Effects of isobenzan on soil fauna and soil structure

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Abstract
The insecticide isobenzan‡ applied to pasture for pest control has had serious side-effects on beneficial soil animals and soil structure. In a field trial at Halkett, on Hatfield fine sandy loam, the populations of all recorded faunal members, except nematodes, were drastically reduced and soil bulk density significantly increased after application of isobenzan at 2 lb active ingredient per acre. A mat of slowly decomposing plant debris formed above the soil surface in treated plots, and nearly 90% of absorptive roots developed within this organic layer. Plant growth was retarded and capacity of soil to absorb and retain moisture reduced. These effects were still evident more than four years after application of isobenzan. In another similar trial on the more fertile Taitapu silt loam at Halswell, with 1 lb active ingredient per acre, the effects of isobenzan on soil fauna, though less severe than at Halkett, were greater than expected from the lower dosage rate. In this trial changes in soil structure were relatively small and in the treated plots pasture mat-formation was not evident but soil organic carbon was considerably increased.