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Rural Industries Research & Development Corporation
Soil Structure, Fungi, Fauna & Phosphorus
in Sustainable Cropping Systemsby David Dumaresq and Richard Greene
September 2001
RIRDC Publication No 01/130 RIRDC Project No ANU-30A
A number of strategies have been recently developed in the search for more sustainable agricultural practices to relieve the combined pressures of environmental and economic degradation on farming enterprises. The development of conservation tillage systems and alternative farming practices such as organic farming are ways to overcome these pressures. It has been suggested that such alternative systems as organic farming offer strategies for reducing some of the adverse environmental impacts of agriculture and may also be superior in terms of soil quality and sustainability than conventional agricultural systems. This study shows that an organic cropping system can provide superior economic, environmental and soil structural outcomes than a comparable conventional approach.
The project provides a comparative study of the agronomic outcomes and the environmental consequences of alternative cereal cropping practices for south-eastern Australia’s sheep-wheat belt.
The study compares a range of measurements taken in first year wheat crops following pasture on a long established organic farming system with crops from a similar conventional system. Field samples and measurements were taken over two years across seven different wheat crops on the two farms; four on the organic farm, and three on the conventional farm.
The crops sampled were the routine commercial crops grown by the two farmers using their respective conventional and organic methods. Farmers were not asked to plant crops as treatment for this project, rather the project sampled those crops grown as the result of the farmers’ commercial decisions.
Subsequently a range of combinations of slightly different cropping systems were sampled as the farmers "fine tuned" their systems in response to changing markets, seasonal conditions and costs.
Soil and crop measurements taken were: i) soil chemical analyses (N, P, CEC, pH, cations); ii) soil organic carbon; iii) aggregate stability; iv) steady state infiltration rate; v) % root length infected by VAM; vi) soil invertebrate populations; vii) crop root length; viii) bulk density and total soil porosity; ix) crop and weed biomass.
Farm and paddock histories were established and agronomic data collected from the farmers, including yield, input costs, and farm management details.
Key similarities and differences for the organic and conventional farming systems are summarised in the two tables below:
1. Farming System - Similarities
| Systems Characteristic Similarities | Organic | Conventional |
| Enterprise
mix
Tillage practices Cropping cycle Soil types Total P Total & available N Soil infiltration |
same
similar similar same similar similar similar |
same
similar similar same similar similar similar |
2. Farming System - Differences
| Differences |
|
|
| Agrochemical
use
-fertilizers -herbicides Rotation length Fallow length Yield Costs† Returns† pH Cation exchange capacity Extractable P Organic carbon Soil invertebrate diversity VAM presence Soil porosity Soil macroaggregation |
RPR + gypsum, lime none 6-9 years 7-8 months lower* lower* higher* higher * higher* lower* higher higher* higher* higher higher |
DAP, superphosphate 1-4 applications per crop 2-3 years 1-4 months higher higher lower lower lower higher lower lower lower lower lower |
*Significant difference (p= 0.05). †Not actual costs & returns – modelled gross margins only.
The organic cropping system had on average substantially lower yields accompanied by lower input costs. However, through the availability of substantial premium prices for organic wheat, the organic system achieved substantially higher economic returns for all its crops.
The organic farm also achieved consistently better environmental outcomes from its system. This was in terms of soil physical and biological characteristics, and through its greater reliance on on-farm biological processes and lower reliance on off-farm chemical inputs.
Some quantifiable differences in system outcomes are summarised below as a set of comparative indices (conventional=100).
System Outcomes
| Systems Characteristic |
|
|
| Yield
Costs ($/ha)† Return (organic wheat @ $215/t )† Return (both systems @ $119/t)† VAM presence Soil faunal diversity (at harvest) Soil macroaggregation (at harvest) Soil porosity (at harvest) |
74 148 61 174 148 105 110 |
100 100 100 100 100 100 100 |
†Not actual costs & returns – modelled gross margins only.
If the results presented here are applicable more widely across the Australian sheep-wheat belt, they indicate that broadacre organic cropping systems for wheat can provide an existing alternative farming strategy to current high input approaches. These organic systems can produce better economic returns than some conventional cropping methods through lower input costs and higher commodity prices.
They can enhance environmental sustainability through reduced cropping system dependency on agricultural chemicals, in particular herbicides and soluble N and P fertilisers. They also maintained soil structural properties at levels equal to, or better than, conventional reduced tillage systems; and can enhance soil organic carbon levels and biological activity.
This study demonstrates that broadacre organic cereal cropping systems can make a significant contribution to Australian agriculture through providing an alternative strategy for sustainability.
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