Estimated reading time: 6 minutes
Potatoes form part of rotation cycles together with grains and pastures, including maize, soya beans and cereals. For this reason, Potatoes SA and Grain SA have joined forces to address soil health together with researchers from the University of Pretoria, the Western Cape Department of Agriculture (WCDA), and the Agricultural Research Council (ARC).
In the quest for more efficient and affordable potato production systems, the spotlight is turning towards soil health and quality. The definition of soil health and quality is complex as it relates to numerous factors associated with the measurement of its physical, biological and chemical properties. Many debates have taken place and research has been conducted into the measurement of soil health. The ultimate outcome, however, is the ability of healthy soil to produce a viable, healthy, good quality potato harvest.
During the previous season, growers encountered potato quality challenges which are believed to have been triggered by environmental impacts rather than primary diseases. These triggers include fluctuating water levels and water quality during tuber initiation. This was probably also aggravated by a weakening of the soil’s biological profile due to the excessive use of broad-spectrum soil-applied crop protection products. These all tend to impact beneficial soil organisms. Pathogens such as Rhizoctonia solani, Streptomyces scabiei and Fusarium spp. manifest through a secondary infection of weakened plants.
What is healthy soil?
The United States Department of Agriculture (USDA) recommends that healthy soils should function as a living ecosystem by sustaining plant and animal life, and providing crucial functions which include regulating water, filtering pollutants, recycling nutrients and offering physical stability to the plant (Figure 1). This provides a base to suppress plant diseases, improve plant health and improve a plant’s ability to recover from stress (Figure 2).
Current research
Two interesting long-term field studies are currently underway in two diverse production areas. The aim is to evaluate the long-term evolution of soil health based on different production systems with the objective of improving pack-out quality and yield. Potatoes SA is currently managing a total of 11 soil health projects (Table 1).
Two of these are long-term projects with a focus on:
- The impact of minimum cultivation systems in the sandy, low-carbon soils of the Sandveld production area.
- Crop rotation systems with summer crops in the dryland production systems.
Cultivation systems
In a nine-year study overseen by Dr Jacques van Zyl of the WCDA in the Sandveld production environment, it was demonstrated that soil biological activity improved because of minimal cultivation practices. The paraplough and rip system was compared to standard, more intensive cultivation practices with a mouldboard plough. This study also looked at rotation systems incorporating rye, triticale (korog) and barley.
The study suggests that cover crops including triticale and rye, along with fodder barley, can be recommended in rotation systems. Additionally, the study shows that soil biological activity varies with growth stages, peaking ten weeks after emergence. The impact of minimal cultivation systems on beneficial free-living nematode counts was also the highest in minimum till and rip treatments. The minimum tillage practices resulted in a slight increase in soil carbon levels.
In the Sandveld, triticale and rye would ultimately be recommended as cover crops along with fodder barley planted in winter, the year before the potato planting. The selection of cover crop mixtures with different rooting depths and growth periods would potentially also be of value in promoting soil quality at different soil depth levels.
Rotation systems
In a long-term, multi-disciplinary field study in the Eastern Free State led by Prof Martin Steyn from the University of Pretoria, long-term crop rotation focused on soil health, physical and chemical conditions in rotation systems typical to the Eastern Free State was studied. The rotation system, which included sunflowers, was found to aggravate the plant parasitic nematodes with Pratylenchus zeae being dominant. Sclerotinia wilt also became an issue, specifically in a sunflower system. Rotation treatments appeared to support biological control agents, particularly those involving maize, soya beans, fallow, and potatoes.
Dr Mariette Marais from the ARC Plant Health and Protection Research Institute observed an evolution of the nematode population. This included an improvement of the free-living species as well as a change in the plant parasitic profile. While nematodes such as Meloidogyne incognita and Meloidogyne javanica are a concern in potato production, the study emphasised the importance of identifying the nematode species in each field to tailor rotation crops, control solutions and cultivar selection.
Although the perception is that only Meloidogyne species are considered a problem in potato production, the succession of plant-feeding nematodes associated with the different crops clearly illustrates the importance of the entire complex of nematodes, as illustrated by the prominence of P. zeae and the presence of large populations of Helicotylenchus dihystera specimens observed inside potato tubers, thus emphasising the concept that plant feeder nematodes are always a complex in the soil and roots of plants.
The focus on soil health in South Africa’s potato industry is not only about protecting potatoes and grain crops; it is about safeguarding the future of sustainable agricultural soil.
The regulations on crop protection remedies are increasing, and traditional crop protection remedies are under pressure. It is becoming critical to integrate control solutions, including the use of biological compounds in innovative farming practices. This needs to be well understood and includes matching the pathogen and pest biology with the potential control remedy.
The perception is often that yield and quality are initially negatively influenced by conservation tillage. This was not observed in the Sandveld and Eastern Free State study. The Sandveld, with its low carbon levels, improved over time when minimum tillage was applied.
It does make sense to consider rotation with different rooting depths and growth periods to promote soil carbon levels. By nurturing soil to improve biological activities, specifically in the root rhizosphere, we are laying the foundation for a healthier, more resilient production system. Conservation farming practices are a viable practice for potato production. Improving soil takes time and patience.
For more information, contact Dirk Uys at dirk@potatoes.co.za.