A Review of the Green Manuring Work Conducted at the South African Sugarcane Research Institute over the Past 80 Years.
Ruth Rhodes, South African Sugarcane Research Institute, 170 Flanders Drive, Mount Edgecombe, South Africa
Farmers have practiced green manuring--growing crops to be returned to the soil as a green mulch, for over two thousand years. This fact alone bears testimony to its value. In the early days of the sugarcane industry in South Africa, inorganic fertilizers were not widely available commercially, and legumes and other green manures were the only means of returning nutrients to the soil. The earliest published discussion of green manuring work in South Africa's sugar industry was presented in 1925. The author underlined the importance of maintaining sufficient organic matter levels in the soil, especially during dry conditions, and stated that the success of any farming enterprise depends on no factor more than the maintenance of soil fertility. Other researchers, too, recognised the decrease in organic matter in sugarcane-growing soils of South Africa, and promoted green manuring to try to combat this decline. Interestingly, researchers as far back as 1925 recognized the importance of green manures to feed azotobacter, free-living N-fixing bacteria. Trials conducted on green manuring in the early days at the Sugar Association in South Africa were rather basic – a few species of crop were grown, and results presented in a subjective manner. Researchers did, however, identify promising varieties for use in our industry, and promoted the practice of green manuring to break the sugarcane monocrop. As manufacturing industries progressed, however, and chemical fertilizers became more easily available and economically viable to buy and apply, so the practice of green manuring gradually declined, and from the 1950s until the 1970s it was practiced very little in our industry. Research at the South African Sugarcane Research Institute (SASRI) in the 1920s focused on green manures and their benefits, but as their use died out, so did the research into this practice. A sugarcane yield decline first identified in the 1970s, however, prompted farmers and researchers alike to question the viability of long-term monocropping. This yield decline, defined as ‘the loss of productive capacity of sugarcane growing soils under long term monoculture', kick-started researchers into once more exploring the benefits of crop rotation and fallow cropping in the sugar industry. Once again, SASRI research mirrored the needs of South Africa's growers, and green manuring once more became an important aspect of our research programme. Since 1998, then, a fair volume of research has been conducted into green manuring and its benefits. Various trials have been carried out to evaluate the effects of leguminous and non-leguminous green manure crops on soil properties and yield response in sugarcane. Trials were planted on various soils with a range of different green manure crops. Researchers hoped, by conducting these trials, to develop a strategy for amelioration of yield decline in sugarcane through the use of green manures. Although researchers experienced periodic setbacks with their trials, such as herbicide drift from neighboring farms, several positive results ensued. Various green manure crops were found to grow very well without fertilizer, and it was found that fertilizer could, in some cases, be reduced in the following cane crop. Interesting results included those which showed a significant increase in cane yield following a green manure crop of lupins (Lupinus angustifolius) which had been taken to seed, and the seed harvested; common knowledge dictates that harvesting green manure seed leads to lower subsequent sugarcane yields than in fields where the crop was not taken to seed. More recently, green manures have been tested for other properties such as weed suppression (by smothering or allelopathy), pest and disease suppression and overall soil health improvement. Current work continues to focus on the best green manure species for various seasons and areas within our sugarcane industry, and the most efficient and practical ways to establish and manage these crops. Soil biology and soil health are receiving increased attention from sugarcane growers and researchers alike, and green manuring forms part of a system of maintaining soil health and biodiversity. Future work at SASRI needs to reflect the changing nature of our clients, the sugarcane growers of South Africa. Land reform and democracy have increased the contribution of small-, medium- and large-scale growers from all backgrounds, and green manures and cover crops need to be studied with resource-poor communities in mind. We need to broaden our thinking in order to best serve the needs of our clients.