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This report describes 55 000 hectares of predominantly red sands and sandy loams — locally referred to as Cockatoo Sands — on the Bonaparte Plains about 70 kilometres north of the Ord River Irrigation Area, Kununurra. It builds on soil surveys conducted in 2009 and 2015, which identified significant areas of red soils in the Kununurra area that could be suitable for agriculture.

The Cockatoo Sands generally have loamy, sand topsoils that grade to sandy loam with depth. The soil profiles are very deep, well drained to rapidly drained, and highly permeable. These soil characteristics support a large range of annual and perennial crops over the wet and dry seasons.

The areas identified for agricultural development are predominantly Cockatoo Sand, loamy phase, and deep red earths that are classified as Haplic Mesotrophic Red Kandosols. Soil profiles typically have loamy, sand topsoils that grade to clayey sand and sandy loam within a metre.

In general, the landscape is almost level, with most slopes less than 2%. Sandstone and limestone hills border the south and west of the study area, respectively. The land slopes to the north and east where ephemeral drainage lines and fringing freshwater springs meet coastal mudflats.

Soil analysis indicates that Cockatoo Sands are strongly weathered with the clay fraction dominated by kaolin and quartz with minor haematite and goethite. The soil reaction trend is neutral, with topsoil pH in the range of 5.5 to 6.5 and subsoil pH in the range of 6.5 to 7.0.

The upper 30–60cm of the soil profile has loamy sand to clayey sand soil textures that have a very low soil buffering capacity. This, combined with their low nutrient-retention ability, means that these soils will require careful management if they are used for irrigated agriculture. Under irrigated agriculture, soil acidity may increase over time, depending on the pH of the irrigation water and fertiliser management.

The underlying sandy loam subsoils are compact, with soil bulk density in the range of 1.7 to 1.8 megagrams per cubic metre. To address this, deep-ripping prior to seeding is recommended for deep-rooted annual and perennial crops.

Water erosion is a major risk because the region has a monsoonal climate. Under intense rainfall, sheet and rill erosion is likely to develop, particularly on long slopes of greater than 1% gradient. Therefore, development of the land requires consideration of the length of crop rows and slope. Slopes greater than 3% are considered not sustainable for annual cropping, while 5% slope is the upper limit for perennial crops where cultivation is infrequent and a protective groundcover can be maintained.

This investigation identified 34 947ha of Cockatoo Sands (normal phase and loamy phase) with a moderate to high capability for irrigated agriculture. The potential for irrigated agriculture on the Bonaparte Plains depends on the development of suitable water supplies for irrigation.

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Bonaparte Plains, East Kimberley, land capability, soils, irrigated agriculture


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