Factors associated with a healthy soil in sugarcane production in Kwazulu Natal.

LONG-TERM mono-cropping of sugarcane can lead to the degradation of soils and a decline in yield. In Australia, yield decline has been linked to changes in the population structure of soil micro-organisms, in particular to the build up of pathogenic fungi such as Rhizoctonia. While a lack of moisture is a major growth-limiting factor in many of the poorer soils under rain-fed conditions, this does not explain the overall reduction in yields and loss of sugarcane ratoons that have been reported in South Africa, even for good soils under irrigated conditions. It was therefore decided to re-examine the wide range of soil factors that could impact upon cane growth. Information from this project will be used to rate the health of soils in the South African sugarcane industry, in order to advise growers on suitable agricultural practices to maintain or improve the health of their soils and, thereby, to reverse the impact of long-term sugarcane yield decline. A field situated near Glenside in the Natal Midlands, known to be highly variable in cane yield, was identified and a 100-point grid was established across the field. The ten best and ten worst yielding grid points were identified, from which yields were determined. Soil from these high and low yielding grid points was sampled and analysed with the goal of correlating yield with soil health factors. Analyses included a range of soil physical, chemical and biological parameters. Biological factors that were positively associated with higher yielding grid points in this study included high levels of microbial respiration. High counts of the nematode, Xiphinema elongatum, were associated with lower yielding grid points. Aggregate stability (in the range 0.5 to 1.0 mm) was one of the physical factors that was associated with high yields. Soil chemical characteristics that were associated with high yielding plots included higher average pH values, lower Al and Na levels and higher P levels. Introduction Repeated mono-cropping of sugarcane can lead to the degradation of soils and a decline in cane yields over time (Garside et al., 1997). Chemical and physical (abiotic) factors in the soil fulfil an important role in soil health. A literature review has revealed an upsurge in research dealing with the role of soil micro-organisms in the soil food web (van Antwerpen et al., 2005). The soil food web is a potential indicator of soil health (Roper and Ophel-Keller, 1997). Soil microbial measurements have not received much attention in the South African sugar industry other than the pioneering studies of Anderson (1959) and Roth (1965, 1971). In Australia, yield decline