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Did you know? ... Maximum rooting depth

The roots of a wild fig tree (Ficus natalensis) at Echo Caves, near Ohrigstad, Mpumalanga, South Africa, reach 120 m deep (Guinness World Records).

Even apart from this record-holder, though, many species regularly put roots down to more than 10 m, and specimens of Boscia albitrunca (the shepherd’s tree) and Acacia erioloba (the camel thorn), both of which grow in southern Africa, can put down roots to more than 60 m.

Critical to survival

Rooting depth is critical to plant survival where water is limiting – the normal situation in Australia. Whether you are growing turf, wheat or forest, the plant’s roots must be able to reach the soil water or the plant will fail to thrive or even die.

The depth of roots in relation to available soil water has a critical effect on the balance of water, nutrients and carbon in an environment. Deep roots can return deep water to the atmosphere where it has the potential to fall again. This continual cycling of water – although not necessarily deep water – is the basis for the high rainfall of tropical forests, notably those of the Amazon Basin. Deep roots are also known to lift deep water and “leak” it into shallow soil, where roots of the same plants and other plants can take it up.

Deep nutrients that might have been washed down through the soil profile by occasional heavy rain are similarly recycled when they are drawn up into the above-ground parts.

Carbon sequestration

At a time when carbon sequestration is attracting a lot of attention as a partial solution to global warming, deep-rooted crops and trees offer huge potential: when the deep roots inevitably die, their carbon remains buried in the ground.

A news story from the ABC quotes Tim Wiley, a pasture agronomist from the WA Department of Agriculture and Food, as saying “Drought-tolerant perennial pastures could make a big dent in Australia’s greenhouse [gas] emissions by helping soils to soak up carbon.” He has found that naturally drought-tolerant pasture species “appear to have an exceptional ability to build up carbon in the soil.” Some of this carbon may be present in mycorrhizal (root-associated) fungi, rather than in the roots.

Drought survival

The deepest roots are typically found in desert environments and tropical grasslands, where seasonal or regular drought means that the topsoil is usually dry. Provided a young plant can get its roots down quickly enough while the topsoil still holds water, it can then survive on deeper water during the next drought.

Experiments in Western Australia by the Forestry and Forest Products Research Institute of Japan have shown that seedlings planted in open-ended plastic tubes about 1 m long establish better in dry areas than those allowed to root normally.

Although you might expect that loose or sandy soil is most conducive to deep roots, in fact clay or even bedrock is not an impediment to some roots, which follow natural cracks to deeper water. The continual death and regrowth of roots keeps these networks of cracks and channels open.

Encouraging deeper roots

Careful scheduling of watering can encourage plants to put down deep roots. Thorough deep watering alternating with long dry spells (but before plants permanently wilt) will force plants to put down deeper roots.

Breeding for deeper roots

Work in Japan has shown that dryland rice puts down deeper roots than paddy rice, and that the difference is genetically controlled. If the genes responsible can be incorporated into paddy rice (without the genes responsible for the lower yields of upland rice), then rice farmers will be able to grow paddy rice with less water – an important consideration in a drier world.

Further reading

J Canadell et al. 1996. Maximum rooting depth of vegetation types at the global scale. Oecologia 108: 583-595.

T Wiley, R Grima. 2007. Drought proofing grazing systems. WA Dept of Agriculture and Food.

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