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A comparison of organic and conventional farming systems with particular reference to wheat production and soil fertility in Western Australia

Deria, Ahmed M. (2000) A comparison of organic and conventional farming systems with particular reference to wheat production and soil fertility in Western Australia. PhD thesis, Murdoch University.

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Organic production systems are possible alternatives to the current conventional farming systems in South Western Australia. However, the impact of these systems on soil fertility, and their sustainability largely remain untested. The primary objective of this research was to compare organic and conventional wheat production during the crop phase of the rotation to determine effects on soil fertility and to establish a scientific basis for assessing the sustainability of organic wheat farming in the Western Australia wheatbelt.

Organic broadacre fanners registered in Western Australia with the National Association for Sustainable Agriculture Australia (NASAA) were surveyed to characterize the farm size, soil type and the nature of the organic farming operations. From the respondents of these surveys seven sites in the northern and central wheatbelt of Western Australia were selected for further study. The organic sites were paired with adjacent conventional fields where soil types, cropping history before conversion of the organic paddock, and where possible, the farm manager were the same, so that the management system for wheat production was the primary object of difference. Soil types of the studied sites varied from Red Earth to Yellow Duplex soils representing a range of wheat growing soils in Western Australia. Organically managed fields were generally cropped after 3 years pasture while the conventionally managed fields were cropped almost every year.

Yield performance of organic and conventional wheat was compared, with particular emphasis on selected chemical, biological and physical properties of these soils as possible causes of yield differences. Most of the soil chemical and physical properties of the paired soils were not significantly different. When compared to the paired conventionally managed sites, the grain yield of the organic wheat in the northern and central wheatbelt of Western Australia was: (i) increased by 17% at one site (site 2), the longest managed organically (> 8 years), (ii) decreased by an average of 27%, at three sites (3, 4 and 5), managed organically between 4 to 5 years, and (iii) was not changed in another three sites (1,6 and 7), managed organically < 4 years.

The lower yield in the organically grown wheat at the three sites could not be attributed to any single factor, but appeared to relate to combinations of two or more factors: lower Colwell-extractable-P, later sowing, decreased nitrogen supply. In addition increased soil strength and weed competition may be involved, but insufficient data was available to draw firm conclusions.

Soil biological and microbial properties of the above sites were studied in situ and in laboratory and glasshouse studies, including microbial biomass carbon C, microbial nitrogen N, microbial phosphorus P; microbial activity (respiration), and; potential N-mineralization. The fields were sampled before the break of the season, and at anthesis. Microbial biomass carbon (MBC) and microbial activity determined as soil respiration were higher in the organically managed fields than the conventionally managed fields at all sites, consequently microbial biomass N and microbial P pools associated with soil microorganisms were also higher in the organically managed soils. Similarly, ammonification of arginine amino acid by soil microorganisms was higher in the organically managed fields than in the conventionally managed fields. However, cotton strips tests used as an index of microbial cellulose decomposition activity showed no differences between organically and conventionally managed fields. Vesicular-arbuscular mycorrhizal root colonization in the field grown wheat and in clover grown in a glasshouse bioassay was high in both organic and conventionally managed fields. Thus, the present results suggest that organic wheat production system could possibly stimulate microbial biomass and microbial activity and the associated pool of nutrients, but have little effect on the levels of VA mycorrhizal root colonization.

Organic (NaHCOs-Po, NaOH-P0 and HC1-P0) and inorganic (NaHC03-Pi, NaOH-Pi and HCl-Pj) P-fractions in six paired organic and conventionally managed soils were measured using sequential-extraction procedures at two sampling times (anthesis and before the break of the following season). There was build up of more resistant organic P fractions (NaOH-P0 and HC1-P0) in the organically managed fields than in the paired conventionally managed fields at the longest organically managed site, or in the more recently converted sites for organic production. The labile inorganic P-fraction (NaHCOs-Pj ) was the only fraction, which responded positively to the Pfertilizers applied in the conventionally managed soils.

To investigate the possible factors that contributed to the increased microbial biomass and microbial activity in the organically managed fields observed in the field, glasshouse and laboratory experiments were set up. Soil samples used in the glasshouse experiment were collected from four paired sites. Organically managed fields of two of these sites contained plant available nutrients greater than that in the conventionally managed soils before sowing crops. By contrast the organically managed fields of the other two sites contained lower nutrients than the conventionally managed fields. Soil samples were treated with two levels of straw (1 t/ha and 10 t /ha) and Pfertilizers (North Carolina reactive rock P, K2HPO4), and composted chicken manure at 20 kg P/ha then sown with wheat followed by clover and finally clover or wheat. Colwell-extractable-P; pH; microbial biomass C and P; microbial activity (measured as respiration and mineralized NH4-N); and, N and P-uptake were determined after harvesting the first crop and the third crop. Results of this experiment show that increased C-input in the organically managed fields due to extended pasture phase and crop residue left on the fields are likely to be the main driving force of the increased microbial biomass and microbial activity in those fields (organically managed fields). Moreover, microorganisms stimulated by providing carbon substrate were able to cycle P from the native soil P and that in the P-fertilizers added (rock P, and K2HPO4) or manure: significant proportions of the cycled P was available for the following crops.

In summary, the main findings of the present study are: (1) Broardacre organic wheat production in the central and northern wheatblet can be as profitable as their counterpart conventional wheat crops; (2) Broadacre organic wheat production relies on the pasture phase for the supply of the plant available nitrogen (N), and on soil reserves of phosphorus (P), potassium (K) and sulphur (S), a strategy that will eventually cause depletion of the plant available nutrient pools and deficiencies of these which in turn will reduce N input from pastures and reduce soil organic matter levels, and; (3) the major cause of the stimulated biological fertility in the organically managed fields is the increased amounts of carbon entering these fields during the pasture phase. Hence, adding carbon to the organic or the conventional fields can stimulate soil microbial activity, which in turn enhances nutrient cycling within the systems.

There are many key research issues that still need to explore about the agronomic and environmental benefits of organic broadacre farming in WA, and sustainability of the systems developed. It is important to understand the impact of the pasture phase on the biological functioning and nutrient cycling. A possible option to manage soil biological fertility in the organic and conventionally managed fields, especially plant available P would be to fertilize the fields during the pasture phase with reactive rock P, so that P could be cycled by microbial cells. Alternatively, where it is possible, organic fertilizers (manure), and rock P together could be applied to maintain sustainable levels of plant-available P._Future research on the organic farming system should aim to understand nutrient management, particularly P cycled through the microbial pool in the organically fields where the external inputs of P-fertilizers are currently minimal. Such research may also generate spinoff benefits for management of plant-available P in the conventionally managed fields. Research is also needed to understand the impact of the organic farming system on soil quality and on economic benefits to farmers. Further, extension of organic agriculture research needs to investigate the performance of the organic farming systems in the medium to high rainfall areas of the state (WA). Any future research should consider the complexity of the organic farming system and requires a system approach.

It is concluded that supplementing soil P levels in the organically managed soils is a priority for maintaining productivity of the system.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Bell, Richard and O'Hara, Graham
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