Improved Procedures for Adjusting Soil pH for Pot Experiments

Abstract

Liming is an effective method of correcting soil acidity, but the cost and other factors, such as sub-soil acidity, topography, and nutrient availability may limit its benefits. An alternative option is to grow acid tolerant plants. Pot experiments are an effective way of identifying plants with the genetic potential to grow in acidic soils. These experiments require a range of acidity levels. The soil used in this study was a sandy loam. Increments of aluminum sulfate [Al₂(SO₄)₃.18H₂O] and calcium carbonate (CaCO₃) were added to derive dose-pH response relationships. The derived relationships were used to estimate the quantity of aluminum sulfate and lime required to change the pH_Ca (1:5, w:v, soil:10 mM CaCl₂) of the soil from 4.4 to 3.89 and 4.4 to 5.2, respectively. The estimated quantities of aluminum sulfate (380 g) and calcium carbonate (48 g) were thoroughly mixed through 100 kg batches of the soil, which was potted (2 kg/pot). Then the acidified soils were leached to remove excess salts. The initial pH_Ca achieved was 3.86 and rose to 3.94 after leaching. A wide range of soluble aluminum (Al) concentrations (1:5, w:v, soil:10 mM CaCl₂) was obtained from this study, e.g., 52 mg Al/kg soil in acidified soil and 1.8 mg Al/kg soil in limed soil. The variation in pH and EC between replicate pots was negligible (range 0.05 pH and 0.02 dS/m) and these remained relatively unchanged after six months. During the next six months some pots were held in the laboratory with 3 wetting/drying cycles; however, most were placed outdoors where they were sown with seed of Austrodanthonia spp. During this time, the pH_Ca of the acidified soils increased slightly to: 4.10 (laboratory), 4.18 (outdoors without plants), and 4.20 (outdoors with plants). A similar increasing trend was also observed in limed soils. It was concluded that adding aluminum sulfate and calcium carbonate followed by washing with water are quick and convenient methods for adjusting soil pH for pot experiments.

Keywords:

• pH adjustment
• Aluminum sulfate
• Calcium carbonate

Acknowledgments

The authors thank: Dr. Mark Conyers for valuable preliminary discussion; Dr. Lindsay Campbell for assistance in preparing the manuscript; Jenni Tarleton for valuable assistance in organizing the figures; Dr. Remy Van de Ven and Ms. Helen Nicol for statistical advice; Ian McKenzie for access to “Mobla Station” Binnaway, NSW to collect soil; and, Dr. Yohannes Alemseged, Sue Betts, Geoff Millar, David Pickering, Noel Preston, Sue Priest, Jenni Tarleton, Geoff Wilson, and Jim Zeylemaker for assistance in preparing and potting the soil. The research was funded by Acid Soil Action and NSW Agriculture.