After many of us have just experienced an unusually wet couple of Winters which brought about a unique set of challenges, now is the time to reflect on what did and didn’t work and what we want to do differently with the Winter forage and cropping season fast approaching, while keeping in mind that no two seasons will be the same. This encourages us to consider the importance of building adaptability and resilience in our farming systems so our environment and finances can handle what comes our way.  

While we consider our fertility programs for the Winter crops, we encourage farmers to keep the long-term outcomes and goals in mind while deciding on the most appropriate management strategies for the close of one growing season and the opening of another. I have summarised an article and included number of direct quotes below (highlighted in green) which provide a very interesting insight into the importance of soil carbon and biological populations as we strive to build long term resilience and productivity in our farming systems. 

The article ‘Understanding soil biota and biological functions: Management of soil biota for improved benefits to crop production and environmental health’ delves into the different roles of soil biota, and how we can use this knowledge to support and enhance biological processes in the soil to improve overall soil health, productivity, and profitability in a changing climate.

We can no longer assess the fertility of our soils through the chemical/nutritional profile alone without acknowledging the biological and structural characteristics. Any factor that influences one will inadvertently influence the others, and so we need to consider the complex interrelations where the individual components (which are also complex within themselves) should not be considered or addressed in isolation. The purpose of this article is to bring to our attention the importance of soil biology when we are considering things like nutrient cycling, fertility programs, crop selection and disease suppression in the upcoming season, and how we can support these biological populations to help us on our sustainable cropping journey. 

‘The successful functioning of many soil biological processes requires a balance of biota interactions in a complex soil biota community.’

Every one of our management decisions will have a positive, negative or neutral impact on soil biology, so it is our job to understand the impacts/consequences of what we’re doing so we can make informed decisions that are in alignment with our long term environmental and financial goals. FARM Agronomy like to say that every tool is in the toolbox, but we need to be aware of the short and long-term impacts of using any one of those tools.  

For example, tillage-induced shifts in the fungi:bacteria ratio influences the rate of organic matter decomposition and nutrient availability. Reduced tillage systems support a fungal-based food web (accumulator organisms) whereas conventional tillage systems support a bacterial-based food web.’

An important place to start is understanding the role of biological diversity in our pasture and cropping systems (see table below for more specific information on biological populations and their role in a healthy soil). 

Microbial biomass and organic matter:

‘Organic matter in soil is the most important fraction that supports microbial populations, especially the biologically available portion of soil organic matter. Microbial biomass (MB), the living component of soil organic matter, constitutes 2-7% of the organic carbon in soils. Microbial biomass acts as the engine for organic matter turnover and nutrient release, but it is dynamic and living and thus is more sensitive to management practices than total soil organic matter.’

Microbial biomass and nutrient cycling/availability: 

‘Microbial biomass is a storehouse of plant essential nutrients. For example, nitrogen levels in microbial biomass range from 15 kg to 150 kg N/ha. Microbial biomass also holds 5-15 kg of sulfur and 10-45 kg phosphorus per ha. Nutrients held in microbial biomass are not prone to leaching, are tied up only temporarily, and are released for plant uptake as a result of predation by microfauna and the death of microbes during soil drying.’

This is an extremely important concept to understand – if we want to get out of the cycle of relying on synthetic nitrogen (and phosphorus) inputs, then we need to focus on building organic matter and creating an environment that supports microbial diversity. This will in turn significantly improve nutrient cycling and availability for our pasture or crop. In addition to this, these nutrients aren’t subject to the losses we experience with synthetic inputs from volatilisation (gaseous losses) and leaching.   

Microbial biomass and carbon cycling:

‘It is the interactions between microorganisms and organic matter in the soil that largely determine the fertility and overall quality of the soil. Soil biological function in most Australian cropping soils, particularly southern and western Australian dryland cropping soils, is regulated mainly by the amount of available carbon. Therefore, it is extremely important to use farm management practices that maintain organic matter levels, especially biologically available organic matter, in our soils.’

‘Plants are the major source of available carbon for biological activity, so soil biodiversity and biological activity depend on the quality and quantity of carbon inputs from plants, through root exudation and above- and below-ground plant residues, and plant-induced changes in soil physical and chemical properties. Pastures are composed of mixtures of plant types (legumes, grasses, C3, C4) so are considered to have a greater potential to influence diverse biological processes. However, the availability of carbon in grazed systems is mediated strongly by grazing management, due to above- and below-ground plant growth in response to grazing.’

Plant growth promotion:

‘Rhizosphere (root zone) microorganisms have one or more specific associations with plants that influence plant growth. Among the many mechanisms associated with plant-rhizosphere microorganism interactions, the production of biologically active metabolites is one of the most important ways that rhizosphere microbiota influence plant growth.’

Disease suppression/biocontrol: 

‘Disease suppressiveness of soil is the ability of a soil to suppress disease severity even in the presence of a pathogen, host plant and favourable climatic conditions.For the disease suppression known as ‘general suppression’, the inhibition of pathogenic populations is related to either the activity of the total microflora or diverse microbial-faunal interactions. The increase in disease suppression against a range of cereal root diseases, documented at Avon SA, was related to increased soil carbon inputs from stubble and roots derived from higher yielding crops and greater cropping frequency.’

‘Increased carbon inputs result in changes to the composition and activity of the soil microbial community over time. These changes result in greater competition for soil resources that, along with predation and inhibition of pathogens, lead to increased suppression of many soil-borne fungal diseases. All soils have the ability to suppress soil-borne root diseases to some extent through the activity of soil microbes, so disease suppression is not an absolute characteristic but a continuum from highly suppressive soils to poorly suppressive (disease conducive) soils.’

Management impacts on microbial biomass: 

  • Direct effects on the populations
  • Changes in the soil habitat (e.g., physical and chemical properties), microclimate and carbon (energy) sources
  • Influences on above ground productivity and community composition. Crop rotation influences soil biota directly by choice of plant types and indirectly by associated agronomic practices. 
  • Some of the agronomic practices that impact on populations of biota include
    • Stubble retention
    • Tillage
    • Application of herbicides
    • Insecticide and fungicide application
    • Application of manures or fertilisers
    • Application of chemical amendments or waste products
    • A number of papers have been published describing the effects of management practices on soil biota even for Australian conditions.

Agrochemicals: 

‘With increased adoption of stubble retention and reduced till practices and the introduction of new herbicides, herbicide use will remain an essential practice in the near future. Non-target effects of herbicides on soil biological activities may cause undesirable effects on essential transformation processes such as reduced nitrification and nitrogen mineralisation, or result in unexpected damage to crops through increased diseased incidence.’

The article mentioned that different herbicides had different effects on soil biota and biological processes, some had considerable negative consequences while others didn’t appear to impact soil biology negatively at all. 

‘Soils with a healthy biota could recover from short-term negative effects of herbicide application. Appropriate use of herbicides could be less destructive to soil biota if management practices that improve biological activity are promoted. Landholders need to modify management practices to avoid application of specific herbicides. An appropriate recovery period for soil biota should be allowed between herbicide applications.’

Conclusion: 

This season, we encourage you to consider the impact of your management decisions on the incredibly important life under our feet. These could consist of what inputs you use, how and when you apply them, crop rotations, inclusion of cover crops, species diversity, disease management, ground cover, row spacing, grazing management, herbicide applications and many others. Soil microbes rely on living root systems and soil carbon/organic matter as a food source and home and striving to increase both things in our soil is fundamental to building resilient systems and reaching our long-term financial and environmental objectives.   

Appendix: 

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