Final report SRDC Project CSR024 Improving the environment for sugarcane growth through the amelioration of soil acidity

Abstract

Most soils used for growing sugarcane in wet tropical northern Queensland are highly acidic. Comparisons between new cane land and land that has been growing sugarcane for many years have demonstrated that our soils have become degraded under continuous sugarcane monoculture and that many of the changes in soil chemical properties are associated with soil acidification. Continued acidification, due to heavy applications of nitrogen fertilizer and the removal of base cations in the cane sent to the mill, will not only further acidify surface soils but will also progressively acidify the lower parts of the soil profile, making amelioration difficult and costly. Low soil pH not only reduces the availability of some nutrients to plants but also reduces soil surface charge resulting in a permanent reduction in the capacity of the soil to hold nutrients. Since many soils in the wet tropics already have a low cation exchange capacity, further reductions in cation exchange capacity (CEC) due to accelerated acidification may lead to sub-optimal levels of exchangeable calcium, magnesium and potassium, which will have a direct impact on sugarcane yields. Current industry recommendations for applying lime are based on perceived economic crop responses to calcium and are based only on the level of soil exchangeable calcium in the surface layer. Whilst this philosophy may be appropriate for soils with very low cation exchange capacities and suboptimal levels of exchangeable calcium, where frequent lime applications would be required to maintain soil calcium levels, it does not offer a sustainable management solution for highly acidic soils with adequate exchangeable calcium levels. Over 85% of cane growing soils in the Herbert River District fall into this category, having exchangeable calcium levels above the critical level and yet having an average soil pH of less than 5. This project aims to enhance the sustainability of the sugar industry by investigating and developing strategies for ameliorating soil acidity and thus making soils more amenable not only for sugarcane production but also for leguminous fallow crops which are now considered to be an important part of a sustainable sugarcane production system. Replicated experimental trials involving five rates of lime and three rates of gypsum were established on farms in the Herbert River District with contrasting soils that were highly acidic but had exchangeable calcium above the critical level. A fourth trial site was included later in the project with very low exchangeable calcium levels. Cane yields and ccs were monitored and soil samples taken from different depths in selected treatments in each trial were analysed in order to monitor changes in soil chemical properties.