Plate 1: Wash area, tapering at 6.5 m, retaining soil and nutrients on the slope

A demonstration site on a Gold Coast Hinterland turf farm clearly showed the value of turf grass in retaining sediment on slopes and reducing nutrient movement into waterways.

The project monitored soil losses during an eight week period in January and February 2011 including a time of extensive flooding in South East Queensland. Sediment movement was captured in three monitoring troughs from adjacent slopes that were either fully covered with turf mats, left bare or sprigged. Estimates of phosphorous and potassium losses in off-site soil movement were calculated from a spot test in late January.

Full sod - what a difference

• Turf mats protected exposed slopes almost instantly, even prior to rooting.
• Sod was 100 times more effective at keeping soil in place than exposed soil during two high rainfall periods (100 mm and 90 mm) and for the term of the monitoring period (see table).
• Sod retained around 19 tonnes/hectare of soil for each rainfall event.
• Where break-through erosion occurred, the sod trapped sediment minimising its travel down slope.

What we found

Soil Loss

Soil loss 52 days after 427 mm of summer rainfall (6 January 2011 to 28 February 2011).

Bare earth (7.3° slope)	Sprigged Area (6.4° slope)	Turf Mats (7.5° slope)
60.5 tonnes/ha lost	35.8 tonnes/ha lost	only 0.55 tonnes/ha lost
9.1 kg/ha phosphorus lost	3.2 kg/ha phosphorus lost	only 0.1 kg/ha phosphorus lost
Trough regularly filled with sediment	Sediment levels fell as ground cover increased	Trough was near empty
Extensive slope repair and re-levelling required	Herbicide was washed away, causing a breakdown in weed control	Slope was well protected

The bare slope area showed obvious and worsening damage from sheet and rill (channel) erosion during the monitoring period (Plate 2).
Plate 2: Unplanted area, showing sheet and rill erosion (left) and adjacent turfed area (right), 31 January 2011

The sprigged area was increasingly effective in retaining soil as the turf grass closed over. However, every bit of turf cover helped:
11% ground cover 2.7 tonnes/ha soil saved following 87.5 mm rainfall	48% ground cover 3.5 tonnes/ha soil saved following 30.5 mm rainfall	86% ground cover 10 tonnes/ha soil saved following 119 mm rainfall

Sediment trapping

In late January, a breakthrough area was found on the low earth mound designed to deflect water and sediment movement from the bare area upslope of the demonstration area. Water and sediment entered the fully turfed area. The ability of turf grass to trap sediment was well illustrated, as the wash area tapered away at 6.5 metres (see Plate 1). As a result, the soil was retained on the slope, and the sediment levels in the monitoring trough remained low.

Implications

The work demonstrated the vulnerability of bare slopes to erosion during the summer rainfall period. It highlighted:

• The need to minimise the time that steeper slopes are left unprotected through high intensity rainfall periods (October to March in South East Queensland).
• The effectiveness of turf grass cover as a means of preventing soil and nutrient losses from slopes, and thereby protecting nearby waterways.
• That although the best results are achieved with 100% turf cover, some level of slope protection is afforded by lesser amounts of cover such as turf buffer strips laid on the contour, sprigs or plugs. Planting of sprigs or plugs prior to high rainfall periods will afford full slope protection in around eight weeks.

More Information

For copies of the fact sheets "Monitoring Sediment Loss in Turfgrass Production" and "Improving Soil Retention in Turfgrass Production: The Benefits of Turfgrass Cover" contact: cynthia.carson@deedi.qld.gov.au.

Acknowledgements

This Department of Employment, Economic Development and Innovation project was run in conjunction with Turf Queensland and John Keleher, Principal of Australian Lawn Concepts. It was funded under the Queensland Government Healthy Country programme.