Interrelationships between edaphic factors potentially limiting cereal growth on alkaline soils in north-western Victoria

Abstract

Calcarosols of the Victorian southern Mallee comprise subsoils that are typically saline, sodic, and have high concentrations of soluble boron (B), which have the potential to restrict growth of rain-fed grain crops. This paper reports relationships between various soil factors, from 150 soil profiles over a survey area of 3600 km², to determine if field texture, pH_1:5, electrical conductivity (EC_e), and Na⁺ could be used to estimate exchangeable sodium percentage (ESP) and B. Assessment of soil profiles across 5 layers to 1 m (n = 750) showed that exchangeable Na⁺ correlated well with both ESP (r = 0.96) and B (r = 0.88). High correlation also existed between ESP and EC_e (r = 0.71) and between B and pH_1:5 (r = 0.70). Using linear and asymptotic regression functions, ESP overall was defined by: ESP = 1.47 + 2.68 × Na⁺ (r² = 93.9) or ESP = 26.53 – 29.84 × 0.84^ECe (r² = 75.5). Boron was described by: B = –0.34 + 3.93 × Na⁺ (r² = 76.7) or B = 3.2 × 10^–6 × 6.11^pH1:5 (r² = 68.5). Inclusion of multiple explanatory variates, using stepwise multiple regression, did not account for more variation; hence, prediction using several variables simultaneously appeared unnecessary. Rapidly determined Na⁺, by ion-specific electrode, could also accurately predict sodicity: ESP = 1.31 + 0.03 × Na⁺ (r² = 95.1). Soils with a pH_1:5 <8.1 were shown to have B levels not potentially toxic to cereal growth. Soil texture also provided valuable estimation of B. Soils in the sand to clay loam texture range did not have levels of B toxic to cereal growth, unlike soils in the light to heavy clay range that frequently had levels of B potentially toxic to cereal growth.