In-depth look at biological amendments
GroundCover™ Issue: 137 November - December 2018 | Author: Andrew Cooke
CSIRO and the University of Western Australia have carried out the biggest-ever study of the use of biological soil inputs in Australian grain growing
A comprehensive three-year project testing the effectiveness of biological amendments such as microbial inoculants, biostimulants and alternative fertilisers in Australian broadacre grain cropping has found limited direct positive effects from these soil inputs.
The GRDC-invested joint CSIRO and University of Western Australia study is believed to be the biggest controlled examination of a wide variety of biological soil inputs in the Australian grain-growing context. It involved a thorough review of current literature; laboratory testing and glasshouse screening of more than 60 inputs including manures, humates, composts and biochars to quantify their chemical variability; field testing of 38 inputs across five states to study their efficacy and impact on soil microbial communities; and glasshouse experiments to probe the mechanisms underlying biological input modes of action.
“The use of biostimulants in agriculture has been estimated to be growing at an annual rate of 12.5 per cent, with the global market set to reach US$2.24 billion by 2018. The microbial inoculant industry is also rapidly expanding,” the project’s final technical report says. “It is clear that while many of these inputs have been available for a long time, questions about their effectiveness and return on investment remain.”
Despite finding wide variability in biological inputs, both within and between product classes, the report says “few significant results” were found either in the glasshouse or in field experiments across all three GRDC growing regions.
“Further mechanistic research found no positive effect of biostimulants or humates on the capture of nitrogen by wheat, either from soil or from an isotopically labelled legume residue. Though the research was conducted only in the short term, we found limited evidence to support efficacy of biological inputs in the Australian broadacre dryland grains context.
“We recommend that growers thinking about using biological inputs consider local constraints to production, and carry out controlled on-farm tests before full adoption.”
Project leader Dr Mark Farrell, a CSIRO soil biogeochemist, says the field trials included the application of “50 per cent district practice fertiliser to supplement each alternative amendment treatment”.
“Each site also includes a fertiliser (nitrogen, phosphorus) response curve to compare any yield response to what can be achieved from conventional fertiliser alone. The fertiliser response curve ranged from zero addition to 200 times district practice,” Dr Farrell says.
“We then had 14 biological amendments, four of which were repeated across all eight sites – a biostimulant, a humate, a microbial inoculant and an alternative fertiliser,” he says. “Finally, we had four local treatments that were mostly organic amendments but in one case, in Western Australia, there was a treatment of an alternative fertiliser that is commonly used in that state.”
The report says that even for more established amendments such as manures and composts, multiple years of reapplication may be required before significant results are observed.
“Across the 38 biological inputs examined over two years at eight field sites, there were only four significant yield responses,” it says. “In the first year of trials, two significant reductions were observed after application of one biostimulant and one humate treatment at the Paskeville site in South Australia. In the second year of trials, those treatments did not negatively impact yield again relative to the control.
“More positively, chicken litter (three tonnes per hectare fresh weight) had a significantly positive effect on grain yield at two sites in NSW, though this was likely related to high nitrogen availability within that particular litter. More in-depth analysis of soils from several of the treatments at each of the field sites revealed only sporadic and generally minor responses in terms of various soil health and fertility indicators, including microbial community structure.”
Dr Farrell says the chicken litter achieved “quite astonishing (yield) results” during the second year of the trials in NSW (but not in the first year).
“But another source of manure applied at the same site didn’t achieve those results, nor did other poultry litters at other sites,” he says. “And it was not immediately obvious from looking at things like micronutrients why one litter achieved those results … which left us with a bit of a puzzle as to why there was such a stark response.”
Due to the limitations of the short-term (one to two-year) field experiments, the project included two mechanistic studies to investigate: i) how biostimulants and humates affect wheat’s capture of nitrogen from legume litter and, ii) how biochars, humates, composts and manures impact on nutrient and carbon cycling and release. The glasshouse experiment investigating nitrogen uptake utilised 15N isotopic labelling to establish whether nitrogen within the plant was coming from the legume litter or the bulk soil.
“Intriguingly, despite conditions optimised for growth, we saw no positive and indeed some negative impacts of biostimulants on capture both of bulk soil and legume-derived nitrogen … It is apparent that in the short term, neither biostimulants nor humates are likely to enhance uptake of nitrogen by wheat,” the report says.
Dr Farrell says the key message from the project is that growers must fully understand all of the constraints facing their system before considering which amendments to use, if any. “The other half of the puzzle then would be knowing what each of the amendments would do to address those constraints, which is not easy on a product-by-product basis because that information is not readily available.
“There is so much that really isn’t known well enough in field conditions. I struggle a bit with the concept of how you can apply a product which will have significant impact on the bulk soil microbial community to the point where it remains in its new state and that benefits plant yield, given the highly adapted populations that are already present.”
It is crucial to have a mechanistic understanding of how various products work in order to evaluate their efficacy, Dr Farrell says. “Even if you see a response in the paddock in one year, you can’t say for certain whether it would happen in the same paddock under different climatic circumstances the next year.
“Ultimately, this is where the project got to and I think this is too much of an intractable problem that targeted research is going to provide a silver bullet across the sector. It might do it for a single product, but what we need is a database that will start giving us a signal that certain classes of products seem to work in specific soil types and with specific constraints.
“There are literally hundreds of these products out there, and we tested about 90 of them. We barely scratched the surface, and even during the project some of the products changed, including new variants and so on. It is hard to put the same rigorous framework over it as you would with fertiliser response trials.”
Work being done by CSIRO researcher Dr Lynne Macdonald is aiming to improve root growth and crop water-use in sandy soil types that suffer from high soil strength (compaction) and poor nutrient supply. The research includes trials that are evaluating whether the inclusion of organic amendments can help achieve longer-term benefits compared with physical amelioration alone (strategic deep tillage or ripping approaches).
“Lynne recognises that it’s not just the use of organic matter itself, but what type of amendment and how they are incorporated into the profile,” Dr Farrell says.
In addition to literature reviews, the project produced a practical guide to on-farm testing of biological inputs and two web-based calculators. The guide summarises the various input product classes and which amendments might be most likely to have the modes of action needed to ameliorate production constraints in a grower’s particular situation. It then details how on-farm testing should be carried out, particularly with regard to replication. The first web-based calculator is a simple statistical analysis of treatments, while the second provides a tool for economic scenario analysis.
The report’s authors acknowledge that further trials would be necessary to identify whether soil amendments might have cumulative effects in the medium term. “We do not discount this possibility where a biological input that is likely to have an effect beyond one growth season is reapplied annually.”
Dr Mark Farrell
08 8303 8664
Dr Lynne Macdonald
08 8273 8111
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