The effect of 72 years of sugarcane residues and fertilizer management on soil physico-chemical properties

• Long-term residue and fertilizer management has not affected soil carbon content.
• Mulching has not increased aggregate stability compared to residue removal.
• Long-term fertilizer application significantly decreased soil aggregate stability.
• Soil aggregate stability is controlled by base cations and not carbon.

This study, carried-out in KwaZulu-Natal, South Africa, investigated changes in selected soil properties and their effect on aggregation induced by 72 years of residue burning or mulching, with and without fertilizer application on a sugarcane trial arranged in a split-plot design with four replications. The main plot treatments were a) green cane harvesting with all residues mulched, b) cane burnt prior to harvest with cane-tops left scattered evenly over the plots and c) cane burnt prior to harvest with all the residues removed from the plots. Split-plot treatments consisted of fertilized and unfertilized plots. Soil samples for physico-chemical and aggregate stability analysis were collected at depths of 0–10 and 10–20 cm from 24 plots. In comparison with burning, significant effects of mulching were only observed on total nitrogen and exchangeable potassium and sodium, mainly at 0–10 cm. Aggregate stability estimated by mean weight diameter (MWD), exchangeable cations (especially calcium and magnesium) and pH were significantly affected by fertilizer application. An increase in acidity and a decrease in MWD and exchangeable calcium and magnesium on fertilized plots were attributed to mining of nutrients by sugarcane, nitrification and subsequent base cation leaching. The significant positive correlation between calcium and magnesium and MWD, and the lack of correlation between organic carbon (OC) and MWD, indicated that bases contributed more to soil aggregation than OC. Total carbon and OC showed no differences across all treatments. It was concluded that (i) annual fertilizer applications may lead to soil structure deterioration under sugarcane regardless of the harvesting method practiced and (ii) increasing additions of organic matter (through mulching) do not always correspond to an improvement of soil aggregate stability and related soil properties.