VEGETABLE growers are well aware of the key role organic matter plays in maintaining fertility, productivity and health of their soil.
Increasing production intensity however provides little room for replenishing soil organic matter through green manure or perennial break crops, leaving the use of external organic soil amendments such as manures and composts for achieving these goals.
Growers often use these products merely to enhance soil physical properties without taking much notice of the substantial contribution they can make to plant nutrition, foregoing significant benefits and missing out on cost reduction opportunities.
A recently completed federally funded research and development project [1] examined the degree to which the use of raw and composted feedlot and layer chicken manure can substitute the use of mineral fertiliser, particularly nitrogen (N), and also reduce gaseous N losses.
The field trial in Queensland’s Lockyer Valley (irrigated black cracking clay) compared use of standard mineral fertiliser application rates with management practices where organic amendments were used annually in addition to mineral fertiliser with and without accounting for additional nutrient inputs from organic amendments.
Therefore, application rates reflected the expected N supply capacity of applied products (eg. 95 kg N per ha and year, rather than an arbitrary volume or tonnage rate per hectare).
Use of organic amendments without reducing standard fertiliser application rates did not result in increased crop yields in the assessed production system (green beans – sorghum [catch crop] – broccoli – lettuce – sweet corn – broccoli – lettuce – sweet corn).
Conversely, the reduced use of synthetic fertiliser did not result in reduced crop yields where nutrients from organic amendments were accounted for and mineral fertiliser rates were reduced.
Hence, nutrients in organic amendments can partially substitute the use of mineral fertiliser.
Growers who use organic soil amendments should account for nutrients supplied with these products and adjust mineral fertiliser application rates to avoid over-fertilisation and capture all benefits offered by using organic soil amendments.
This can be done best by employing a multi-year nutrient budgeting tool, as nutrient release from organic amendments occurs gradually over several years.
Potassium (K) and phosphorous (P) in organic soil amendments have high fertilising efficiency.
Virtually all K becomes plant available in the first year after application, while P becomes available over a two to four year period.
P and K supplied in organic amendments should therefore be fully accounted for in nutrient budgets.
The same is the case for calcium (Ca) and associated liming effects. Accounting for N contributions from organic soil amendments is less straightforward, as only a small part of the total N content, ammonium and nitrate, is immediately available for plant uptake.
The larger part (65 per cent - 100pc) is tied up in organic compounds that release N only over time as they break down, a process that depends on product and soil properties, and also on management and environmental conditions.
The integrated supply of nutrients from organic and mineral sources needs to be considered when organic soil amendments are used.
The conceptual model for Integrated N Supply (see graph) shows that, with this approach, organic nutrients supplied from manure or compost are topped up with nutrients through mineral fertiliser to meet crop requirements.
This model needs to be transformed into a user-friendly, integrated decision support tool that allows growers to accurately account for plant nutrients supplied by organic soil amendments and to reduce mineral fertiliser inputs accordingly, resulting in more nutrient efficient and more profitable farming operations.
The field trial showed that accounting for N supplied from raw and composted feedlot and layer chicken manure led to a reduction of synthetic N inputs (770 kg N ha-1 during 2.5 years) by between 25pc and 38pc without affecting yield when compared to standard fertilising practices.
Accounting for N, P and K supplied via organic soil amendments over the 2.5 year trial period resulted in fertiliser cost reductions of between $1450 and $2150 per ha.
Overall, gaseous N losses as nitrous oxide (N2O) were relatively small (< 0.5pc of total N applied) in the assessed production system.
Nevertheless, use of organic amendments in addition to mineral nitrogen fertiliser increased N2O losses compared to use of mineral fertiliser alone, regardless whether amendments were raw or composted.
N2O emissions were lower when mineral N was applied at reduced rather than standard rates and when composted rather than raw animal manures were applied.
Hence, use of composted manures in combination with reduced mineral N fertiliser rates resulted in N2O losses that were similar to those seen when mineral fertiliser alone was used.
The trial results indicated that factors other than nitrogen are the main drivers for N2O emissions in horticultural production systems, with total and labile carbon inputs most likely having a strong influence on N2O emission rates.
In summary, this study showed that:
- Use of manure and compost without adjusting mineral fertiliser rates represents inefficient nutrient management practices;
- It is possible to use an integrated organo-mineral fertilisation strategy where nutrients from repeat application of organic amendments allow partial substitution of mineral fertiliser without compromising yield;
- Use of composted amendments in combination with reduced mineral fertiliser rates does not result in N2O losses that exceed those of current standard fertilisation practices by much, and (iv) that a user-friendly decision support tool for the integrated use of organic and mineral nutrient sources is needed for growers to take full advantage of these findings.
Johannes Biala worked at the Institute for Future Environments, Queensland University of Technology, and is now working at the University of Queensland.
- [1] Composting as a Means of Minimising Greenhouse Gas Emissions from the Manure Supply Chain, part of the National Agricultural Manure Management Program, which was funded by the Australian Government Department of Agriculture, Australian Pork Limited, Meat and Livestock Australia, Australian Egg Corporation Limited, Rural Industries Research and Development Corporation and Dairy Australia.
