Invasive plant species are those that have been introduced into a specific area, and that displace and outcompete indigenous species or negatively influence the growth and production potential of ‘beneficial’ alien species. In this case ‘beneficial’ alien species can include ornamental plants, but mostly refer to plants that are used for food, fibre and medicinal compound production.

A wide range of detrimental effects are associated with invasive plants and include:

  • Negative effects on biodiversity.
  • Growth of surrounding or affected plants.
  • Water quality impacts.
  • In some cases, soil health.

Many invasive species have no negative effect on soil health but certain species, such as those discussed below, have very distinct impacts.

What is soil health?

Soil health is broadly defined as the sustained potential of a soil to function as a growth medium for plants and soil organisms, through the provision of nutrients, water and habitat, for the long-term health and benefit of man and the environment.

This concept is very broad and has to be applied with caution. The natural soils of South Africa vary significantly along climatic (rainfall) gradients, geological zones, topography, organisms (all living organisms – big and small) and landscape age. The specific conditions have, under natural conditions, led to the very distinct natural heritage of South Africa that is characterised by a wide range of plants, animals and soils.

These soils therefore have very specific combinations of characteristics that make up the localised ‘soil health’ features. It is within these specific areas that invasive plant species can detrimentally impact on plant or animal species.

The effect of agriculture

With the introduction of people, the natural landscape was altered with the most prominent change being the introduction of extensive and intensive agriculture. In this context the natural soil health characteristics have to change to accommodate the new land use through the removal of competing plants (both indigenous and invasive), optimisation of soil parameters for crop production and in many cases, an increased water content and throughflow resulting from irrigation.

Within the concept of ‘soil health’, an almost invisible component plays a very critical role. This is the concept of an active and healthy soil microbiome. Soils are biologically very diverse with a large diversity of bacteria, fungi, nematodes, mites, etc. A very small fraction of these function as plant pathogens (killing plants directly or indirectly), with the bulk functioning as beneficial organisms.

These organisms play critical roles in plants’ capacity to develop roots, access nutrients, fight off pathogenic organisms and develop healthy above-ground structures. These organisms are themselves dependent on a wide range of soil properties and can therefore be manipulated, or ‘helped’, to provide for healthier soil and improved plant production.

Benefits of micro-organisms

All plant roots form associations with beneficial micro-organisms in the soil. Although the plant and organism interactions are numerous and varied, this concept will be illustrated with an example.

A new root with fine root hairs is shown in Figure 1a. When roots are inoculated with a beneficial fungus such as Trichoderma asperellum (Real IPM strain TRC900) shown in Figure 1b, this fungus grows into the roots as well as around the root, forming a sheath. The association is referred to as an ‘endophytic rhizosphere fungus’. In this case T. asperellum has been shown to firstly increase plant growth and health, secondly improve nutrient uptake for the plant due to the large increase in effective root volume, and thirdly suppress plant pathogens (such as pathogenic fungi and nematodes) that are killed or antagonised by the fungus.

Figure 1: (a) Young chrysanthemum root with root hairs and (b) chrysanthemum root inoculated with Trichoderma asperellum (Real IPM strain TRC900).

This is but one example of numerous beneficial interactions between plants and soil micro-organisms that occur in all natural soils. The main requirement for optimising such interactions is a healthy soil in respect of nutrients, physical conditions and organic matter.

Degrading soil health

The main impacts of certain invasive plant species are the systematic degradation of soil health through:

  • Soil acidification.
  • Competition with other plant species, thereby decreasing above-ground and below-ground diversity.
  • Release of allelopathic* compounds suppressing growth potential of other plants.
  • Changing of the soil microbiome through the alteration of fungal and bacterial associations.

What are allelopathic compounds?

They are chemical compounds released by one plant into the environment, most often soils, that inhibit the growth or germination of seeds of another plant.

A well-known example of an invasive species that has a detrimental impact on soil health, is the black wattle (Acacia mearnsii). Black wattle stands are characterised by the complete disappearance of other plant species under its canopy as well as the distinct degeneration of soil structure and organic matter content.

In Figure 2, examples are provided of soil profiles under black wattle. These profiles are compared to a low potential soil profile (not ideally suited to crop production and prone to poor structure) under maize, that is also an introduced plant, where root development and good tilth is much more pronounced. ‘Tilth’ refers to the tilled condition of soil and the suitability of a soil for the sowing of seeds or plant production.

Figure 2: Soil profiles under black wattle (left) exhibiting poor root development and distinct bare topsoil compared to a maize field profile (right) with good root development and managed bare surfaces.

Although both conditions exhibit introduced plants, the maize field is managed through dedicated fertiliser and soil amendments to boost soil health. In addition, the maize field, although also exemplifying ‘destructive’ land management, is part of a managed landscape that involves fallows and rotations with cover crops to improve long-term soil health and productivity of the soil.

Conclusion

Invasive species require management and eradication under certain conditions. Crop, fruit and fibre production invariably leads to the introduction of alien plants into natural landscapes. Where the plants are controlled, the soil health can also be controlled through dedicated land management and the introduction of regenerative agricultural practices.

These include the minimisation of tillage, establishment of living covers and the improvement of soil biology through amendments and beneficial organisms. In cases where plants have detrimental effects on soil health, the negative impacts will spread to other spheres such as water resources and natural areas.

It is imperative that landowners/managers be aware of such impacts and that they act proactively and with purpose to ensure their long-term livelihoods by applying regenerative agricultural principles.

For more information, email the author at johan@terrasoil.co.za. – Johan van der Waals, PhD Soil Science (Pr.Sci.Nat.), Real IPM South Africa