Effect of Tree Spacing on Root Distribution of 9-Year Old ‘Washington’ Navel Oranges1

The effect of tree spacing on the root distribution of 9-year old ‘Washington’ navel orange trees on ‘Troyer’ citrange rootstock was studied. Spacings ranged from 9 ft x 15 ft to 22 ft x 22 ft. Roots were washed from soil cores (3-ft deep, 4-inch diam) taken at 2-ft intervals along 2 transects, 1 across the row and 1 along the row and beneath an irrigation furrow. The distribution of both small roots (1.5 mm in diam or smaller) and large roots was affected by tree spacing and by location of the irrigation furrow. At close springs, many soil cores contained about 4 g dry wt of small roots, suggesting that the soil contained as many feeder roots as possible under those conditions. Wider spacings had fewer roots per core. Evidence suggests that considerable overlapping of root systems occurred at close spacings but not at the wider spacings. It is likely that root competition will limit yield during the next decade at the closer spacings, whereas considerable room for root growth exists at wide spacings.