Prof Hennie Snyman’s research found a teaspoon of salt is enough to kill slangbos without oversalinating soil. He spoke to Roelof Bezuidenhout.
Young stands of slangbos
can be easily and cheaply controlled by sprinkling small amounts of farm salt around the plant stems. This raises the salinity in the root area enough to inhibit growth without damaging the soil, says Prof Hennie Snyman of the Department of Animal, Wildlife and Pasture Research at Free State University. Seriphium plumosum (known as slangbos, bankrotbos, vaalbos or Khoi-kooigoed) is a huge problem in the Eastern Cape, Free State, Mpumalanga, North West and Gauteng. This unpalatable, low-quality shrub invades productive grassveld, but it doesn’t grow in clayey, vlei areas with high organic material, and soils with a high sodium content. This gave the researchers the idea of using salt and nitrogen fertiliser to eradicate it (see box: Nitrogen: the other secret weapon).
With a pinch of salt
In a study near Thaba Nchu in the Free State, Prof Snyman had 100% success in treating slangbos plants of 400mm and smaller by placing just one teaspoon of salt around the stem. The plants took 14 days to die, at a cost of only 0,2c/plant, and salt prevents seedlings re-establishing in the same spot. At a density of about 1 400 plants per hectare, 100kg/ha of salt was used at R42/50kg bag. The salt can be applied by hand as granules or dissolved in water using a dose gun. It isn’t broadcast over the whole area, but around each plant. If necessary, applications can be increased before the end of the growing season. Ideally, salt should be applied from October so the rains can wash it into the soil for the roots to absorb.
But while bushes of 400mm and smaller can be treated with salt, Prof Snyman advises using the herbicide Molopo (active ingredient Tebuthiuron) on bigger plants. “Controlling larger bushes takes much more salt, which is as expensive as chemical control and can lead to soil salination,” he cautions.“We’re not yet sure of the long-term effects of applying salt applications or what effect rainfall would have on soil salination, but at this stage we don’t recommend applying more than 300kg of salt per hectare.”
Molopo can be applied at a rate of 4mâ„“/bush for the bigger bushes at 12c/bush. But if plant density is too high, aerial application or tractor spraying become cheaper than single-plant treatments. Where Molopo is applied aerially in granular form, application rates will depend on the soil’s clay content.
The integrated approach
“Integrating all the options might be the best control method, ecologically and economically,” says Prof Snyman. Treatment with residual herbicides is effective for up to three years, but it’s expensive so it must be done properly. Prof Snyman warns mechanical control (chopping) is labour-intensive and requires follow-up action to eradicate new growth. Chopped bush must be burned to kill remaining seed, and incorrectly timed burning can worsen the problem. With no biological agents available, fighting slangbos remains tricky, requiring follow-up treatments to prevent reinfestations from neighbouring areas. Before tackling dense infestations, it’s best to first treat relatively mildly infested patches to stop further invasions. “The salt remedy hasn’t been tested in different soils or climates – we’d really appreciate feedback from farmers who experiment with other application rates,” Prof Snyman concludes.
E-mail Prof Hennie Snyman at [email protected] or call (051) 401 2221. |fw
