Two long-term field trials in South Australia were used to detect and characterise changes in soil biological properties that were a consequence of different agricultural management. The properties examined were total bacteria, fungi, and actinomycetes; total pseudomonads; cellulolytic bacteria and fungi; mycorrhizal fungi; plant root pathogens (Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Pythium irregulare); bacterial-feeding protozoa; soil mesofauna (collembola and acari); earthworms; microbial biomass; C and N mineralisation; in situ CO2 respiration; cellulose decomposition; and soil enzyme activity (peptidase, phosphatase, sulfatase). The sensitivity of these biological properties was assessed to tillage (no-tillage v. conventional cultivation), stubble management (stubble retained v. stubble harvested), crop rotation (continuous wheat v. wheat-sown pasture), and N fertilisation (nil v. 80 kg N/ha applied during the crop phase). Tillage, stubble management, crop rotation, and N fertilisation significantly (P<0.01) affected C mineralisation and microbial biomass. Tillage with stubble management significantly affected root pathogenic fungi, protozoa, collembola, earthworms, and cellulose decomposition. Crop rotation affected mycorrhizal fungi, protozoa, and soil peptidase activity, and N fertiliser had a significant effect on mycorrhizal fungi, protozoa, and cellulose decomposition. As these biological properties are responsive to agricultural management, they may have potential as bioindicators. Total bacteria, fungi, and actinomycetes, cellulosedecomposing bacteria and fungi, soil phosphatase and sulfatase activity, and N mineralisation were less affected by these treatments and may therefore have limited potential as bioindicators.

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Field experiments were conducted in 1991, 1992 and 1994 at three locations in South Australia to investigate the involvement of root pathogenic fungi in the poor establishment and growth of annual medics (Medicago spp.) in naturally regenerating pastures. The severity of root disease across the three sites, rated on a 0–4 scale, was 2·4 (1991), 2·7 (1992) and 1·7 (1994). Medicago emergence and/or herbage production increased in the presence of potassium phosphonate applied as a soil drench or as a postemergence spray at 1·0 kg to 3·0 kg a.i. ha−1 in every year and with metalaxyl applied as a postemergence spray (0·83 kg a.i. ha−1) in 1992 and as a soil drench (5 kg a.i. ha−1) in 1994. The severity of root disease was reduced in the presence of both fungicides. Metalaxyl specifically reduced root infection of Medicago by Pythium spp. whilst potassium phosphonate showed a broader spectrum activity, reducing infection by Fusarium spp., Phoma spp., Pythium spp. and Rhizoctonia solani. Fungal isolates showing pathogenicity towards Medicago littoralis cv. Harbinger in growth cabinet experiments included Fusarium acuminatum, F. avenaceum, F. culmorum, F. crookwellense, F. equiseti, F. graminearum, Phoma medicaginis, Pythium irregulare and Rhizoctonia solani.

Under prevailing climatic conditions, treatment of winter-sown wheat with metalaxyl (seed dressing, granule, or soil drench applications) significantly limited seed infection by Pythium species but did not give a significant yield response in separate field trials conducted on 2 contrasting soil types in South Australia. However, following irrigation of trial plots (which raised the soil-water matric potential from about -0.09 MPa to -0.02 MPa) during the first 3 weeks of wheat growth in 1989, metalaxyl granule treatment was effective in limiting seed and root infection by the 2 dominant Pythium species present, P. irregulare and P. echinulatum, and gave a significant increase in grain yield on both soil types. This effect of soil-water matric potential was confirmed in growth cabinet experiments in which metalaxyl was shown to be more effective in controlling infection of wheat by P. irregulare in both soil types when the soil-water matric potential was maintained at -0.02 MPa than at a soil-water matric potential of -0.1 MPa.