Living within the uppermost 5cm of soil and invisible to the unaided eye lies a bustling metropolis of a vital yet unsung motley crew of microorganisms.
Working quietly and without reward, these microscopic residents are central to soil health. They orchestrate a delicate balance that sustains life above and below the surface.
Their silent work, uncelebrated but essential, is a testament to nature’s intricate design and services. It has been said that to restore our soils, we need to feed the microbes, and that the measure of soil health is its ability to support a diverse and productive ecosystem for these microorganisms.
Just what do these soil microorganisms do?
First, they help plants acquire essential nutrients. Bacteria are among the most common soil microorganisms. One bacterial group, diazotrophs, can convert atmospheric nitrogen into a form that plants can use directly. In other words, having these bacteria in our soils provides “free” nitrogen fertiliser for our plants, literally out of thin air. This natural process is called nitrogen fixation and can help minimise the use of chemical fertilisers.
Furthermore, some fungi and bacteria can solubilise two other important plant nutrients: phosphorus (P) and potassium (K). These microbes can secrete organic acids and enzymes, such as phosphatases, to break down the complex compounds of insoluble P and K into available forms for plant uptake. This is also a natural process, particularly important in our acidic tropical soils, where P and K are often unavailable to plants.
Secondly, beneficial soil microbes secrete plant growth regulators or phytohormones to enhance root growth and promote water and nutrient uptake.
Auxins, such as indole-3-acetic acid, are a common phytohormone responsible for this. Plants with established and vigorous roots will be more stable and resistant to lodging, and extended roots will be helpful in acquiring more water and nutrients, making them more tolerant to drought and other environmental stresses.
Thirdly, some fungi and bacteria are effective decomposers of organic matter. Adding organic matter to soils is fruitless if the organic matter does not decompose. These soil microbes help achieve this goal.
Cellulolytic bacteria secrete cellulase enzymes to hydrolyse cellulose, a primary compound in plant cell walls, thereby accelerating the decomposition process.
Fourthly, beneficial soil microorganisms can serve as biological agents that reduce the incidence of pests and diseases. Some soil bacteria and fungi can produce specific compounds that inhibit or disrupt the life cycle of insect pests, and similar mechanisms can be found for pathogens.
This group of soil microbes has several advantages over chemical alternatives, one of which is their high specificity. This specificity minimises their impact on non-target organisms and the plant host, making them a more selective and often the preferred option.
Finally, soil bacteria and fungi secrete polysaccharides and biofilms, which bind soil particles and stabilise the soil.
Earthworms, too large to be called microorganisms, are nevertheless a part of the soil biota that is essential for soil health.
They break up compacted soil and increase soil porosity through their movement in the soil, and their casts are nutrient-rich and feed microorganisms. The sight of earthworms in soil is to be celebrated because only healthy soils are able to host them.
So, what can gardeners do to encourage beneficial soil microorganisms?
Introduce inoculants of beneficial microbes
Numerous microbial-based products, including effective microorganisms (EM), are available for online purchase.
They comprise many beneficial microbes, such as Bacillus spp., Azospirillum spp., Rhizobium spp., and Glomus spp., which can be applied to soil.
Recycle plant residues as mulching
Mulching helps preserve soil moisture and increases the population of soil microbes. Instead of discarding garden waste, it can be recycled through composting or mulching.
Plant legumes
Legumes, such as Arachis pintoi (yellow peanut, yellow glory, perennial peanut, or pinto peanut) and Trifolium repens (microclover, white clover, or lucky clover), can be planted.
They are low-lying plants, and they are attractive and easy to care.
They provide multiple benefits, namely supplying nitrogen to the soil through biological nitrogen fixation, preserving soil moisture, reducing weed growth, and protecting bare soil from erosion.Add compost, vermicompost, and organic fertilisersComposted plant and animal residues or organic fertilisers will help to encourage soil microbial growth.
Minimise chemical use
Excessive or unnecessary use of chemicals (fertilisers and pesticides) reduces soil microbial populations. Soil microorganisms form the backbone of soil health.
Embracing practices such as adding beneficial microbes to soils, recycling plant residues, planting legumes, and minimising chemical use can foster a thriving ecosystem beneath our feet.
Adopting microbe-friendly practices is not only beneficial for the garden, but also an investment in the future health of the soil and the environment.
Dr Ali Tan Kee Zuan is a senior lecturer in the Dept of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. His field of expertise lies in soil microbiology. All views expressed are entirely his own.
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