Technical Yearbook 2023

FIGURE 1. Soil food web.

in the soil. Roots would rather go around a hard or dense object than go through it. This restricts root growth and distribution within the compacted layer. 26 Furthermore, compaction causes anaerobic or anoxic conditions. Root respiration requires oxygen. The anaerobic conditions created as a result of compaction lead to root death and as a result, reduced root growth. 3,26 Compaction within the soil should be addressed in order to allow for the optimum development of roots throughout the soil profile. Compaction can be addressed by implementing controlled traffic principles, tillage and the use of appropriate cover crops. The distribution, health and functioning of the root systems are also strongly influenced by the prevailing soil water dynamics. 24 Depending on soil type, over-irrigation and poor drainage can result in waterlogged conditions. This much like compaction results in anaerobic or anoxic soil conditions. As discussed earlier, roots require oxygen to respire and as such do not grow optimally in such conditions. The constant availability of water can lead to the development of a shallow root system. Readily available water near the surface results in roots not needing to go searching for water. This causes the majority of the roots to remain near the surface instead of developing throughout the soil profile in search of water. 26 A deep-rooted plant is a resilient plant. Plants with deep roots are able to survive periods of water stress as deep roots are able to access water from areas deep within the soil. To develop a deep root system, the water must be managed carefully to allow for periodic water stress which will encourage the roots to go searching for water. This may result in short-term water stress and affect yields, but will result in a deep-rooted plant that will perform better

through periods of water stress. Although in irrigated fields, water stress can theoretically be controlled, periods of unplanned water stress may occur due to the erratic electricity supply coupled with the possibility of water shortages in South Africa. Land use planning, irrigation scheduling and monitoring irrigation water quality are important irrigation management tools that can be applied to prevent waterlogging and stimulate deep-root development. Chemical soil health Soil acidity is considered to be one of the largest crop limiting soil constraints in rainfed agriculture. The presence of excess aluminium (Al) in the soil and increased iron and manganese solubility create a toxic environment for plants. In addition, it causes a deficiency of base cations, particularly calcium (Ca) and magnesium (Mg). Excess Al can also lead to reduced phosphorus availability. Additionally, the uptake of Ca, Mg, potassium (K) and some micronutrients can also be reduced. Soil acidity in agriculture is caused mainly by the excessive use of nitrogen fertilisers and the loss of Ca, Mg and K when the crop is removed at harvest. 21, 22, 23, 27 The effect of soil acidity on plant roots is dependent on the crop grown. In contrast to soil acidity, soil salinity is an excess of freely available Ca and Mg ions in the soil. Soil salinity reduces water uptake by the roots by creating an unfavourable concentration gradient between the soil solution and the root. 4 When the salt content in the soil solution is much greater than that of the root cells, the plant is unable to produce sufficient sugars to compensate. As such, the plant has a reduced capacity to absorb water. 4, 10 Under extremely high salt loadings

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TECHNICAL YEARBOOK 2023