Large Farms

Addressing Encroacher Bush

The Perivoli Rangeland Institute is establishing a number of pilot projects in Namibia to showcase best practices for harvesting encroacher bush and the application of wood and biochar onto the rangeland.

Our expectation is that this will encourage moisture retention and the return of perennial grasses, leading to the building of Soil Organic Carbon (SOC).

…and in the process justify the earning of carbon sink credits and, in time, biodiversity credits for the farmer.

Farmers Association

Our goal is to establish a Perivoli Rangeland Farmers Association of landowners interested in drawing upon our science-based learnings.

Bush Harvesting

The larger farm projects start by harvesting encroacher bush through selective thinning and along contour strips by teams of bush harvesters equipped only with manual tools, such as axes and machetes.

Sustainable bush removal is calculated on a Tree Equivalent (TE) or Evapotranspiration Tree Equivalent (ETTE) basis. Based on local research a “rule of thumb” stipulates that the median ETTE/ha and TE/ha that can be supported in a specific rainfall region without adversely affecting the grass layer should not exceed 10 and 1.5 times the mean annual rainfall respectively.

Simply put, on average across the central bush encroached region of Namibia, this translates to roughly 30-50% tree and bush coverage, increasing as the average annual rainfall increases in a north-easterly direction.

The bush species that are cut back are mainly Acacia mellifera, A. reficiens, A erubescens, A. fleckii and Dichrostachys cineria.

The bush harvesting staff are equipped with protective clothing and paid at above market hourly rates.

Bush Filters

The branches of the harvested bushes are laid out on contour lines, known as bush filters.

The branches slow rainwater runoff and provide shade, reducing evaporation. They afford cover from the impact of grazing cattle and game, allowing new seedlings to establish. They also attract termites and microbes that puff up the soil which ponds water upslope, improving infiltration.

Over time, the leaves of the cut bush and those deposited by wind or rainwater runoff reduce soil temperatures before they compost down to release nutrients for uptake by plants.

In addition, some of the branches of the harvested bush are strategically placed in rills, rivulets and gullies to divert or slow the flow of water. In some cases bush is suspended from wires across a rivulet so as to slow the flow of water and divert it onto nearby terrain, improving water retention.

Pyrolysis

Burning

The larger parts of the bush stems are stacked for a few weeks to dry out and then converted to biochar in mobile kilns specially designed by the Perivoli Rangeland Institute for the purpose.

The design of the kilns allows for the burning wood stumps to reach high temperatures (500 degrees centigrade) at which point very little smoke is released, the resulting gases being reburned within the kilns.

Wood is loaded into the kilns in layers to maintain as high a combustion heat as possible, while the bottom layers are deprived of oxygen, thus promoting the anaerobic gasification process known as “pyrolysis”.

Quenching

Once the stumps have burned down to hot coals, water is pumped up into the kiln from the bottom in a process known as “quenching”.

The quenching causes a steam explosion which cracks and opens the biochar’s carbon crystalline structure expelling the pyrolytic oils.

The contents of the kiln are allowed to sit for roughly half an hour to cool. The liquid is then drained back out of the bottom of the kilns and used as “quench water”, a pest repellent and partial fertiliser that farmers are encouraged to apply after dilution in water to vegetable patches and flower beds around their homesteads.

Biochar

During the quenching process, the hot coals fragment into a crystalline latticed structure known as “biochar”.

The surface area of one gram of biochar is said to be equivalent in size to the surface area of half a full-sized football pitch.

The kilns are designed to be tipped on an axis, allowing for the biochar to be piled onto the ground for applying to soil.

The kilns can be loaded onto a utility vehicle or trailer and moved relatively easily around the terrain. The aim is to cover as much of the land as possible with biochar.