2017 - Sustainable farming for biologically active soils

In this week’s hub update, guest blogger, Jamie Stotzka, Science and Project Manager for PlantWorks Ltd, discusses the role and importance of soil microbes to plant and soil health.

Healthy soils are the foundation and future of sustainable farming. An abundance of beneficial soil microbes, such as symbiotic fungi and bacteria, is the cornerstone of this foundation.

The services rendered by soil microbes are critical to plant health: they include solubilisation, acquisition and transport of nutrients, protection from stresses such as drought and pathogens, as well as boosting the plants’ immune system and water use efficiency.

“Plants colonised by arbuscular mycorrhizal fungi are healther and significantly more efficient.”

One particularly important and well-studied group of microbes is that of arbuscular mycorrhizal fungi (AMF). This remarkable fungus associates with over 80% of all land plants and 90% of all crop plants. Fossil evidence proves the existence of this symbiosis for over 460 million years. By growing into the root, as well as accessing large areas of soil around the plant, the fungus effectively increases the uptake surface area of plant roots by up to 700 times. Additionally, plant defence mechanisms are improved by the partnership. Put simply, plants colonised by AMF are healthier and significantly more efficient at collecting water and nutrients from the soil.

“Conventional farming techniques can disrupt ecosystem stability and deplete soils of beneficial biological components.”

Plant Growth Promoting Rhizobacteria (PGPR) are another group of important soil microorganisms. This type of bacteria has the ability to fix atmospheric Nitrogen and unlock soil bound Phosphorus, allowing subsequent transport to host plants by AMF. Additionally, PGPR further support a robust plant immune system for healthier crops and produce phytohormones such as auxins and cytokinins, aiding growth and development of plants.                                                                                                                                                             Conventional farming techniques can disrupt ecosystem stability and deplete soils of beneficial biological components. Such detrimental techniques include soil disruption (ploughing), fallow periods and cropping with non-mycorrhizal plants, as well as over-application of agro-chemicals.

The pillars of Conservation Agriculture, including no-till practices, continuous crop cover and crop diversification are in direct support of the maintenance of a healthy balance of soil biology. Nonetheless, certain conditions can disrupt soil ecosystem balance and biodiversity. This includes the occasional need for ploughing to address for example compaction or problematic weed populations.  Likewise it may be desirable or necessary to grow non-mycorrhizal plants, either as main cash- or cover-crops. Plants that are unable to form the symbiotic relationship with AMF include those belonging to the families of Brassicaceae and Ameranthaceae amongst a few other minor groups. Oilseed rape is one important crop that falls into this category, along with mustard, a frequently used brassica cover crop. Other examples of commercially valuable non-mycorrhizal crops are all types of beet, which belong to the Goosefoot family of plants (Ameranthaceae). Some agro-chemicals and in particular fungicides are known to negatively impact AMF.

“Using a wider range of plant species in cover crops will enhance microbial biodiversity.”

Commercially available inoculants of AMF and PGPR such as the ‘Smart Rotations’ product range by PlantWorks Ltd. can help to alleviate problems caused by any of the circumstances described above to build and restore microbial balance in arable soils. Inoculation should take place at time of planting and in combination with a suitable host plant. In order to gain the most benefits from this type of inoculation, careful crop management is recommended. Crop rotations that support the maintenance of AMF should be planned along with the use of cover crops and minimal soil disturbance. As a general rule, using a wider range of plant species in cover crops will enhance microbial biodiversity. Cover crop mixes that include a number of different plant species, and particularly legumes, are most effective at building up and restoring AMF communities. The use of inoculation in cover crops can help to not only restore soils after fallow periods or non-mycorrhizal crops, but can also serve to build up soil biology for follow-on cash crops.

When planting a non-mycorrhizal crop, such as oilseed rape, inoculation with a preparation of PGPR can help to maintain biological diversity as well as supporting crop growth and productivity. To speed up recovery and re-establishment of AMF this should then be followed by a highly mycorrhizal crop, with the option of treating this with a mycorrhizal inoculum.

Soil ecosystems with a large amount of microbial biodiversity are the best foundation for successful and sustainable farming. The use of conservation farming techniques coupled with carefully planned crop rotations and quality inocula can help to maintain and build such balanced ecosystems.

Cavagnaro et al, 2015

Water and nutrient uptake effects of AM fungi. These benefits are further enhanced in the presence of plant growth promoting bacteria.