Optimizing water and nitrogen inputs for trickle irrigated melons

Abstract

Rising water costs and concern for groundwater contamination are forcing melon growers to improve irrigation and nitrogen fertilization efficiency. The research objectives were: 1) to determine quantities of nitrogen and water applied through a subsurface drip irrigation system to cantaloupe and watermelon which would optimize fruit yield while minimizing losses of nitrogen and; 2) to develop specific monitoring techniques for assessing the water and nitrogen status of melon crops throughout the growing season. Four years of field research were conducted from 1988 to 1991 at the University of Arizona Maricopa Agricultural Center, Maricopa, AZ. The first two years of research determined which varieties of cantaloupe and watermelon would grow best under drip irrigation and the feasibility of using plant tissue tests to aid in N fertilizer scheduling. The last two years of research used a factorial design with levels of N and target soil water tension to determine response surfaces for fruit yield and net return. Information from tensiometers was used to schedule irrigations. Watermelon petiole nitrate levels at critical growth stages were used to recommend application rates of nitrogen fertilizer. In 1991, an N difference method was used to estimate N which was unaccounted for in this watermelon cropping system. Petiole nitrate levels were highly responsive to N fertilizer treatments and accurately quantified visual observations of crop N status. Petiole nitrate results also indicated that preliminary nitrogen fertilizer management guidelines using a tissue nitrate test was reasonably accurate in predicting optimum nitrogen 'management. Market able yield showed a soil water tension by N interaction. Maximum marketable yield was estimated to be 101 Mg ha⁻¹ at 7.2 kPa tension and 336 kg of applied N ha⁻¹. Maximum net return was estimated to be $8 250 ha⁻¹ when average soil water tension was 10.6 kPa and applied N was 243 kg N ha⁻¹. Unaccountable N was estimated to be 300 kg N ha⁻¹ when N rates were 500 kg ha⁻¹ and average soil water tension was 4 kPa. A response surface was estimated through the transformation and summation of yield, net return and unaccounted for N response variables. The optimum average soil water tension and rate of applied N were 12.6 kPa and 181 kg N ha⁻¹, respectively.