While taking off from a forestry strip the aircraft performance achieved was less than expected. Concerned about terrain proximity after lift off the pilot operated the dump lever. This did not prevent the landing gear from striking a log beyond the end of the strip. The aircraft contined to fly and the pilot diverted to Albury Airport. During the subsequent landing roll the right main landing gear leg collapsed. Soft sand had been put on portions of the centre of the forestry strip and it appeared this had adversly affected the takeoff performance. A spreader was fitted to the aircraft to assist in distribution of the hopper load. When the dump was attempted the spreader severly reduced the effectiveness of the dump and only about 20 per cent of the load was jettisoned. Significant Factors: The following factors were considered relevant to the development of the accident: 1 The presence of soft sand on the strip surface. 2 The pilot did not anticipate the effect the sand would have on the takeoff performance. 3 Fitment of the spreader resulted in a severe reduction in the performance of the dump system.