Sappi Southern Africa
Corporate Citizenship Report

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Making the most
of mulching

Background

Plantation residue – bark, foliage, branches – is generated during the harvesting process. Burning of post-harvest residue (slash) is a common practice, as it reduces fuel load and the risk of wildfire. It also facilitates preplanting, planting and post-planting activities. However, burning reduces the content of soil organic matter (SOM). Burning can also increase the risk of soil erosion. Studies on nutrition of trees have shown that nutrient loss during burning followed by wind erosion is a concern with sensitive soils.

Although we still practice burning, because of the benefits we have observed, mulching is being rolled out to more of our plantations each year.

Why is SOM important?

  • Improves the water holding capacity of the soil
  • Plays an important role in improving soil texture and maintaining soil fertility
  • Regulates soil temperature, facilitates water penetration and serves as a habitat for soil organisms

What is mulching?

Mechanical mulching of the post-harvest residue is a costlier alternative to burning. Because warmer temperatures brought on by climate change will lead to drier soils, mulching helps to mitigate the impact of climate change.

What are the benefits of mulching?

  • Increased survival and growth of trees
  • More environmentally sound practice
  • Value of the estimated additional timber produced exceeds the current mulching cost.
  • Moderates soil temperature and positively modifies the physical and chemical properties of soil
  • Conserves soil moisture, reduces evaporation and increases retention of soil water
  • Protects soil against erosion and compaction
  • Inhibits weed growth
  • Reduces plant stress – high temperatures of bare soil can scorch seedlings and cause fine root mortality
  • Acts as a slow release fertiliser, releasing nutrients as it decomposes
  • Improves access for fire prevention and control
  • Retards coppice regrowth consequently reducing soil water use, leaving more water available for the new crop

FAQ

Q: Sappi has a 30% share in a biomass plant at Ngodwana Mill that uses biomass residue from the surrounding plantations. Won’t this impact the soil?
A: While biomass is being collected, the foliage and fine branches, which contain most of the nutrients, are being left behind, so the impact on soil fertility is minimal.
B: Biomass will not be removed from areas with sensitive soils.

Summing up

The benefits of mulching compared with burning have been demonstrated in various research trials, with improved growth, survival and uniformity recorded across all sites to varying extent.

Further research is under way using paired plots to quantify the benefit operationally. This study aims to directly measure the impact of different residue treatment options on the temporarily unplanted period and understand the mechanisms behind the improved growth response associated with mulching by recoding soil temperature, soil moisture, and nutrition.