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In glasshouse experiments, Meloidogyne javanica reproduced on Brassica juncea, B. napus and BQMulch, a commercial biofumigant crop consisting of both B. napus and B. campestris. Although nematode multiplication rates were only 3–23% of the rate on a highly susceptible crop (tomato), they were high enough to suggest that Brassica rotation crops will increase root-knot nematode populations to potentially damaging levels in situations where temperatures do not limit nematode reproduction. A field study showed that this problem could be prevented in subtropical climates by restricting the growth of Brassica crops to June, July and August, when low temperatures limit nematode development and eggs are not produced until about 10 weeks after planting. Field and pot experiments in which M. javanica was introduced into soil following incorporation of Brassica residues failed to demonstrate a 'biofumigation' effect, possibly because the soil was too dry or the degree of tissue disruption was insufficient to maximise isothiocyanate release. Numbers of root-knot nematodes were reduced when Brassica roots, leaves and stems were incorporated into soil at 17 t DM/ha, but it is unlikely that this was due to the production of nematoxic chemicals, as large numbers of free-living nematodes multiplied during the decomposition process. From a practical perspective, these results suggest that green-manured Brassica rotation crops are unlikely to be useful for controlling root-knot nematodes on some subtropical horticultural crops that are currently fumigated for nematode control. The susceptibility of brassicas to M. javanica and the need to grow them during winter limits their potential in the vegetable industry, while difficulties in obtaining a biofumigation effect in dry soil will reduce effectiveness in non-irrigated pineapple soils. Brassicas are likely to be most useful in the ginger industry, as they can be grown during the winter break between ginger crops and fields can be irrigated before the Brassica crop is incorporated into soil.

A field experiment was established in which an amendment of poultry manure and sawdust (200 t/ha) was incorporated into some plots but not others and then a permanent pasture or a sequence of biomass-producing crops was grown with and without tillage, with all biomass being returned to the soil. After 4 years, soil C levels were highest in amended plots, particularly those that had been cropped using minimum tillage, and lowest in nonamended and fallowed plots, regardless of how they had been tilled. When ginger was planted, symphylans caused severe damage to all treatments, indicating that cropping, tillage and organic matter management practices commonly used to improve soil health are not necessarily effective for all crops or soils. During the rotational phase of the experiment, the development of suppressiveness to three key pathogens of ginger was monitored using bioassays. Results for root-knot nematode (Meloidogyne javanica) indicated that for the first 2 years, amended soil was more suppressive than non-amended soil from the same cropping and tillage treatment, whereas under pasture, the amendment only enhanced suppressiveness in the first year. Suppressiveness was generally associated with higher C levels and enhanced biological activity (as measured by the rate of fluorescein diacetate (FDA) hydrolysis and numbers of free-living nematodes). Reduced tillage also enhanced suppressiveness, as gall ratings and egg counts in the second and third years were usually significantly lower in cropped soils under minimum rather than conventional tillage. Additionally, soil that was not disturbed during the process of setting up bioassays was more suppressive than soil which had been gently mixed by hand. Results of bioassays with Fusarium oxysporum f. sp. zingiberi were too inconsistent to draw firm conclusions, but the severity of fusarium yellows was generally higher in fumigated fallow soil than in other treatments, with soil management practices having little impact on disease severity. With regard to Pythium myriotylum, biological factors capable of reducing rhizome rot were present, but were not effective enough to suppress the disease under environmental conditions that were ideal for disease development.

No abstract

Abstract. The grain-producing regions of northern New South Wales and southern and central Queensland are characterised by cropping systems that are strongly dependent on stored soil moisture rather than in-crop rainfall, and tillage systems that are increasingly reliant on zero or minimum tillage. Crops are grown relatively infrequently and crop rotations are dominated by winter and summer grains (wheat [Triticum aestivum L.] and sorghum [Sorghum bicolor L. Moench], respectively), with smaller areas of grain legumes and cotton (Gossypium hirsutum L.). The grey, black, and brown Vertosols represent the more productive soils in the region under rainfed cropping, and are the focus of work reported in this study.Soil samples were collected from surface soils (0-0.30 m) across the region, utilising sites of long term tillage and residue management studies, fertiliser trials, and commercial fields to enable an assessment of the impact of various management practices on soil biological properties. A number of biological and biochemical parameters were measured (microbial biomass C, total organic C and labile C fractions, total C and N, microbial activity using FDA, cellulase activity, free living nematodes, total DNA and fatty acid profiles), and the response of wheat, sorghum, and chickpea (Cicer arietinum L.) to steam pasteurisation was assessed in glasshouse bioassays. The objective was to obtain an indication of the biological status of grain-growing soils and assess the impact of biological constraints in soils from different regions and management systems.Results showed that biological activity in cropped soils was consistently low relative to other land uses in northern Australia, with management practices like stubble retention and adoption of zero tillage producing relatively small benefits. In the case of zero tillage, many of these benefits were confined to the top 0.05 m of the soil profile. Fallowing to recharge soil moisture reserves significantly reduced all soil biological parameters, while pasture leys produced consistent positive benefits. Breaking a long fallow with a short duration grain or brown manure crop significantly moderated the negative effects of a long bare fallow on soil biology. Use of inorganic N and P fertilisers produced minimal effects on soil biota, with the exception of one component of the free-living nematode community (the Dorylaimida).The glasshouse bioassays provided consistent evidence that soil biota were constraining growth of both grain crops (sorghum and wheat) but not the grain legume (chickpea). The biota associated with this constraint have not yet been identified, but effects were consistent across the region and were not associated with the presence of any known pathogen or correlated with any of the measured soil biological or biochemical properties. Further work to confirm the existence and significance of these constraints under field conditions is needed.None of the measured biological or biochemical parameters consistently changed in response to management prac...