheat and barley to liming on a sandy soil with subsoil acidity. *Field Crops Research, 80*, 235–244. Yeates, J. S., et al. (1984), as cited in McArthur (1991). **Connect with Leonie** Instagram: @agfromthegroundup Website: www.agriculturefromthegroundup.com.au