SUBSURFACE DRIP IRRIGATION AND UREA PHOSPHATE USE IN VEGETABLES ON CALCAREOUS SOILS.

Abstract

Drip irrigation lines placed 15 cm deep or 5 cm shallow below soil surface and furrow irrigation were compared on calcareous soils of southern Arizona. Crops grown were zucchini squash (Cucurbita pepo L.) 1982 and 1983 summers, and cabbage (Brassica oleracea var. capitata L.), 1982 fall. Furrows received a preplant application of phosphorus (P) at a rate of 100 kg P₂O₅/ha. Urea phosphate (UP) (17-44-0) solution was injected twice in drip lines during the growing season to give a total rate of 100 kg P₂O₅/ha on cabbage and 50 kg P₂O₅/ha on squash. Deep drip lines produced 52 and 34% higher yield than shallow drip in squash 1982 and 1983 experiments respectively. Deep drip yields were comparable to fertilized furrow yields with half the water and half the fertilizer rate of the furrow used by the deep drip. Unfertilized furrow yielded least. Application of UP in deep drip raised PO₄-P and NO₃-N concentration in squash leaves to comparable values in fertilized furrow plants. Shallow drip and unfertilized furrow plants had low nutrient concentrations. Soil analysis for NaHCO₃ extractable P and NO₃-N showed higher levels under drip than furrow treatments. Available P increased to 20 cm from emitters. EC was lower under drip than furrow. Soil pH was reduced by 0.5 units around emitters. Soil moisture in root zone was highest under furrows. Deep drip had more moisture than shallow drip. Injection of UP reduced water pH to 1.8 which prevented P precipitation. Cabbage yields were comparable under all fertilized treatments. Deep drip yielded 19% higher than shallow drip. Tissue analysis showed higher nutrient concentration under drip than furrow treatments. Soil analysis showed higher available P and NO₃-N and lower EC and pH under drip than furrow treatments. Mobility of PO₄ from UP was about 20 cm in soil columns. Rate of P fixation was high. Reduction in pH followed same pattern of P mobility. Root growth and distribution was more extensive under deep than shallow drip. Roots concentrated around deep UP zone in transparent cylinders.