The fungal-bacterial ratio for soil health

Soil is a complex, integrated ecosystem. Microbiotic populations, essential to soil functioning, are indicators of soil health, says Dr Sarina Claassens.

The fungal-bacterial ratio for soil health
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A gram of soil contains up to a billion bacterial cells and one million individual fungi. The biodiversity of soil exceeds that of aquatic environments and other above-ground ecosystems by several orders of magnitude. Soil biota is the ‘biological engine of the earth’ and microbes contribute significantly to the maintenance of fundamental soil processes and overall soil health.

The biomass, activity and community structure of soil micro-organisms can be used as indicators of soil health because they perform critical functions. The improvement of soil health and agricultural sustainability requires the optimal use and management of the biological and physico-chemical properties of soil. 

Soil health describes the integrity and quality of a soil. It reflects the capacity of soil to respond to agricultural intervention, so that it continues to support agricultural production and other ecosystem services. A simple definition of soil health is the capacity of a soil to function. Healthy soils are key components of successful agriculture because they have the capacity to sustain biological productivity and to maintain plant, animal and human health.

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Soil is a biological system and the micro-organisms that live in the soil environment are essential to the functioning of this system and contribute in many ways to the health of the soil.

Organic material
An important factor in soil health is soil organic matter. This increases plant nutrient availability and water-holding capacity and improves soil structure. Microbial biomass forms the living part of the soil organic matter and is mainly composed of fungi and bacteria.

These micro-organisms play crucial roles in the maintenance of important ecosystem functions, including: nutrient cycling of nitrogen and phosphorus, for example; maintenance of soil structure by aggregation and particle transport; biological regulation of pests and diseases; and transformation of carbon through the decomposition of plant residues and other organic matter.

Both fungi and bacteria are responsible for the decomposition of plant residues and the release of plant nutrients. However, they have different roles in the recycling of nutrients due to their different choice of habitats within the soil and the different types of organic matter they consume. Therefore, the balance of fungi and bacteria in the soil is important for optimal functioning. Organic matter decomposition underpins all other soil functions, so damage at the microbial level is likely to have wide implications.

The ratio of fungi to bacteria can be used to detect detrimental changes in the soil and to prevent further degradation. In general, the higher the microbial biomass, the healthier the soil. There is a shift in dominance from bacterial to fungal biomass in mature ecosystems. In highly productive agricultural soils the fungal-to-bacterial ratio will approach one (F/B=1), meaning that the biomass of the fungi and bacteria is approximately even. In contrast, healthy grassland systems tend to show bacterial dominance.

In other words, while it is difficult to attribute a specific ratio to a specific soil in order for the soil to be considered ‘healthy’, it is possible to monitor fungal and bacterial composition in order to detect adverse changes in the soil environment.

Diversity above and below
Soil properties such as pH, temperature and soil moisture affect the composition and activity of the microbial population. It has been found that a low pH is associated with fungal dominance, whereas a high pH might be related to bacterial dominance. Fungal and bacterial activity follows seasonal fluctuations, with a peak during optimal conditions of temperature and soil moisture. Of greater concern is the effect of human-induced influences on soil.

Fungal-to-bacterial biomass is affected by a number of soil and agricultural management practices. These practices disturb abiotic (non-living) and biotic (living) components of soils with a consequent decrease in soil health. Fungi and bacteria differ in their responses to agricultural management, with fungi being more sensitive to these changes. For example, tillage destroys large amounts of fungal hyphae and as a result, the stability of soil aggregates is compromised.

An increase in the intensity of tillage will lead to a decrease in microbial diversity and biomass, while no-tillage enhances fungal biomass with a consequent improvement in the quality and quantity of soil organic matter. Other management practices that may negatively affect fungi and bacteria include the application of pesticides, lime and fertilisers.

As the above-ground ecosystem complexity decreases, the soil ecosystem diversity will also decrease. Thus the diversity of soil micro-organisms in a crop monoculture will be lower than in undisturbed soils. Crop rotations help to increase diversity, fungal-to-bacterial ratios and accordingly soil health. To increase the activity and biomass of fungi and bacteria, the addition of organic materials and maintenance of high soil organic matter is of great importance.

When the microbiological component of soils is considered, direct and indirect benefits for agricultural production can follow. These include economic and environmental benefits. More efficient decomposition and nutrient cycling processes, water storage and movement translates into reduced input costs. Yield and crop quality may improve, especially through controlling pests and diseases and enhancing plant growth.

Reduced use of agro-chemicals will prevent pollution and land degradation and ensure better maintenance of soil structure and cation exchange capacity. It has been observed that less intensively managed agro-ecosystems, such as those managed by no-tillage practices, more closely resemble natural soil ecosystems. If soil health is enhanced, the improved structure of the soil results in greater water infiltration, less runoff and erosion, and reduced incidence of flooding and sedimentation.

Soil is a complex, highly integrated system, and soil health depends on a number of interrelated factors. An impact that alters any one function or component will inevitably alter the dynamics of others. The health of a soil is more than the sum of the contributions of various components and soil management should aim to take different aspects of soil health into consideration.

Phone Dr Sarina Claassens of the School of Biological Sciences, North-West University, Potchefstroom Campus, on 018 299 2321 or e-mail [email protected].