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Is continuous cropping sustainable on red duplex soils?

G.W. Ford

Victorian Institute for Dryland Agriculture, Private Bag 260, Horsham VIC 3400

Hardsetting red duplex soils are extensively cropped in northern Victoria. Major constraints to improving the efficiency and sustainability of dryland cropping on these soils are the continuing decline in soil structure and nitrogen status with traditional cropping practices. As many farmers wish to continue intensively cropping these fragile soils without using ley farming systems, there is a demand for the evaluation of the likely long-term impact of "conservation cropping" options on productivity.


A rotation-tillage trial was established in 1986 to assess tillage intensity (conventional compared with minimal), stubble management (burn compared with retain), and the use of traditional bare winter fallowing (cultivated or chemical). Two three-course rotations (wheat-barley-field peas, fallow-wheat-field peas) were compared. Treatments were assessed by their effects on grain yields, soil fertility and structure, and on cereal root diseases.

Results and discussion

Grain yield data will be presented from the "traditional" (conventional till, stubble burnt) and "conservation" (minimum till, stubble retained) treatments over the initial cycle (1987-89) of both rotations. Conventional tillage significantly increased grain yields, especially on non-fallow wheat (wheat-barley-field peas); stubble management (burnt versus retained) had no effect. During 1986-89, crop yields were similar under both rotations; thus continuous stubble cropping was the most profitable practice.

Cereal root diseases (root rots and nematodes) were a major problem (especially Rhizoctonia in stubble-sown barley), even where pre-sowing tillage was used. A survey of commercial cereal crops in north-west Victoria during 1990 indicated that these root diseases were widespread on red duplex soils.

Soil fertility and structure were not improved by any treatment. The periodic application of gypsum, even with direct drilling and stubble retention, failed to improve aggregate stability sufficiently to control surface sealing. In both rotations, inclusion of a grain legume (field peas) did not slow the continued decline in soil organic carbon and nitrogen (0-10 cm) and available soil nitrate at sowing during 1986-90. An associated study in 1988 of the value of sap nitrate tests for predicting yield responses to nitrogen fertiliser by wheat confirmed that crop yields were probably limited by available soil nitrogen. This suggests that these rotations are unlikely to prove sustainable.


This project was supported by the Wheat Industry Research Committee of Victoria.

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