Conserving soil health for future generations

Terry Wolhuter receiving his prize – a brand new chainsaw – from Hayden Hutton of Stihl.


Terry and Belinda Wolhuter of 92 Farming (Pty) Ltd are NCT’s Commercial Tree Farmers of the Year for 2023.

Terry is the sixth generation of the Wolhuter family farming on Eiland Spruit Farm in New Hanover in the KZN midlands. The farm was established in 1851 by Mathys Wolhuter, and was historically utilised for raising cattle while crops were cultivated in the flatter areas.

It was Terry’s father, Peter Wolhuter, who started growing wattle on the steeper areas of the farm with sugar planted on the flatter areas.

The farm is 500ha in size and is currently planted with 250ha of sugar cane, 110ha of Acacia and 40ha of Eucalyptus. The remaining hectares are managed as open areas, valleys and waterways which are well maintained with seasonal work being done to ensure alien invasives are eradicated.

All timber compartments are being re-established along the contours to prevent soil erosion.

Terry is very aware of his responsibility as the custodian of the land and the importance of ensuring the viability of the farming operation for the next generation, so conservation of the natural resources - especially the soil - is of fundamental importance to his operational planning. Hence the move to ‘regenerative agriculture’. All timber compartments that are harvested are being re-established along the contours; cool burns are practiced to reduce the harvesting residues. This is only done when the weather conditions are conducive to a cool burn, and after the local community has removed firewood from the harvest sites.

Pesticide usage is kept to a minimum and weed control is done by means of line hoeing followed by a modified slasher that uses chains instead of blades. This creates a mulch in the inter-row that conserves moisture, reduces weed germination and protects the soil from sun, wind and heavy rain storm events.
Terry uses his Nguni cattle to graze under the canopy thus reducing the fuel load for fire protection, and promoting weed control.

The farm’s neighbours are corporate timber growers and NCT commercial timber growers.

Regenerative agriculture in the sugar cane blocks is done by planting the fields due for re-establishment with a cover crop seed mix that includes Japanese Radish, Stooling Rye, Fescue grass and Oats. The resultant crop is used for grazing by the Ngunis – the manure they leave behind is a bonus for the soil. After this operation, maize is planted that is either sold or used for feed.

Terry is discovering the benefits of leaving a two-year fallow period between sugar cane crops which he says increases the microbial activity in the soil and results in improved growth when the sugar cane is replanted. Due to the current situation with more sugar cane being carried over than usual, Terry is feeding this to the Ngunis so these blocks are receiving an addition bonus of manure before the sugar cane ratoons or is planted with the cover crop.

Green wattle (Acacia decurrens) stand sown with the specially designed seeder.

Terry’s passion for his farm doesn’t stop at his adoption of regenerative agricultural operations. Innovation is what has assisted Terry in the timber operation, with the creation of a unique wattle seeder as well as a modified ripper with a duck’s foot that has improved stand survival and uniformity.

The wattle seeder, built by Terry’s mechanic Tewis, has reduced the quantity of seed used per hectare and created a uniform dense hedge of young wattle seedlings that are thinned 12 to 18 months after sowing to 2 500 SPHA and then down to 1 800 SPHA at 24 months. Where site conditions allow, conventional Acacia mearnsii (Black Wattle) seedlings are planted. This is where Terry’s ripper and duck’s foot combination comes to the fore. This piece of equipment creates a rip line, and the seedlings are planted into it after is has been marked to the correct espacement. The addition of the duck’s foot behind the ripper’s tine shatters the soil underneath the surface, while the suspended weight automatically closes up the rip line ensuring that soil moisture is not lost due to drying out. This replaces the conventional pit planting system.

Being a sugar cane grower and owning an earth moving business specialising in cane contouring and water way construction, Terry knows the importance of a well-maintained road infrastructure. All the main access roads throughout the farm are gravelled. Contour roads and water ways are all grassed to prevent erosion. Stream crossings are constructed with pipes and concrete so vehicles can cross easily and silting up of the streams is prevented.

A composting operation on the farm reduces the need to purchase synthetic fertilizers to boost growth of the sugar cane crops. Compost is made from a mixture of cane tops, Mila sourced from the local cane mill and chicken litter. The ingredients are mixed and left to break down into a healthy compost that enriches the soil and boosts growth.

The composting operation reduces the need to use synthetic fertiliser, and is central to the regenerative agriculture approach.

Social responsibilities are as important as any other operation on the farm, and apart from assisting with firewood, Terry has loaned TLBs to the community and sponsored a local soccer team.

Terry he attributes the success of the farm to everyone working together, and he says it wouldn’t be the success that it is without the assistance of his wife, Belinda, especially when it comes to all the admin work.

Responsible pesticide use is essential … all pesticides are locked safely away when not in use.

Red flag over deteriorating health of forest soils

Planting successive rotations of the same tree species on the same piece of land over and over is likely to have a negative impact on soil health and long term productivity of the site will decline – especially when the residue from the previous harvest has been burnt off, leaving the soil unprotected from the elements, as is the case with this community forestry enterprise in Eastern Cape.

Planting the same tree species over successive rotations carries a high risk of deteriorating soil health, say FABI researchers …

It is widely accepted in agriculture circles that practicing crop rotation is beneficial for soil health. This is because the continuous cultivation of the same plant species on the same piece of land over and over again impacts negatively on soil health, and leads to a build-up of harmful micro-organisms. There is plenty of scientific evidence to support this theory.

To address this issue, good farmers all over the world practice crop rotation, planting different crops over successive rotations, or do inter-cropping where for instance they will plant a legume in between rotations to improve soil health and biodiversity.

Many small-scale tree farmers in Zululand practice inter-cropping, planting mielies, beans or peanuts in between their Eucalyptus seedlings when they are still small, and before the canopy closes. This could be beneficial for soil health.

So why not treat short rotation tree crops the same way – and if we don’t, and we keep on planting the same species rotation after rotation, will we eventually find that soil health deteriorates to the point where the trees will no longer grow properly?

Common sense would suggest that this will be the case, but there is no ‘conclusive’ scientific evidence to support this theory. In any event crop rotation in forestry is not so easily done as rotation lengths are long (8 – 20 years), land available for forestry is limited (in South Africa) and only three species are widely grown – eucalypts, pine and wattle – all of which are geared to serve specific markets.

Moreover improvements in silvicultural practices and advances in tree breeding have thus far masked any impacts of deteriorating soil health on successive tree crops, thus reducing growers’ appetite for experimenting with crop rotation...

This Sappi compartment in Zululand has been mulched and the residue retained to cover the soil with a nutrient-rich layer that provides protection, retains moisture and gives the young trees a growth boost.

But now a group of highly regarded researchers have raised a red flag and warned that deteriorating soil health is a real risk for short rotation plantation forestry, as it is practiced in South Africa and in many other parts of the world.

A team of FABI researchers including Dr Tanay Bose, Prof Bernard Slippers, Almuth Hammerbacher, Jolanda Roux and Mike Wingfield, have analysed existing data on soil microbiomes from short-rotation forestry environments around the world which, they say, provides evidence that a build-up of harmful micro-organisms, depletion of beneficial micro-organisms, and deterioration of the physical and chemical properties of soil can result from continuous replanting of the same tree species on the same piece of land.

“Populations of unfavourable microbes can be expected to become more abundant over successive rotations,” state the FABI researchers in an article published in the journal Current Forestry Reports. “This is strongly supported by data from recent soil microbiome studies involving commercially managed forests, which provide convincing evidence of an increase in pathogenic microbes in soils of continuously replanted forests.”

Building on these findings and considering similar approaches in agriculture, the team proposes a number of practical solutions that have the potential to mitigate the deterioration of soil health resulting from planting the same species over successive rotations.

Weza farmer Jon Mackenzie uses a tractor-powered chipping machine to chip the post-harvest residue after clear felling, which is then spread evenly over the site before the re-planting operation begins. This is his strategy to conserve the health of the soil on his farm for the log term.

• Retaining post-harvest residue on the site is likely to result in healthier soils over successive rotations. This is why many growers have stopped burning the post-harvest residues and have opted for mulching or chipping.
• Application of biochar has considerable potential to enhance soil properties, nutrients and microbes in continuously replanted forests. Biochar is a carbon-rich, stable organic product made from the pyrolysis of organic biomasses such as leaves, sawdust, animal dung and wood. During carbonization, biochar releases phosphate into the soil along with other mineral nutrients, improving its fertility. Biochar also improves the physical properties and microbial biodiversity of the soil, which could further increase soil nutrient availability and carbon storage. However more research is needed to assess the impact of biochar on plantation soils.
• Crop rotation and intercropping could alleviate the negative consequences of continuous replanting of the same species in short-rotation plantation forestry. For example, rotating between eucalypts, black wattle and pine species could prevent the accumulation of harmful soil microbes detrimental to these trees. Rotating nitrogen fixing leguminous tree species such as Acacia with eucalypts or pines has the potential to further promote both soil and tree health increasing the availability of nitrogen in the soil and improving the quality of plant litter.
• Practicing agro-forestry, where different crops are inter-planted on the same piece of land, would also have a positive impact on soil health.
• Innoculation of tree seedlings in the nursery with beneficial microbes.
“Continuous replanting practised in short-rotation plantation forests is likely to be accompanied by a high risk of ‘replanting syndrome’ in plantations. While long-term monitoring programs to document the changes in soil microbiomes are still lacking and should be urgently initiated, the available evidence suggests that short-rotation forest plantation enterprises could be restrictive when successively establishing plots with the same or nearly the same genotypes,” the FABI team concluded.

Mulching after clear felling is an expensive exercise, but many growers have opted for this approach instead of burning off post-harvest residues, in the interests of boosting soil health for the long term.

For the full Report see: Current Forestry Reports.

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