Compost is mostly the home gardener’s friend, but when used in agriculture it’s wide-ranging characteristics make it unpredictable.
But the large-scale application of compost in agriculture – as an alternative to mineral fertilisers – has a long list of potential benefits, including improving soil health, crop yields and soil carbon content. Using compost could also reduce the amount of food and organic waste entering landfill and creating greenhouse gas emissions.
Getting farmers to change their minds about compost might happen if they were provided with “designer compost.”
A study co-led by the University of Queensland’s School of Agriculture and Food Sciences has found the smart application of compost could boost the annual global production of major cereal crops by 96 million tonnes, or around four percent of current production.
The research is published in Nature Food.
University of Queensland Professor Susanne Schmidt, an author of the study, says “compost is mostly used as a generic product … you buy compost from your local supply and that can be a city council composting or a commercial company. And you don’t really know what you’re getting.”
The characteristics of compost – its carbon and nitrogen content, acidity or alkalinity – can vary wildly depending on the source. How well compost performs is also dependent on other factors like soil type, crop type and cropping method, as well as climate conditions like temperature and rainfall.
This creates uncertainty for farmers about how much or what type of compost to apply, and concern about what it would mean for crop yields.
To address this problem and better understand the science behind the successful use of compost in agriculture, the researchers undertook a global meta-analysis of 256 compost studies, incorporating more than 2,000 observations.
They found the key drivers for crop yield were nitrogen supply, the compost’s carbon-to-nitrogen ratio and the compost pH (whether it was acidic, alkaline or neutral).
These traits vary depending on the source. Wood chips for example would be mostly carbon, whereas household food waste has a much higher nitrogen content.
To address the variability, the researchers have proposed a “precision compost strategy” underpinned by their findings, matching the right compost for the crop and climate.
Schmidt says a precision compost strategy starts from knowledge about the local climate and soil conditions, the needs of a particular crop, and a farmer’s overarching goal, whether that’s higher production, or increasing soil carbon.
The compost can then either be selected for certain traits, or in some cases mixed with mineral fertilisers to achieve the desired outcome.
“We can then formulate this compost from a generic product into a designer product,” she says.
The benefits of doing so are greater in some of the worst soils, Schmidt says. This science-based approach can yield as much as a 40% improvement in food production compared to conventional practices.
The targeted use of compost in agriculture could also help mitigate climate change.
“In Australia alone, more than 7 million tonnes of biowaste ends up in landfill every year where it generates huge amounts of avoidable greenhouse gases and other undesirable effects,” Schmidt says.
“If we repurpose it, we can restore crucial carbon in cropland topsoil.
“There are cost benefits too – diverting just 15,000 tonnes of biowaste could save a local council as much as $2-3 million a year.”
Far North Queensland sugarcane farmer Tony Rossi says his family’s company V. Rossi & Sons had been using precision compost for seven years with great success.
“We’ve been able to almost halve our fertiliser use which is so much better for the environment, and our crop yield is the same,” Rossi said.
Petra Stock has a degree in environmental engineering and a Masters in Journalism from University of Melbourne. She has previously worked as a climate and energy analyst.
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