Crop rotation effects on soil and plant health in a dryland production system in South Africa

Abstract

Soil and plant health are key limiting factors of crop production in conventional tillage systems under rain-fed conditions in South Africa (SA). Historical evidence shows that different crop rotation practices result in soil health legacies that are linked to crop type and functional soil biota. On drylands in the eastern Free State (EFS), crop rotation with conventional tillage is commonplace but overlooks soil and plant health. The objective of this study was to highlight how soil and plant health in a conventional tillage field experiment can be assessed for four crop rotation sequences through a description of spatio-temporal changes in soil nutrient composition, pathogen DNA concentration, and crop yields in EFS dryland systems as well as in greenhouses. A dryland field study on the potato (Solanum tuberosum) and maize (Zea mays L.) uniformity trials in season 2015/16 and season 2016/2017, indicated that field soils were relatively uniform with no abiotic imbalances. The maize uniformity trial in season 2016/17 provided the baseline data for a long-term field experiment investigating soil and plant health under dryland cultivation in the EFS. Greenhouse studies of plant-soil feedbacks (PSFs) using two-phase experiments showed indirect effects with positive plant growth recorded for all crop species tested. PSF results suggest that sugar bean (Phaseolus vulgaris L.) rotations were strongly associated with specific beneficial microbiota. In terms of PSF crop sequences, sugar bean in sunflower (Helianthus annuus L.) rotations offers soil resilience to pathogens associated with sunflower monoculture. Results of pot experiments showed that field-collected soils from Potatoes-Maize-Sunflower-Maize were significantly more suppressive to Fusarium graminearum but significantly more conducive to the negative effects of Rhizoctonia solani AG 2-2 IIIB on test plant metrics. The low severity of root rot disease in broad beans (Vicia faba L.) planted in the field-collected Potatoes-Maize-Maize-Teff soil, indicate that grass-dominated rotation promotes suppressive soil conditions against R. solani AG 2-2 IIIB. Sunflower seems to have the potential of increasing fungal to bacterial PLFA biomass composition ratios, supporting saprophytic microflora and microbial enzyme activities. Furthermore, the inclusion of sunflower, as the third rotation crop, will possibly create legacies of a less compacted subsoil and a sharp-increase in R. solani AG 3-PT population in the following season’s maize crop. The incorporation of maize or teff (Eragrostis tef) in maize sequences in the final season, increased soil inoculum of R. solani AG 3-PT DNA. This suggests that these grasses minimized the resilience of field soils against build-up of R. solani AG 3-PT DNA levels within four years. In the final season (2018/19), the soybean (Glycine max L. (Merr.)) rotation sequence (Potatoes-Maize-Soybean-Maize) significantly reduced R. solani AG 3-PT DNA legacies. There is an urgent need, in the sandy soils of the EFS under rain-fed conditions, to raise soil pH and increase soil organic matter and micronutrients to assist growers to increase crop yields and improve soil health. The accurate detection and quantification of R. solani AG 3-PT would enable farmers to link disease thresholds with the development of effective integrated management plans for dryland crop rotations.