The interactions between biotic and abiotic factors that influence the sustainability of tomato production in South Africa

Abstract

Tomato production was an important economic activity in the Limpopo Province of South Africa. A clear tomato yield gap existed between South Africa and the other countries in Southern Africa. Understanding the reasons behind tomato crop failures and successes in South Africa could increase tomato production in the fast-growing tomato markets of Angola, Mozambique and Zimbabwe, thereby improving food and nutrition security for smallholders and the population in general. In this study, the i) economics of tomato production in South Africa was investigated and compared to similar production systems in the USA, Turkey and India, ii) the interactions between biotic and abiotic factors that limited tomato yield and quality within three climatically distinct planting windows in the Limpopo Province of South Africa were examined, and iii) the correlations between three commercially available soil health metrics (i.e., a microscope-based method for estimating the biomass of soil bacteria, fungi and protozoans; nematode community profiling based on counts and trophic group classifications and related indices; polyphasic soil health testing based on soil biological, physical and chemical variables) and tomato yield were assessed. Meta-analysis was used to explore yield variation in open field production systems in the international context. The main yield-limiting factors were identified as planting times, planting density, soil-water relations, and synthetic/organic nitrogen fertilization. The focus of the study shifted to commercial tomato production in the Limpopo Province of South Africa. Since 2003, these tomato producers practiced intensive open field tomato production using a combination of synthetic and organic soil, crop and pest management technologies. A review of tomato production economics revealed that within a period of six years, South African tomato production cost per hectare more than doubled but the profit margin halved. The importance of tomato quality as an economic factor was demonstrated. Economic pressures forced these tomato producers to intensify production, which underscored the need for the continued development of sustainable tomato production systems. To achieve this strategic goal, the primary biotic and abiotic factors that limited tomato production were identified. The results indicated that complex interactions between biotic and abiotic factors explained yield and quality variation. Climate variation dominated crop productivity, especially in unsuitable planting windows. Soil and crop management variables, notably synthetic fertilizer and pesticide usage, ensured high quality yield. Soil biology management was an important aspect of sustainable agriculture and the use of appropriate soil biology metrics facilitated soil biology management at field scale. All three soil biology metrics were sensitive to distinguish between three types of disturbed soils commonly encountered in the open field tomato production context: natural, disturbed and cultivated soils. The microscope-based method used for quantifying bacterial, fungal and protozoan biomass and numbers was unsuitable for explaining yield variation. Nematode community profiling, in conjunction with polyphasic soil health testing, was very useful for explaining yield variation. In particular, soil pH, boron, aggregate stability, Paratrichodorus spp. and the balance among soil cations (especially exchangeable K and Mg) explained yield variation. In conclusion, sustainable open field tomato production depends on the integrated use of synthetic and organic crop nutrition and protection technologies, optimum planting times, disease-resistant genetic material, and cultivation on healthy soils. The findings of this study will benefit policy development in support of sustainable vegetable production in the rural areas of Southern Africa.