Southern Zone of Rejuvenated Drainage

Posted on Jan 9, 2015


Southern Zone of Rejuvenated DrainageLandResources

Erosional surface of gently undulating rises to low hills. Continuous stream channels that flow in most years. Colluvial processes are active. Soils formed in colluvium or rock-weathered in-situ.

SWCC Strategic Priority

Threats to be addressed:

  • water erosion
  • soil structure decline and/or compaction

DAFWA – Report Card (September 2013) The Southern Zone of Rejuvenated Drainage, for the purposes of DAFWA’s Report Card, is included in the Agricultural Soil Zone 7. Zone of Rejuvenated Drainage. Agricultural Soil Zones are based on repeating patterns of soil and land types. The Ag Soil Zones are used for reporting on the following themes:

  • Soil acidity
  • Wind erosion
  • Water erosion
  • Soil organic carbon
  • Soil compaction
  • Water repellence
  • Nutrient status (phophorus)

Hydrozones are based on grouping areas with similar hydrology and are used for reporting on:

  • Dryland salinity

The Southern Zone of Rejuvenated Drainage is identified on its own as a hydrozone.

Key Findings for this Zone

Soil Acidity

Soil Acidity

Summary – A large proportion of surface soils are below target. Although the trend for surface soils is stable, the subsurface will continue to acidify.

Condition and Trend –Poor and Deteriorating (confidence in condition and trend: Adequate high-quality evidence and high level of consensus in both).

This zone is dominated by deep sandy duplex soils comprising 55% of the area. In this soil type 87% of topsoil samples collected had pH below the recommended target. This degree of acidification reflects the generally higher production associated with higher rainfall, compared to zones further east and the low capacity of the surface sandy soils to resist change in pH (low buffer capacity).

The other two soils – gravels and deep loamy duplexes and earths – had somewhat lower proportions of surface samples below the target pH. However, more than half were below target. The situation for this zone improved with depth, with only 30-40% of samples from the 20-30 cm layer having a pH below the target. This result reflects the change in soil type with depth, characteristic of duplex soils.

A summary of the percentage of samples below target for depths 0-10 cm, 10-20 cm and 20-30 cm for the Ag Soil Zone 7. Zone of Rejuvenated Drainage (DAFWA Report Card, 2013).

A summary of the percentage of samples below target for depths 0-10 cm, 10-20 cm and 20-30 cm for the Ag Soil Zone 7. Zone of Rejuvenated Drainage (DAFWA Report Card, 2013).

Source: Gazey C, Andrew J and Griffen E (2013). ‘Soil Acidity’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Soil Acidification in the south-west of Western Australia go here.

Wind Erosion

Wind Erosion

Summary – Moderate hazard overall but has areas of high hazard in the west where high stock numbers are retained on poor pastures in drought years.

Hazard and Trend – Moderate (meaning 2 yr in 4 below target hazard – yrs 2009-12) and variable (confidence in hazard and in trend: limited evidence or limited consensus in both).

Click on the image below to enlarge. Table below (from the DAFWA Report Card) shows that in 2011, there was a high average wind erosion hazard for the zone (Zone 7 – Zone of Rejuvenated Drainage).

Wind Erosion – Hazard Summary for 2009-12 (DAFWA Report Card, 2013)

Wind Erosion – Hazard Summary for 2009-12 (DAFWA Report Card, 2013)

Source: Carter D and Laycock J (2013). ‘Wind Erosion’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Wind Erosion in the south-west of Western Australia go here.

Water erosion

Water erosion

Summary – Excessive grazing pressure on sloping land in some parts perpetuates risk of erosion. Cropping land is largely stable.

Hazard and Trend – Moderate and stable (confidence in hazard and trend: limited evidence or limited consensus in both).

Source: Galloway P and van Gool D (2013). ‘Water Erosion’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Water Erosion in the south-west of Western Australia go here.

Soil organic carbon

Soil organic carbon

Summary – SOC levels vary with rainfall gradient. Possible decline where cropping is increasing and pasture declining. Soil constraints can limit biomass production.

Abundance and Trend – Moderate and unclear trend (confidence in abundance: limited evidence or limited consensus; confidence in trend: evidence and consensus too low to make an assessment).

Source: Griffen E, Hoyle F and Murphy D (2013). ‘Soil Organic Carbon’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Soil Organic Carbon in the south-west of Western Australia go here.

Soil compaction

Soil compaction

Summary – Soils mainly resilient to compaction, but small areas of yellow soils are a problem as cropping becomes more common.

Hazard and Trend – Moderate (meaning soil hazard moderate and current land use likely to cause compaction) and stable (confidence in hazard and in trend: limited evidence or limited consensus in both).

Source: Carter D, Davies S and Schoknecht N (2013). ‘Soil Compaction’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Soil Compaction in the south-west of Western Australia go here.

Water repellence

Water repellence

Summary – The higher rainfall western portion of the zone has water repellence in sandy gravelly areas.

Condition and Trend –Poor (meaning water repellence common) and deteriorating (confidence in condition and in trend: limited evidence or limited consensus).

Source: Carter D, Davies S Blackwell P and Schoknecht N (2013). ‘Water repellence’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

Dryland Salinity

Dryland Salinity

Summary – Salinity is extensive. Groundwater levels are mostly stable. The risk is moderating as catchments approach equilibrium.

Risk and Groundwater Trends – Moderate (Likelihood of occurring: likely; consequence: moderate) and stable (confidence in condition and in trend: adequate high-quality evidence and high level of consensus).

Most monitored catchments appear to be approaching equilibrium but there are some bores with significant rates of groundwater level rise adjacent to valley hazard areas, so an expansion of the area affected by salinity is likely. Localised impacts are expected to be moderate as fresh to brackish groundwater resources occur. The moderate salinity risk in this zone is likely to be realised within the next 20 years.

Source: Simons J, George R and Raper P (2013). ‘Dryland Salinity’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Dryland Salinity in the south-west of Western Australia go here.

Nutrient status (phosphorus)

Nutrient status (phosphorus)

Summary – Soil P fertility is well in excess of optimal range. Ceasing P application would see P levels very slowly decrease to optimal levels.

Condition and Trend –Well in excess (meaning P fertility index > 1.5) and stable (confidence in condition and in trend: limited evidence or limited consensus in both).

Source: Weaver DM and Summers RN (2013). ‘Nutrient status (phosphorus)’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.

To find out more about Nutrient status (phosphorus) in the south-west of Western Australia go here.

Projects:

SWCC has developed two videos to help you get started with soil health. Part 1 describes a process for investigating soil health and identifying what constraints to manage first. Part 2 answers some common soil health questions, such as how to manage soil organic matter and soil biology, impacts of synthetic fertilisers on soil biology, and organic soil amendments such as compost and biochar.

Source:

  • Department of Agriculture and Food 2013, Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.
  • N. Schoknecht, P. Tille, B. Purdie (2004). Soil-landscape mapping in south-western Australia. An overview of methodology and outputs.
  • Van Gool, D., Vernon, L. and Runge, W. (2008). Land Resources in the South-West Agricultural Region. A shire-based summary of land degradation and land capability. Department of Agriculture and Food Western Australia.

 

Header and thumbnail images supplied by Peter Clifton

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