Episode 14: Do You Actually Know What pH Means? The Number on Every Soil Test Part 1

Send us Fan Mail You already know your pH. It is on every soil test you have ever paid for. This episode is not rates, products or a lime program. It is what that number is actually measuring. Leonie starts with a question most of us cannot answer cleanly: what does pH stand for? There are two answers in common use; potential hydrogen and power of hydrogen, and both are useful. One tells you which ion is winning in the soil water. The other tells you the score is kept in multiples of ten. A drop from 6 to 5 is not “one point.” It is a tenfold increase in hydrogen ions. Then the mechanism. Agricultural lime is calcium carbonate. The part that does the work is the carbonate: it accepts hydrogen ions and turns them into water and carbon dioxide; the same fizz you get in a field acid test, just slowed down over years. The calcium left behind evicts aluminium from the exchange sites, which is often the bigger prize for roots. On some Swan Coastal Plain soils a single application held for about a decade. pH is the master switch sitting upstream of phosphorus, traces, nitrogen cycling, root depth and water repellence. Next episode: why your number sits where it sits, what buffering is, and how to read an acid result differently from an alkaline one. References are in the episode notes. Thank you to Williams Woolshed in Williams and Naked Bean Coffee Roasters in Albany. **References** 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/ 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. 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. Yeates, J. S., et al. (1984), as cited in McArthur (1991). **Connect with Leonie** Instagram: @agfromthegroundup Website: www.agriculturefromthegroundup.com.au