Irrigation of tree plantations with wastewater in Victoria.  2. Distribution, length and weight of roots in young Eucalyptus grandis (flooded gum) plantations at two sites in North-western Victoria.  P. J. Baldwin and H. T. L. Stewart.  July 1985.  13 pp. (unpubl.)

SUMMARY

The root systems of Eucalyptus grandis W. Hill ex Maiden (flooded gum), irrigated with municipal wastewater at two sites in the Mildura Region, north-western Victoria, were studied by excavation and coring. Trees at Robinvale were four-years old and were irrigated using micro-sprays (one per tree) that covered only 70% of the ground surface area around each tree, whereas at Mildura wastewater was uniformly applied to six-year-old trees by flood and sprinkler irrigation. Trees were planted in rows spaced 3 m apart. Measurements were made of the length and weight of roots and the number of root intercepts in the top metre of soil at Mildura. The effect of distance from a tree row on each of these parameters in the top 30 cm of soil at both sites was also studied.

At Mildura where roots were excavated from a 2.80 m x 2.80 m x 1.20 m block of soil, a total root length of 1193 m.m^-2 and a total root weight of 3.1 kg m^-2 were estimated in the top metre. For roots > 1 mm diameter, 77% of intercepts were at 0-30 cm, whereas only 5% were in the 50-100 cm soil horizon.

At both sites where roots in the top 30 cm were studied by coring, the vertical distributions of root intercepts, length and weight were similar. Root length was greatest in the 0-10 cm soil horizon at both sites, and intercepts of roots < 1 mm diameter comprised 73% and 81% of all roots at Mildura and Robinvale respectively. Roots < 1 mm diameter contributed 85% of total length at both sites, though only 19% and 21% of total weight, at Mildura and Robinvale respectively. The horizontal distribution of roots differed at the two sites. With uniform application of wastewater at Mildura, root intercepts and length were concentrated in the central region between rows of trees whereas at Robinvale, the concentration occurred closer to the tree row, due mainly to the different method of irrigation. Root weight at both sites was highest within 50 cm of the tree row.

Root densities of 0.11 to 0.57 cm.cm^-3 were estimated for the two plantations; these were similar to root densities measured in Pinus radiata D. Don (radiata pine) plantations up to 46 months old, but were considerably lower than those estimated for pastures in other studies. The implications of the results for the management of irrigated plantations of eucalypts are discussed.