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Understanding your soils (Part 1): Arbuscular mycorrhizal fungi

Written by Gjizelle Nel (Agronomist)

The most important resource in any crop farming system is the soil. Soil health is therefore essential to ensure crop success, season after season. Soil health is influenced by different factors of which microbiology is one. Understanding the microbial networks in the soil and the symbiosis between the crop roots and these networks could greatly benefit the farming system.

Soil Microbial diversity

Over years of conventional cultivation, we have upset the balance of the soil to such an extent that the biological diversity of soils has been minimized. Tillage practices (such as ploughing and discing), expose soil micro-organisms to damaging environmental factors - such as sunlight and heat. Microbial communities take years to develop, and these communities can be disturbed through tillage practices.

What does arbuscular mycorrhizal fungi (AMF) do?

Arbuscular mycorrhizal fungi, are organisms which enter the roots and penetrate the cortical cells of vascular plants. Arbuscules, vesicles and hyphae are formed in the plant cells. The arbuscules are highly branched, facilitating nutrient and water exchange with the plant. AMF are especially beneficial for phosphorous exchange as they have access to phosphorous that is not accessible to the roots themselves.

Arbuscular mycorrhizal symbiosis refers to the symbiotic relationship between the roots of different plants and the microbial life within the soil. This symbiotic relationship is only evident in the roots of approximately 70% of terrestrial plants, however it is greatly beneficial to these plants. AMF hyphae interlink the different root systems of different plants to form a mycorrhizal network. These hyphae act as elongations of the root hairs (up to 10cm) and enable the plants to acquire nutrients with low mobility in the soil (i.e. P and Zn). The acquisition of these elements is made possible by acid excretions from the fungal hyphae which solubilizes these elements. The fungi also form networks with other microbes in the soil to gain access to other organic forms of nutrients. In return the plant caters for the carbon needs of the fungi.

Soil texture influences many characteristics of the soil. These include the water infiltration rates of irrigation or rainwater, the water holding capacity of the soil, the build-up or leaching of nutrients, the swelling and shrinking properties of the soil and many more.

Stabilizing soil aggregates can result in increased infiltration rates, increased water holding capacity and decreased leaching of nutrients among other things. Although the science is not understood fully, we now believe that an excretion from arbuscular mycorrhizal fungi; known as glomalin-related soil proteins, are responsible for the integrity of soil aggregates and could in a sense be considered the super glue of the soil.

These proteins are water insoluble and resistant to microbial decay. It can last in soils for a period of 10-50 years and only becomes soluble at 121°C. Glomalin-related soil proteins act as protection against the loss of water and nutrients from the fungal hyphae. It is believed that once the hyphae of the fungi stop transporting nutrients, the glomalin-related soil proteins are released from the hyphae and attach to the soil particles around it. This aids in the stabilization of soil aggregates in no-till, undisturbed systems. Glomalin-related soil proteins are therefore essential to combat wind and water erosion and to ensure that nutrients and organic matter in unstable aggregates are not lost. By stabilizing the soil aggregates these proteins also play a role in increased water infiltration, increased water holding capacity close to the rootzone, improved nutrient cycling and reduced compaction. These are the same benefits often seen from cover cropping.

The link between soil microbial populations and cover crops

By using cover crops in a farming system, it ensures that there are living roots within soils for the longest possible time. Incorporating a cover crop after the main cash crop could aid in reduced erosion risk, increased soil health, as well as additional feed for the animal component on the farm (if applicable).

Most plants have mycorrhizal associations, however some crops such as Brassicas (Canola, broccoli, cabbage and cauliflower) do not, which means that they do not aid in producing glomalin-related soil proteins. By diversifying cover crop mixtures, the microbial diversity of the soil can also be increased. This is due to the different relationships of the different plant roots with different soil microbes. The populations of these beneficial organisms can be increased by including different plant functional groups (i.e. grasses, grains, legumes, chenopods and brassicas). Some species have the ability to suppress fungal pathogens and increase the disease-suppressive bacterial populations.

All of these factors work together to ensure a healthy, thriving and living soil, which in turn can assist your main crop to perform to the best of its ability and yield more profitably.

Cover crops also reduce the need for inorganic fertilizers. The application of these inorganic fertilizers has a negative effect on the microbial life within the soil. It would be more beneficial to have living roots for as long as possible. This will allow a build-up of glomalin-related soil proteins which in time would stabilize the soil aggregates and favour the development of well textured soils.

After years of cultivation, biodiversity has almost been eradicated from our soil biosphere. Now we stand at a crossroads, where we can either choose to go down the same old path and face ever increasing soil related challenges, or we can choose to take the time and effort to remediate our soils and ensure many more years of profitability and food sustainability.

4 beginner steps to success:

  1. Decrease tillage

  2. Reduce inorganic inputs

  3. Plant cover crops – living roots for as long as possible

  4. Plant mycorrhizal crops – crops with symbiotic relationships between the roots and soil microbes

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