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Published: 10 Jun 2022

Soil biological health underpins soil functioning and can do a power of work in productive agriculture. But, in most conventional cropping agricultural systems there will often be significant disturbances to the soil.

In an agricultural context, soil biology has several key roles that support our food systems.

Rainwater infiltration and storage is dependent on soil biology. A healthy soil with a good carbon content, will allow more water in, and will hold on to it, like a sponge, for crops to use as they require.

Many aspects of crop growth and health are related to soil structure. Good structure is built and maintained by the microbiology of soil and is great for plants and can even reduce machinery use through requiring less intensive cultivation and lighter machines.

In conventional cropping agriculture most of the crop’s bulk nutrient requirements are applied via synthetic fertilisers. However, only a fraction of the applied nutrient is used by the plants. A healthy, high functioning soil can improve this efficiency and prevent nutrients leaching into waterways. The soil community also cycles the many micronutrients and trace elements essential to the crop.

Cultivation, synthetic inputs, fungicides, and pesticides, monoculture plant stands, and the movement of vehicle traffic are features of conventional agriculture, and generally have a negative impact on soil biology.

Theresa Chapman, also a soil carbon Field Officer with AgriProve, is exploring soil health and the impacts of mixed cover crops in her PhD subject Diversity and Function of Cover Crops.

“Our vegetables and grains feed Australia and the world. We have some great soils – and others with certain challenges – and fantastic land and business managers working with complex food and processing chains,” Theresa said.

“These valuable commodities rely on our soil resource, and many growers and businesses are working towards either reducing detrimental practices or reducing their impact through better recovery.”

Cover cropping is used widely across Tasmania, with annual rye grass being the most widely used.

“It’s cheap, readily available, easy to terminate and breaks down fast. It grows a beautiful fibrous root system, provides excellent soil protection, and looks gorgeous,” Theresa said.

“But resilient, productive natural systems don’t do monocultures. And we may be missing out on a range of benefits that a more diverse cover crop could deliver.

“A diverse stand of plants differs from a monoculture in having more diverse plant root architecture, a more complex and possibly larger rhizosphere, increased microbiological diversity, and an ability to use more of the available resources and niches via complementarity.”

These features are potential mechanisms for services that have been linked to diverse cover crops such as increased disease suppression, improved nutrient cycling, carbon dioxide draw down and productivity (above and belowground biomass).

There is strong evidence that cover crops can:

  • Reduce topsoil erosion
  • Increase soil carbon, or at least prevent its decline
  • Improve soil structure
  • Increase plant available nitrogen and phosphorous
  • Supress weeds and pathogens
  • Improve water dynamics
  • Increase yield of following commercial crops

So, can a cropping system access any of these benefits from a short window of plant diversity, during the cover crop window?

To address this question from a northern Tasmanian context Theresa has established two trial sites – one on a ferrosol of north-west Tasmania’s Forth river valley, at the TIA Vegetable Research Facility. The second is in the northern midlands on a duplex soil – which is a challenging soil type, also known as texture contrast soil.

“It has a reasonably shallow topsoil, and the subsoil is clay rich and often somewhat impenetrable. But these soils are important because they dominate a lot of the areas of Tasmania where new irrigation systems are facilitating more intensive cropping.”

So, should a diversity of cover crops be used?

Backed by her reading, Theresa said diversity can have a positive effect on many soil health outcomes, but she can’t link that to her field trial yet.

“Soil is difficult to study, time consuming, lots of variables, that is part of the reason we still know so little about it.”

This article was published in Tasmanian Country newspaper on 10 June 2022.