Soil biotic and biochemical factors in a long-term tillage and stubble management experiment on a vertisol. 1. Seedling inhibition by stubble

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In a long-term experiment on a vertisol in southern Queensland, depression of vegetative growth of barley ( Hordeum vulgare L.) by stubble retention was far greater with zero tillage than with mechanical tillage of the fallow. The possible phytotoxic effects of stubble on barley and wheat ( Triticum aestivum L.) were investigated in seedling bioassays. Stubble collected from field plots just prior to planting did not reduce germination of wheat or barley seed but markedly reduced coleoptile length at 4 days. This effect of stubble became less apparent after 6 and 8 days, and was overcome by increasing water volume in the bioassay dish. Stubble absorbed 4.5–6 times its own weight of water and thereby competed with the germinating seed. Filtrates of stubble macerates in water, collected either before or after incubation of the saturated stubble, had no effect on coleoptile length indicating the absence of a water-soluble phytotoxin. Brown lesions on wheat coleoptiles (most apparent at 8 days) and roots were decreased by stubble but increased by more water in the bioassay dish. Alternaria sp. and two types of bacteria were associated with the coleoptile lesions, and Fusarium sp. and several types of bacteria were associated with root lesions. Surface sterilisation of seed reduced root lesions but not coleoptile lesions. Filtrates of both incubated and unincubated stubble macerates reduced coleoptile lesions. Poor early growth of barley in the field on zero-till, stubble-retained plots was not associated with incorporation of stubble into the drill slit in contact with the seed. Poor early growth was not overcome by nitrogen fertilizer drilled into the soil 2 months before planting. The quantities of air-dried stubble on the soil surface were capable of absorbing of the order of 4 mm rainfall. It is concluded that neither phytotoxins nor water absorption by the stubble were likely causes of the problem of poor early growth.