Most flood damage caused by the torrential rain in KwaZulu-Natal (KZN) earlier this year occurred in residential areas and informal settlements where soils are disturbed and there is poor drainage infrastructure.

If a surface allows infiltration, the water will be dealt with, but with the water accumulating off areas such as roofs, there was too much per unit area of soil to be absorbed.

This is according to UKZN’s Professor Pardon Muchaonyerwa who was included in a Thompson Reuters Foundation piece on land degradation in Africa which featured the input of experts from all over the continent. The foundation article followed the release of the second UN Convention to Combat Desertification Global Land Outlook report this year.

Added to the astronomically high rainfall in the April flooding, about a third of the area’s annual average was recorded over two days and because of that – coupled with poorly planned settlements on sandy soils, on steep slopes or on disturbed land or floodplains at the end of the catchment – mudslides occurred where soil that had reached its saturation limit flowed like a liquid, destroying homes and trapping people. Additionally, crops, farmlands and vegetation were washed away.

Muchaonyerwa weighed in on the role of soil health in the damage experienced after the April 2022 floods in KZN.

Speaking about the effects of the high levels of rainfall that led to severe damage to infrastructure and tragic loss of life, Muchaonyerwa highlighted the importance of increasing infiltration and reducing the quantity of floodwaters and runoff.

An expert on soil ecosystem function and health and sustainable agriculture, Muchaonyerwa has been at UKZN for 11 years and is recognised for his knowledge and research globally.

With the observance of World Desertification and Drought Day this month, the phenomenon of land degradation is an important consideration – it threatens food production and livelihoods, and plays a role in extreme events.

Describing South Africa’s soils as unique, with a large proportion being predominantly derived from shales and mudstones, Muchaonyerwa said these tended to have a high concentration of silt and very fine sand, making them extremely erodable. Combined with the high rates of overgrazing on South African grasslands, soils are exposed to damage by natural phenomena, including rainfall.

South Africa’s naturally problematic environment for soil erosion and land degradation are exacerbated by human activity in the form of deforestation, intensive livestock grazing and conventional crop production, where soils are heavily utilised with intensive tillage in the summer season and then left fallow and exposed in winter.

‘When we talk of degradation, we’re talking about erosion, desertification and loss of nutrients through erosion, gaseous emissions and leaching, but the main problem is that as we lose the soil, we also lose nutrients and carbon, which stabilises the soil – many of our soils have low carbon, making them susceptible to degradation,’ said Muchaonyerwa.

Some parts of KwaZulu-Natal are characterised by high carbon humic soils, and despite the value of keeping carbon in the soil, current agricultural practices such as tilling overturn the soil and release its stored carbon.

He spoke of the need to reduce practices that result in soil and carbon loss and to make the soil a good habitat for organisms to thrive. Where there is low carbon in degraded soils, Muchaonyerwa said there were conservation agriculture techniques that could improve soil quality, including keeping soil covered with residues for protection, returning organic matter to the soil, rotating crops and making use of cover crops.

Muchaonyerwa said that with the layer of fertile topsoil covering the globe being at most 30cm deep, the current loss of up to 1 cm of some soils every year through unsustainable practices was a major problem, especially considering the rate of soil formation was extremely slow. The fastest-forming soils take at least 10 years to form 1cm, with the slowest-forming soils taking almost a millennium to develop the same depth. Degraded or exposed soils being lost to runoff in high rainfall events contribute to the high rate of soil loss.

He said while there remained a lack of data on rates of soil loss in South Africa, its effects were evident in physically observable dongas, aerial photography and remote sensing data. Efforts to protect the soil resource and regenerate its productivity were ongoing.

Research being done by Muchaonyerwa and PhD candidate Ms Nontokozo Mkhonza linked to conservation agriculture features a comparison of two neighbouring sugarcane farms on the KZN coast that employ different methods of harvesting – one follows burnt cane harvesting, while the other has harvested green cane for 78 years.

Mkhonza, who is examining carbon and phosphorous cycles, has found higher microbial presence and enzyme activity in the soils on the farm using green cane harvesting. Moreover, in the KZN floods, there was no loss of soil to erosion or flooding under green cane harvesting, while the neighbouring farms experienced major damage to fields and infrastructure.

Soil carbon is lost to trash burning and released into the atmosphere as a greenhouse gas. Green cane farms had soils with higher organic matter that could retain water and reduce runoff effectively, making water available during times of drought. Despite a lower sucrose yield in green cane harvests in normal seasons, its yield does not diminish significantly under drought conditions compared to pre-harvest burning of sugarcane.

‘Retention of residues stabilises yields, improves soil quality, and stores carbon, minimising the release of carbon into the atmosphere meaning a positive effect for climate change,’ said Muchaonyerwa.

He noted that the Discipline of Soil Science offered many opportunities for innovation to reverse degradation, including rotation of livestock and crops, using cover crops that could benefit farmers and improving practices in marginal areas, for example through practising no-till and amending the soil with biochar produced from recalcitrant organic wastes.

Words: Christine Cuénod

Photographs: Supplied