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An Introduction to Soil Biology - Part 2: Compost and Compost Tea

In part 2 of his series on soil biology, Joel Williams, Agronomist at Laverstoke Park Laboratory Services, examines the role of microbial activity in the soil.

In part 1 of this article, we introduced the soil health triangle as a fundamental principle of understanding how farming soils function; and how soil biology, chemistry and physics are all of equal importance when it comes to soil management. Filling the gaps in the soil biology side, we considered some of the key micro-organisms that dwell in the vastly diverse underground ecosystems of our agricultural soils.

The various roles of bacteria, fungi, protozoa and nematodes were highlighted and a particular emphasis on the importance of the fungal to bacterial ratio in relation to different crop types was discussed. Just like it is common place for farms to test the soil for its nutrient content (chemistry); the importance of microbial assessment was also highlighted; but the question remains, once microbial deficiencies and microbial balance is determined – what can we do about it?

Figure 1: The surface of compost – note the complete colonisation and coverage of microbial biomass (bacteria, fungi and a large yeast spore). © Soil Foodweb Inc.

Managing Microbes

There are numerous ways to stimulate microbial activity and growth in soils. The three main strategies used to adjust microbial imbalances is via the application of (i) compost, (ii) compost teas and (iii) food sources. Each of these three approaches would be an essay in itself so for the purposes of this article, only the introductory principles of each will be discussed.

Compost

Although people often think of compost as a source of organic matter and plant nutrients; in fact, good quality compost is bountiful with a diverse and thriving community of billions upon billions of beneficial micro-organisms – compost is alive! In particular, bacteria, fungi, protozoa and nematodes (bacterial and fungal feeders and predatory nematodes) can all be present. Figure 1 illustrates the living microbial biomass that is present on the surfaces of good quality compost.

Application of this living biofertiliser can introduce a microbial workforce into the soil environment where they can perform their various beneficial functions. BD500 (horn manure) and CPP (cow pat pit) are also very biologically active substances and can introduce living microbes into the soil in the same way. However, “composts ain’t composts” and many factors influence compost quality and the microbial activity contained within. Oxygen, temperature, moisture and food balance are all of primary importance.

Compost by definition, is ‘aerobic’ decomposition of organic materials into stable humus. This means that oxygen is required for this process as beneficial microbes require this oxygen for their survival. Anaerobic (reduced or no oxygen) decomposition encourages anaerobic organisms and some of these microbes produce a host of by-products that can be harmful to plant growth and also reduce the nutrient content of the compost. Hence, turning a compost pile may be necessary to enable oxygen diffusion into the pile.

However; a gentle, passive turning/mixing action is most beneficial as excessive and aggressive turning and disruption to the pile will reduce microbial action; in particular, fungi. Microbes have an ideal temperature threshold for their optimum growth. Temperatures too hot or too cold will limit microbial growth. If the pile becomes too hot (above 65°C) microbial activity can decline and there will be a necessity to turn/mix the pile to cool it down. The moisture content of the pile is also important – if the pile is too dry, decomposition will slow down while if the pile is too saturated, oxygen will not diffuse throughout the pile and it will turn anaerobic.

If you squeeze a handful of compost, it should be able to form a ball and a small drop or two of water should drip out but not pour out. Add water if it feels too dry or alternatively add more carbon in the form of straw or woody materials to dry the pile out. The balance of ingredients that is used when making the pile is also important. A suitable balance of carbon to nitrogen (browns and greens) is required for optimum compost production.

Generally speaking; higher carbon inputs (browns) include woodchip, sawdust, straw, tree prunings, paper, cardboard while higher nitrogen inputs (greens) include manures, fresh green plant material, fresh grass clippings and green waste. A good starting recipe would comprise 1/3 greens and 2/3 browns but this will depend on the exact final C:N ratio of the mix. Table 1 demonstrates the effect of different compost organic inputs on microbial biomass after an 8-week production cycle. Ultimately, the fungal/bacterial ratio is the key consideration if matching compost quality to crop type (refer to Part 1 of this article).

• Compost 1: 40% Farm Manure, 10% Wood, 50% Green Waste
• Compost 2: 35% Wood, 65% Green Waste
• Compost 3: 20% Zoo Manure, 20% Wood, 40% Green Waste
• Compost 4: 50% Wood, 50% Green Waste

Compost Tea

Another means to increase soil biological activity is via the application of liquid compost extracts also known as compost tea. There are different types and variations of compost tea and different methods of production. The basic recipe is produced by mixing compost with water and extracting microbes into solution. Other organic compounds and soluble nutrients are also extracted during this process. The extracted microbes are aerobic organisms or ‘oxygen loving’ and they require an oxygen rich environment for optimum growth. Consequently, aerating the liquid after extraction by bubbling air into the water is required (much like a fish tank/aquarium aerator).

Figure 2: The leaf surface before (left) and after (right) the application of a compost tea. Note the higher level of glowing green surfaces on the right hand side indicating higher microbial colonisation (particularly fungi in this instance). © Laverstoke Park Laboratory.

This aeration can be achieved with an air blower, venturi pump or via circulation through a Flowform™. To this aerated compost-water mixture, specific food sources are also added to feed and multiply the extracted workforce. This ‘brew’ is then left aerated for 24-48 hours, permitting time for the microbes to feed, grow and multiply.

Experimental work at Laverstoke Park Laboratories has highlighted that closer to 48 hours is required under colder conditions as the microbes will grow more slowly. The brewed, finished compost tea can then be used neat or is often diluted with additional water and applied as a foliar spray or soil drench to repopulate the agroenvironment; thereby, improving soil health and plant growth. Figure 2 depicts a leaf surface before and after the application of compost tea on the Laverstoke Park vineyard. A minimum of 70% microbial coverage is required for adequate disease suppression.

There are many specifics and particularities when it comes to producing quality compost tea which are beyond the scope of this introductory article. I would encourage readers to do further detailed reading on compost tea before they venture into production – there are thousands of pages of reading via Google.

Microbial Food Sources

The application of specific microbial food sources to the soil is also a useful method to increase microbial growth or adjust microbial balance. Applying or incorporating carbon substrates to feed soil life is an ideal way to stimulate the native organisms in contrast to introducing new populations via compost or compost tea. A combination of both strategies is an ideal integrated approach to maintaining soil health and sustaining a healthy microbial population.

Feeding soil life can be achieved via the application of organic materials such as compost, stubble residues, green manures and farm yard manures. As these organic materials decay they feed soil micro-flora and contribute to enhance the fertility of the soil from a biological, chemical and physical perspective. For a more rapid response, the application of highly available food substrates can also stimulate microbial growth.

Microbial biomass is measured in soil, compost and compost tea at the Soil Foodweb lab at Laverstoke Park, Hampshire, UK.

Bacteria prefer simple carbon compounds such as sugars, molasses and fulvic acid while fungi prefer more complex foods such as fish hydrolysate, seaweed extracts and humic acid. Plant teas such as comfrey, nettle or equisetum teas also contain a host of organic compounds beneficial for feeding hungry microbes. These available substances are not long lasting in the soil and will usually rapidly breakdown providing only a shorter term benefit.

Similarly, the balance of foods also influences the types of organisms that grow during the production of compost (as illustrated in Table 1) and also in compost teas. Therefore, the addition of fungal or bacterial foods to soil, compost or compost tea can influence the growth of the required organisms.

Benefits of Microbial Living Soils

With such vast diversity of beneficial microorganism that exist in soils comes a multitude of roles and functions. Among their many services, microbes decompose organic materials including manure and plant residues; fix atmospheric nitrogen and solubilise soil minerals into plant available form; store and recycle soil nutrients; enhance soil aggregation and porosity; degrade pesticide residues; build soil humus and hence increase nutrient and moisture retention; prey on crop pests and even be consumed themselves by higher level predators from the intertwined soil food web.

These benefits to soil and crop health can be achieved via an understanding and careful management of the life in the soil. The organic and Biodynamic movement is well and truly underway and will continue to evolve and gain recognition in mainstream agricultural farming practices due to the regenerative rather than destructive methods used. With the future of high input, conventional agriculture under scrutiny, the low input nature of organic and Biodynamic practices is certainly a more attractive option and may hold the key for maintaining soil fertility and sustainable farming.

An increasing number of farmers are integrating other technologies such as compost tea within these farming approaches, which serves to highlight the evolution and the future potential of organic and Biodynamic farming throughout the world.

For more information on our laboratory services please contact +44 1256 772 815; This e-mail address is being protected from spambots. You need JavaScript enabled to view it or www.laverstokepark.co.uk

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