Legume cover crops ameliorate soil acidity and enhance nutrient availability in South African sugarcane fields

Sugarcane is a drought-sensitive crop that is particularly vulnerable during the vegetative growth stage under rainfed conditions. The study aimed to investigate how climate influences soil nutrients, pH, and exchangeable acidity (EA) in South African sugarcane plantations, and evaluated soil responses to selected legume species. We evaluated six legume species, namely, Arachis hypogaea, Cajanus cajan, Mucuna pruriens, Vicia sativa, Vicia villosa, and Vigna unguiculata. Random forests (RF) were used to capture the relationships between soil nutrients and site, total precipitation (TP) and elevation. Random Forest models revealed strong predictive power for primary (P, N, K) and secondary (Ca, Mg) nutrients, with site, TP, and elevation as key predictors. EA was well predicted (90.4%), while pH showed lower predictability of 30.7%. Site, precipitation and elevation consistently emerged as dominant drivers of soil variability. Mixed-effects models showed that legumes reduced soil N, P, and K, with Vigna unguiculata having the strongest effect on N and P, and Arachis hypogaea on K. In contrast, soil Ca and Mg were increased by four of the six legumes, notably Vicia villosa and Vicia sativa. An inverse relationship between pH and EA was observed, with EA being 67% lower in KwaZulu-Natal Coastal Belt Grassland than in Maputaland Coastal Belt. Exchangeable acidity was reduced in legume-planted soils compared to pre-treatment soils (0.227 cmol/L), with decreases of 51.0% in Vicia sativa and 51.1% in Vicia villosa. Soil pH increased from 4.94 before planting soils to 5.22 to 6.24 under legumes, corresponding to a 6 to 26% increase, with the largest increase observed under Mucuna pruriens (26%). Based on soil responses and climate resilience, we recommend Vicia sativa as a promising legume to enhance soil fertility and pH under changing climatic conditions.