The fourth Talkin’ Soil Health Conference was held at York in March, bringing together soil health advocates, practitioners and researchers from across the country and beyond.
The conference’s keynote speaker was soil health advocate Joel Williams from Integrated Soils, who spoke about increasing diversity in our farming systems to improve properties such as soil carbon, biological activity, soil structure and nutrient availability.
Joel believes that plant diversity has a positive effect on the biomass and activity of micro-organisms (microbes). Diverse crops and their organic inputs may support a more diverse community of microbes and potentially increasing soil functions such as nutrient cycling and soil aggregation.
A more diverse community of microbes is likely to increase microbial biomass, which are constantly dying off and becoming a source of soil organic carbon. Diverse crops could also lead to a direct increase soil organic carbon if total plant biomass increases, particularly if this occurs below ground in the form of roots and root exudates. So, managing soil and plants to maximise root growth, such as removing subsoil constraints or allowing rest periods between grazing so newly grown leaves can fuel root growth, may therefore be beneficial to soil carbon maintenance.
The key take-home messages from Joel were that increasing plant diversity and maximising root abundance and mass are important for soil health and ecosystem function. He suggested that any step towards increasing plant diversity was helpful, including simply changing from monocultures to two-species crops. He suggested running trials on intercropping with legumes to find what species work well together, and encouraged peer to peer communication to maximise progress.
Plant diversity is not the only means of increasing the diversity of organic inputs and exudates. Another way is to apply amendments such as compost extracts, practiced by Nick Kelly at Newdegate. At the event’s field day prior to the conference, Nick spoke about using extracts, and has even changed his source of compost to diversify the input and introduce or favour less abundant groups of microbes. Extracts can be applied at lower rates compared to hard compost. Nick applies one kilogram of compost to make up 80 litres of extract, which is applied at a rate of 70-80 litres per hectare at sowing and 50 litres per hectare as a foliar spray.
Nick’s use of extracts complements a diverse mix of cover crops, which are now being transitioned into a mix of summer active perennial pastures that become dormant during winter cropping. He believes that complex plant mixes fire up soil biology and improves yield.
Plants are considered central to building soil biological fertility, described by Emeritus Professor Lyn Abbott from the University of Western Australia (UWA) as a slow process that increases resilience, but is difficult to measure. She agrees that plant diversity is important, even if this takes the form of increased crop rotations, and said that biological activity around plant roots, whether living or dead, is central. Professor Abbott also noted that rates of amendments don’t necessarily need to be high to increase biological activity, citing a trial on South West dairy farms indicating higher biological “activity” in a compost treatment of three tonnes per hectare compared to six tonnes.
Professor Abbott said that amendments like compost, biostimulants and manure can influence more than one soil property and vary between having longer term effects (biochar, compost) and short-term effects (biostimulants). Importantly, product variability makes it impossible to assume different brands of the same amendment will act in the same way.
Variability in amendments requires farm trials with controls in place to assess their effect on particular soils. Running test strips across paddocks was recommended by Associate Professor Frances Hoyle from UWA, who urged farmers to measure something so that an effect can be assessed quantitatively.
Professor Hoyle spoke about the challenge of measuring soil health and said while critical values are difficult to develop, it was possible to use a traffic light system to categorise data such as good, marginal and bad to indicate overall status. She suggested some short-term indices for soil health (http://soilquality.org.au/factsheets), including:
- Total carbon,
- Soil nitrogen supply,
- Disease load,
- Soil pH at 0-10 cm, 10-20 cm and 20-30 cm,
- Electrical Conductivity,
- Water repellency, and
- Bulk density (compaction, soil structure).
The results can indicate where management efforts need to be allocated.
Professor Hoyle said that micro-organisms in the soil were responsible for lots of ecosystem services that we desire, such as nutrient cycling, aggregate stabilisation (soil structure) and contaminant degradation. These services increase our soil’s resilience to withstand stress and interact with other soil properties to support production.
Professor Hoyle’s research in Western Australia have shown that building soil organic carbon is difficult and should be approached with a long term perspective. Organic carbon is determined largely by climate (which affects net primary productivity and decomposition), soil type (with clay soils protecting more carbon from microbial breakdown compared to sands), and to a lesser extent management. While organic inputs can increase soil carbon, it is initially in a short-lived form (labile or particulate carbon), most of which is utilised by microorganisms as a food substrate and subsequently lost to the atmosphere as carbon dioxide. One long-term research site in WA indicates approximately 15% of carbon input was retained, though this will vary between sites.
Any gain in labile soil carbon pools can be lost if “additional” carbon inputs are stopped. In this case soil organic carbon levels will slowly return to an equilibrium, determined mostly by climate and soil type. The influence of rainfall on net primary production explains why a sandy soil in the high rainfall zone has much more carbon that a sandy soil in the eastern wheatbelt – with inputs of organic matter being significantly higher and supporting a longer proportion of the year under plant cover. With predictions of future climate drying (less and more frequent rainfall, higher temperatures), Professor Hoyle has suggested it is critical to consider whether future climate systems may start to drive soil organic carbon levels down.
The soil health indices described by Professor Hoyle were also mentioned by Buntine farmer Stuart McAlpine. However, Stuart reminded us that there are some other simple methods to assess soil health such as a shovel to dig up plants, observe the roots and smell or feel the soil. Stuart surely won the award for quote of the day:
“If your agronomist doesn’t have a shovel, get another agronomist!”
The common themes were that soil is a critical asset to farming, delivering key services such as nutrient cycling, soil structure and contaminant degradation that underpin agriculture. Some basic steps to protect the resource are essential, such as maintaining groundcover to limit erosion of topsoil, nutrients and soil carbon. While there are different ways to improve soil such as plant diversity and organic amendments, outcomes are difficult to measure and gains in soil carbon are not always observable. However, measurement of some indicators and regular observation combined with controlled trials that put observations into perspective, can help to provide an evidence base and direct farmers towards a more resilient system for their soil resource.
Presentations are available at https://www.soilhealth.com.au/presentations-
Article updated 29 April 2020