Technical Yearbook 2023

and high demand for water, plants may exhibit a wilted appearance despite there being enough moisture available in the soil. 26 The presence of high levels of sodium (Na) and the resultant high Na exchangeable percentage leads to a sodic soil. Sodic soils have a collapsed soil structure which results in blocked pore spaces and limited movement of water through the soil profile. The resultant anoxic conditions created are similar to those created by compaction and waterlogging. The resultant effect on roots (see physical soil health above) is the same. The key to managing nutritional soil health is soil and crop nutrition monitoring through sampling and testing. Avoid over-fertilising by basing fertiliser regimes on regular soil and crop testing. Acidity can be corrected through the use of lime and or gypsum. Gypsum moves more easily through the soil profile and is usually used to address subsoil acidity. Gypsum can also assist with sodic soils. Soil acidity problems are best addressed at planting and regular monitoring ensures that increasing acidity levels are detected early. Regular water quality monitoring can prevent creating of high salt levels in the soil by detecting and addressing the high salt levels in irrigation water. Much like soil acidity, regular soil monitoring will detect increasing salt levels early. Salt accumulation in soils can be addressed through drainage and flushing of soil with excessive water. 26 Biological soil health Biological soil health, as indicated in the name, refers to the living component of the soil. The concept of biological soil health is based on the presence of a soil food web (figure 1). The soil food web consists of varying trophic levels at which different organisms play a critical role. The start of the soil food web is the first trophic level which consists of the producers. These are the plants that are present. Within agriculture, this is generally the crop that is grown, as well as any weeds, grasses or cover crops that are being grown. Other organisms within this level include moss, lichens, photosynthetic bacteria and algae. These organisms are identified by their ability to produce their own food by using the sun’s energy and CO 2 . The next critical component of the soil food web and a requirement for a functioning soil food web is organic matter. Organic matter within the soil is made up of waste, residue and metabolites from plants, animals and microbes. It is usually made up of approximately equal parts of humus and active organic matter. Microorganisms feed on the active fraction of the organic matter. The humus fraction consists of complex organic compounds left behind after decomposition. Not many organisms can decompose this material, so it is referred to as the stable or permanent organic matter fraction. 11 The second trophic level consists of bacteria, fungi and root-feeding nematodes. The bacteria and fungi within this trophic level can either be beneficial or parasitic bacteria and fungi. The abundance of either bacteria or fungi is

dependent on the type of food source available. Bacteria feed on simpler compounds such as root exudates and fresh easy-to-digest plant tissue. Fungi, on the other hand, feed on more complex plant tissue such as fibrous plant tissue, wood and soil humus. The third trophic level includes arthropods, protozoa, and bacterial and fungal-feeding nematodes. These organisms feed on bacteria, fungi and plant parasitic nematodes. The fourth and fifth trophic levels consist of higher-level predators that feed on the lower levels. These levels include omnivorous and predatory nematodes. All of the organisms within the soil food web recycle nutrients within the soil and make them available to plants, through their feeding and the resultant excretion of excess nutrients. Management of biological soil health is two-fold. The first is the reduction of soil-borne pests and pathogens, while increasing the levels of beneficial soil insects and microorganisms. In essence, it is about maintaining the balance of the food web such that the percentage of pathogens and pests is at an acceptable level. It is also about creating healthy soil which in turn creates a healthy plant that is able to withstand the effect of pests and pathogens. As indicated above, the soil food web consists of bacteria, fungi, nematodes and numerous other soil organisms. The rest of this paper, however, will focus on nematodes within the soil food web and their role as both contributors to the soil food web, as well as indicators of soil health and ecosystem functioning. Nematodes are microscopic, worm-like organisms that live in the moisture between the soil particles. A nematode’s main function in the soil food web is organic matter decomposition and nutrient mineralisation. Nematodes, however, do not decompose organic matter or influence the mechanical and physical soil properties directly. 16 Their effect is rather through feeding on the microbial population (bacteria and fungi) in the soil. The role of primary decomposition thus lies with bacteria and fungi. The indirect effects of nematode feeding on the microbes include modification of the microbial community, accelerated turnover of microbial cells and inoculation of new substrates through nematode movement. 9 Numbers and proportions of bacterial and fungal-feeding nematodes are indicative of the numbers and relative proportions of the relevant food source. Plant parasitic nematodes are herbivorous nematodes that feed on the roots of plants and in doing so reduce the root systems in plants, in particular fine roots and root hairs. This is the main transport system of the plant and as such can severely affect root and plant health. The most damaging group of plant parasitic nematodes are the sedentary endoparasitic nematodes. The two main types of nematodes belonging to this group are the root-knot and cyst nematodes. These nematodes travel into the root system and alter the morphology of the roots to no longer serve the interest of the plant, but rather the nematode. The semi-endo parasitic group travels within the roots, as well

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TECHNICAL YEARBOOK 2023