On average, trees with a surface mulch (3.5 inches deep) had 53% greater increase in trunk area than did those trees without a surface mulch.
Organic amendments in the backfill do not improve and may reduce shoot and root growth. A shallow, wide hole with unamended backfill and a surface mulch is an acceptable, if not superior, planting standard for trees and shrubs.
Root growth is often limited as much by the lack of oxygen as the lack of water. In compacted urban soils oxygen deprivation is a serious limitation.
Source: Barba, J., E. Davidson, and J. Tipton, Effects of Planting Practices on Tree Performance, Turfgrass and Ornamental Research Summary, The University of Arizona College of Agriculture, 1997.
? About using oleander and eucalyptus in the compost pile: "There was no apparent phytotoxid effect of either eucalyptus or oleander compost. These results suggest that whatever phytotoxic compounds that may be present in eucalyptus or oleander were destroyed by composting...based on the results of these tests, yard waste compost derived from eucalyptus or oleander should be a suitable component of container medium."
Source: E. Davidson and J. Tipton, Evaluation of Arizona Yard Waste Compost as a Container Growing Medium, Turfgrass and Ornamental Research Summary, The University of Arizona College of Agriculture, 1997.