Bark residue as a soil amendment in broad-acre intensive vegetable

Background: Approximately 40,000 tonnes of eucalypt bark residue are produced by North Forests each year, posing a significant waste issue. Conversely, concern has been expressed over the continual loss of organic matter from intensively cultivated cropping soils on the North West Coast of Tasmania, leading to poor soil structure. This project is investigating whether bark residue could be beneficially employed in vegetable soils.

Objectives: To investigate the costs and benefits of adding bark and compost amendments to intensive vegetable soils on the North West Coast of Tasmania.

Work undertaken to date: A trial was established in 1999 on the TAFE Freer Farm at Burnie and involved four replicate plots (each 210 m²) of each of the following: composted sewage sludge/eucalyptus bark at I 0 and 100 t/ha, eucalyptus bark at 10, 50 and 100 t/ha and an untreated control. A broccoli crop was transplanted and harvested in February and April/May 1999 respectively. Significant weed suppression was noted initially on plots treated with high rates of bark. Treatments had no effect on total yield, although maturity was delayed slightly in plots treated with high rates of bark. An onion crop was sown in August 1999 and lifted in February 2000. From tensiometer readings, plots treated with high rates of bark remained wetter for longer, so bark plots received less irrigation. However at harvest, yield and average bulb size was suppressed by high rates of bark. The suppression is thought to be due to a delay in maturity of the onions in the bark plots, together with reduced water availability at a critical period late in the growth of the onion bulb. Further crops are to be grown, with more detailed assessment of crop development and soil moisture.