YIELD, GROWTH, AND MINERAL-NUTRITION OF YOUNG RAY RUBY GRAPEFRUIT TREES UNDER TRICKLE OR FLOOD IRRIGATION AND VARIOUS NITROGEN RATES

Growth, fruiting, and mineral nutrition of trickle- or flood-irrigated young 'Ray Ruby' grapefruit (Citrus paradisi Macf.) trees on sour orange (C. aurantium L.) rootstock were compared in a 4-year field study. Trickle irrigations (two emitters per tree) were scheduled based on: 1) 0.7 (first 3 years) or 0.5 (4th year) of Class A pan evaporation (TPAN) adjusted to the ground area covered by tree canopies, or 2) tensiometer readings (TTEN) of -0.02 MPa at 30-cm soil depth. The flood irrigations (FLOOD) were scheduled at 50% available soil water depletion at 30 cm (first 3 years) or 30- and 60-cm soil depth (the 4th year). Nitrogen at N0 (none), N1 (20, 40, 80, 160 g N/tree per year in the four consecutive years), or N2 (twice the amount of N1) was injected into the trickle lines from January to August or, under FLOOD, split into January and May soil applications. TPAN and TTEN trees were irrigated with < 10% of the water amount applied to FLOOD trees without negatively affecting tree growth, yield, or fruit size. Growth of the trees was not affected by N fertilization, but fruit count and yield and leaf N concentration were increased by the N1 and N2 treatments in the fourth growing season. Frequent N fertigations under the trickle system provided no benefits over two split-soil broadcast applications under the flood system. Fruit size was reduced by the N2 treatment. Based on the water amounts applied to TTEN trees, irrigation needs under the trickle system were estimated to be 0.75, 0.57, 0.30, and 0.20 of Class A pan evaporation adjusted to the ground area covered by the plant canopies, in the first, 2nd, 3rd, and 4th year of orchard life. The decreasing pan coefficient indicated increasing extraction of water from outside the irrigated zones. Roots of TPAN and TTEN trees grew at least 210 cm past the wetted zones into the row middles. More than half of the roots in the TPAN and TTEN treatments were found at 60- to 230-cm soil depth compared to only 17% in the FLOOD treatment.