Effect of phosphogypsum application on aluminum speciation in acid pasture soils

Purpose: Legume establishment and persistence in New Zealand hill and high-country soils are largely limited by high soil acidity and associated aluminum (Al) toxicity. The present study aimed to evaluate the effect of four rates of phosphogypsum (0, 1, 3, and 9 t ha¯¹) on Al speciation in the soil solution and to examine which species are mostly impacting total dry matter (TDM) yield of lucerne. Methods: Glasshouse and incubation experiments were conducted using three acid soils with different exchangeable Al concentrations: Molesworth, Glenmore, and Lindis Peaks. The distribution of Al species was modeled using visual Minteq. Partial least square (PLS) regression was used to evaluate the relationships between Al³⁺ and other variables in the soil-soil solution system. Results: In the planted and incubated Molesworth soils, Al³⁺ and hydroxylated Al (Al–OH) fractions decreased significantly at 1 and 3 t of phosphogypsum ha−1 compared to 0 t ha¯¹. However, in the planted Glenmore and incubated Lindis Peaks soils, these two fractions remained unchanged. The contribution of variables in Al³⁺ concentration depended on the soil type. However, the loading plot of the whole soil data set (n = 62) showed that Al–OH, base saturation, soil/soil solution pHw, and exchangeable Al were the main explanatory variables for the variation in Al³⁺ concentration. The TDM yield of lucerne was better explained by Al³⁺ , Al-F, and Al-DOM than exchangeable Al. Conclusions: Reasonable amounts of phosphogypsum (1 to 3 t ha¯¹) can help to alleviate Al toxicity in acid soils (pH ≤ 5.3), but higher application rates should be avoided