Labeled Nitrogen Uptake by Drip‐Irrigated Tomatoes¹

In order to evaluate the availability of fertilizer N applied by drip irrigation, tomatoes ( Lycopersicum esculentum Mill.) were grown in the field for two years on Panoche clay loam (Typic Torriorthents). Labeled fertilizer was banded in the soil and furrow or drip irrigated as well as applied through the drip system to measure the effect of different placements and timing on fertilizer N availability. Some treatments received N only at planting while others received split applications through the drip system at different stages of plant development. Nitrogen ( 15 N depleted) was applied to the center harvest rows while commercial (0.36% 15 N) ammonium sulfate was applied to the outside rows of each plot. Plants from the center harvest rows were analyzed for total N and isotopic composition. Plant analyses indicated that treatments in which fertilizer N was applied through the drip system, regardless of timing or growth stage, showed higher percent fertilizer N contents during most of the growing season than those of the furrow‐irrigated treatments. The greatest amount of fertilizer N in the plant at any sampling time was 40 to 50% of the total plant N. Only small differences in total tomato yields were observed among treatments, although drip‐applied did appear to enhance tomato fruit maturity. For high efficiency, fertilizer N should be placed carefully with respect to plant roots, taking into consideration the direction of water movement during irrigations.