Soil repays ground cover and controlled traffic

Penetration resistance

Central Queensland trial results are confirming that retaining ground cover and using controlled traffic do wonders for the soil.

Queensland Department of Natural Resources scientists are measuring the effects of these techniques, supported by growers and the Federal Government through the GRDC. On one site near Emerald, funded by the Cotton Research and Development Corporation, they have measured runoff and soil loss continuously since 1994.

The scientists found the amount of vegetative cover on the ground, alive or dead, had a big impact on the level of runoff and the amount of soil lost. The table (below) shows this in a comparison of two runoff events in 1998. The site has black cracking clay on a 1 per cent slope and is 550 m long. As in all the controlled traffic layouts, the wheel tracks go down the slope.

"The data confirm that designed CTF (controlled traffic farming) layouts which incorporate minimum tillage and opportunity cropping will control soil erosion," said team leader Don Yule.

46 mm rain, rate of 44 mm/hr 25/9/9824 mm rain, rate of 32 mm/hr 9/10/98
Ground cover (%)Runoff (mm)Soil loss (t/ha)Erodability (t/ha/mm)Runoff (mm)Soil loss (t/ha)Erodability (t/ha/mm)
0-20263.320.1391.330.15
20-40223.320.1550.850.17
40-60120.550.0510.060.0

Soil cover decreases runoff

Runoff, soil loss and erodability all decreased dramatically when cover levels exceeded 40 per cent. This means that if annual runoff is 100 mm and soil cover low, soil loss would be around 15 t/ha. If ground cover is high, runoff is reduced to around 50 mm and soil loss to only 2.5 t/ha. (Ground cover is estimated by visual equipment used from a helicopter.)

Compaction another worry

More recently on the same sites, researchers also studied soil compaction in the downslope CTF layout. They did this by measuring penetration resistance — an indicator of soil compaction — in crop zones that had not seen traffic for six years and also beneath wheel track zones.

Penetration resistance levels above two MPa (megapascals) will restrict taproot penetration of cotton. Values over 2 MPa occurred within 5-10 cm below the wheel track and again at 20-40 cm below the wheel track (see diagram below). The increase in penetration was limited to a zone 40-60 cm wide below the wheel track.

"Soil compaction in CTF is confined to the wheel tracks, and as overall traffic is also reduced in CTF, even in the wheel tracks there was only limited compaction after six years," Dr Yule said.

Program 3.4.1 Contact: Dr Don Yule 07 3362 9388, Mr Ken Rohde QDNR Emerald 07 4987 9318

Region North