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Soils and habitat restoration

Habitat loss and degradation is the most important cause of species decline and extinction in Australia. The biodiversity of South Australia’s temperate agricultural regions is in overall decline and a range of restoration actions are needed to halt further decline.

Habitat restoration often focuses on the revegetation of native plant communities, which help improve soil condition (e.g. provide nutrient inputs, soil stabilisation and support microbial activity) and re-establish ecological function (e.g. provide food resources and shelter for animals) and services (e.g. support/attract pollinators).

Many native plants will only regenerate in specific soil types and landscapes that they have adapted to over millions of years. Successful habitat restoration requires an understanding of the plant species’ environmental requirements and tolerances. Effective restoration planning therefore requires a good understanding of the site’s topography, hydrology, geology and soils.

Soil factors to assess for successful restoration include soil type, soil fertility, pH, salinity, slope and aspect.

Native vegetation growing in a paddock. There are grass-trees in the foreground and larger trees in the background. — Native vegetation growing in a paddock, with grass-trees (Xanthorrhoea sp.) in the foreground and larger trees in the back. Image Source: DEW

Soil Type

In early ecosystem establishment, the focus should be on soil stability, cover and appropriate species mix. Knowing the soil type of a revegetation site is crucial for understanding plant suitability and ensuring appropriate plant selection for use in restoration. For example, deep rooted plant species such as Wattles (Acacia sp.) are a good choice for sandy soils, as these plants can access soil moisture stored deeper within the soil profile, whilst providing erosion protection by stabilising soils. Additionally, Wattles are nitrogen-fixers which can improve the condition of sandy soils which are naturally less nutrient rich than finer textured soils (loams and clays).

Generally, early successional plant species, those which readily grow after a disturbance (e.g. following vegetation clearance), are recommended as part of revegetation programs. These native plants are hardy (adapted to poor soils/growing conditions) and fast-growing, providing ground cover and soil stability sooner than other species. These plants also improve soil conditions, making the environment more suitable for the establishment of other plant species over time, improving biodiversity. Pop Saltbush (Atriplex holocarpa) and Ruby Saltbush (Enchylaena tomentosa) are examples of early-successional species native to South Australia.

Knowing the soil type and condition can also help identify potential limiting factors. For example, very compacted or rocky soil may need to be avoided or require mechanical modification prior to planting to facilitate successful germination. Heavy clay or impermeable subsoils can indicate a potential for waterlogging and limit vehicle access for direct seeding.

The susceptibility of soils to erosion should help inform the revegetation method used. For example, direct seeding is less successful on eroding sands, as seeds are likely to be blown away (low seed retention). Similarly, direct seeding is also less successful in cracking clays, where clay swelling and shrinking in response to rains can damage emerging plants and their root systems.

Four vertical lines of different coloured soils. — Different soil types. Image Source: Shutterstock.

Soil Fertility

Changes in soil condition through land management practices may mean that some areas are no longer able to support the ‘pre-European’ (native) vegetation species which were originally present in the landscape. Soils rich in nutrients from agricultural fertilisers may be less suitable for restoration with species adapted to nutrient-poor soils. Australia’s soils are old and highly weathered, and therefore naturally nutrient-poor. Most of Australia’s native plant species, particularly those from heathland and woodland ecosystems, have evolved to thrive in these nutrient poor soils. Consequently, these species are slow growing and can be outcompeted by non-native (weed) species when growing in nutrient rich soil environments. In these environments, weed management needs to be implemented prior to and post-planting, to facilitate successful establishment of native plants.

Soil Slope and Aspect

Uneven soil surfaces and steep slopes can restrict the use of machinery as part of restoration processes (e.g. direct seeding), as it presents a risk for machinery rollover, accelerated wear on axels or excessive vibration. Successful plant establishment on steep slopes may also be limited, as seeds are easily blown away by the wind or washed away through runoff, resulting in low seed retention at the site. However, some site limitations can be managed or overcome through careful planning and engineering. For example, ripping can be applied perpendicular to hillside slopes (along the contour) to create furrows in the soil surface for capturing wind/water transported seed, improving seed retention at the site and germination rates.

Uneven surfaces have the potential to limit machinery use in some instances, but having some unevenness to the soil surface is important for successful seed retention. Depressions in the soil surface provide opportunities to capture moving seed (e.g. as it is blown across the soil surface) and rainfall, providing a microclimate for facilitating seed germination and plant growth.