Mulching of harvest residues is rapidly gaining ground in South African forestry, and is proving to be a game changer …

Deon Redinger of Savithi Mulching is one of a new crop of contractors flying the flag for mulching as a means of managing post-harvest slash in SA. Deon is a passionate believer in the benefits of mulching over burning slash, and local forestry companies – notably Sappi – have opted for mulching over burning in plantations with sensitive soils.

Historically there has been a reluctance on the part of forestry companies to throw their full weight behind mulching, due to the relative cost of mulching vs burning, which is the traditional South African way of dealing with slash.

This reluctance to engage with mulching has been exacerbated by the fact that it is known to be one of the toughest operations in forestry – on both man and machine. Contractors and growers have had to learn some hard lessons in the process of finding the right systems that can meet productivity expectations while delivering a consistent quality of mulch.

Moreover the additional and hidden benefits of mulching are not easily quantifiable. It impacts on almost every facet of growing and harvesting trees, so to appreciate the full benefits you have to consider the bigger picture - not just short term rands and cents.

Productive tree growth requires a healthy soil habitat which is achieved through maintaining the pristine state and balance of soil physical, chemical and biological properties. Numerous local and global studies have demonstrated the extreme negative impact of organic matter loss due to repeated residue removal, burning (and subsequent erosion) on the soil health and productivity of sensitive soils. Residues left behind after harvest contain large quantities of organically bound nutrients and carbon. Mulching compared to burning or residue removal directly conserves the soil health. This retained organic matter feeds the trees and soil microbes as a slow release organic fertiliser and carbon source for much of the subsequent rotation. Healthy soil microbes contribute to tree nutrition and are believed to also act as the soils immune system by outcompeting soil pathogens.

Further benefits measured after mulching are increased soil water due to reduced surface evaporation, reduced weed growth and increased percolation; and stabilisation of soil temperature by eliminating extreme heat and cold. Prevention of soil erosion and compaction through surface protection are further major benefits.

In addition, greenhouse gas emission due to fuel use during mulching is far less than the methane and nitrous oxide release during residue burning.

Residue mulching through these benefits can potentially mitigate the effects of climate change.

No more burning

Sappi was one of the first large grower companies to adopt mulching of slash as a strategy in their Zululand plantations. Work by Sappi’s research team to gauge the cost benefit based on trials concluded that at rotation end the additional growth benefit was three times the cost of mulching. This research was used to motivate for the decision to proceed with mulching in Zululand.

Sappi started doing trials in 2010, and by 2014 were mulching 100% of their Zululand coastal plantations, stretching from Richards Bay to north of Mtubatuba. No more burning of slash takes place on these plantations.

Mulching solved a lot of problems for Sappi in Zululand, reducing temporary unplanted areas dramatically, improving seedling survival and growth, removing old stumps thus paving the way for better access for modernised silviculture operations and fire prevention; as well as more productive future harvesting operations, whilst protecting and nourishing the sensitive soils with a mulch blanket.

Much work has been carried out by Sappi in quantifying the financial benefits of mulching due to improved growth at rotation age. Their research team has installed mulch/burn twin plots to directly measure and compare the effects of mulching on soil water, soil health and tree growth.

This work has led to a study currently being undertaken by Leeshan Mahadeo, a BSc Forestry graduate from Stellenbosch University, to gauge the impact of mechanical mulching on subsequent pitting and planting operations in both pine and Eucalyptus. The study, supervised by Bruce Talbot and Simon Ackerman of Stellenbosch University, is being implemented on sites in Zululand, KZN midlands and the Mpumalanga Highveld.

This study will provide useful data that will help to clarify the operational cost/benefits of mulching while developing drone-based methods for residue load assessment.

Negative impact of residue removal

According to Sappi research scientist, Steven Dovey, productive tree growth requires a healthy soil habitat which is achieved through maintaining the pristine state and balance of soil physical, chemical and biological properties. Numerous local and global studies have demonstrated the extreme negative impact of organic matter loss due to repeated residue removal, burning (and subsequent erosion) on the soil health and productivity of sensitive soils, he says.

Residues left behind after harvest contain large quantities of organically bound nutrients and carbon. Mulching compared to burning or residue removal directly conserves the soil health, he says. This retained organic matter feeds the trees and soil microbes as a slow release organic fertiliser and carbon source for much of the subsequent rotation. Healthy soil microbes contribute to tree nutrition and are believed to also act as the soil’s immune system by outcompeting soil pathogens.

Further benefits measured after mulching are increased soil water due to reduced surface evaporation, reduced weed growth and increased percolation; and stabilisation of soil temperature by eliminating extreme heat and cold. Prevention of soil erosion and compaction through surface protection are further major benefits.

In addition, greenhouse gas emission due to fuel use during mulching is far less than the methane and nitrous oxide release during residue burning. Thus residue mulching can potentially mitigate the effects of climate change, says Steven.

Savithi Mulching

Savithi Mulching, equipped with a fleet of tough-as-teak Tigercat wheeled mulchers, are currently mulching for Sappi in Zululand, and have now started mulching for Sappi in the KZN midlands around Ixopo, Highflats and Bulwer as well.

Deon Redinger established Savithi Mulching in 2010, initially using tracked machines but now has graduated to wheeled Tigercat M726G machines.

“We started slowly and learnt a lot of lessons along the way,” says Deon. “We started with tracked machines, but when we got wheeled machines we came right.”

The three most important ingredients in the mulching business, according to Deon, are the quality of your ‘pilot’ (operator), effective maintenance and spares availability.

It’s no coincidence that he calls his operators ‘pilots’. This is because it takes training, skill and concentration to operate a mulcher properly. The importance of effective machine maintenance and spares availability in this extremely tough operating environment speaks for itself.

Every compartment presents different conditions and different challenges, says Deon. Tree species require different mulching tactics due to the quantity and nature of the slash left behind. Some stumps are much harder to grind down than others; ground conditions, weather, slope and the turnaround space at the compartment edges - all of these factors impact on the mulching operation.

“The challenge is to get the mulch to cover the soil evenly like a blanket (not to mix the mulch up with the soil) and to reduce stumps to ground height.”

He says the mulcher will run over a brushline two or three times to get an even distribution of mulch that you can plant into.

It can take anything from two to six hours to mulch one hectare (in Zululand), depending on conditions, so Savithi generally works on an average of three hours per hectare. He says they average around three hectares per machine per day, and do not operate at night.

Deon says that 350 horse power is the minimum grunt required for an effective mulcher. He’s also convinced that wheeled machines are better than tracked because they can be moved around from compartment to compartment without the need for a lowbed trailer.

He says they are operating right behind the harvesting team, and as soon as they’re done mulching the planting team moves in. They are also mulching the routes that will be used to extract the harvested timber from in-field, so mobility of the mulching machines between adjacent compartments is essential. This operation speeds up the shorthaul, reduces tyre damage and protects the soil from compaction.

All post plant and future harvesting operations are made easier and cheaper in compartments that have been mulched.

Deon says Savithi has mulched 13 000 to 14 000 ha in Zululand over the past few years, and just this year will mulch 3 000 to 4 000 ha in the KZN midlands.

“We can work in fairly steep slopes – where a skidder can go our mulchers can go,” he said.

Savithi has also been mulching old forestry compartments for private farmers that are converting land to other crops like macadamias, avos and pecans.

Deon has been doing a lot of work to find an effective way to accumulate mulch for further downstream processing opportunities that he believes will be viable in future, and likes to refer to mulched material as “unutilised biomass”.

Sappi’s Zululand Area Manager, Sandile Nkosi, says mulching has enabled the Sappi forestry team to keep temporary unplanted areas below 1.5% throughout the year, thanks to the speed of mechanised operations and the extended planting window that mulching has given them. Furthermore it has enabled Sappi Zululand to maximise silviculture mechanisation, improving compartment access for planting, weeding and fire fighting.

He says they have had fewer fires since they started mulching, and are able to put fires out quickly as they don’t spread so fast. He says all the Sappi foresters have observed improved survival and growth in mulched compartments.

Says Deon: “the mulching operation returns organic matter back into the soil and increases plant growth by 5-10%.

“The benefits of mulching harvesting residues are infinitely preferable to burning them. It has taken some time, but the benefits of the mulching process are finally being recognised by the forestry industry.”

The last page of Sappi’s 2019 Corporate Citizenship report focuses on the company’s mulching strategy in their South African plantations, and states: “The value of the estimated additional timber produced exceeds the current mulching costs.”

There you have it! With the right team, the right equipment and sufficient commitment and experience, mulching can be a game changer in the South African forestry environment.

Related article: Mulching gaining ground in SA