MANAGING NITROGEN-FERTILIZATION FOR YEAR-ROUND VEGETABLE PRODUCTION IN PADDY RICE FIELDS

Permanent high bed cultivation systems are primarily used to overcome flood damage in vegetable crops cultivated in many lowland peri-urban production zones throughout Asia. Over-doses of nitrogenous fertilizer s in intensive tropical and subtropical vegetable production have crea ted concern about impact on environment and product quality, and have led to a demand for better N management practices. The N-min-fertiliza tion method has been tested in a 14-month continuous vegetable croppin g sequence with 5 crops in two cultivation systems: flat planting beds and permanent high beds. Permanent high bed cultivation and N-min-fer tilization method were compared with standard practices to test their potential for more resource-efficient productivity. An integrated stud y of soil NO3, plant sap NO3, and crop yield was undertaken for dry se ason crops of carrots and vegetable soybean and rainy season crops of Chinese cabbage and chili. The influence of cultivation system (flat b eds versus high beds) on productivity was much more conspicuous than t he effect of fertilization regimes. Summer crops of Chinese cabbage an d chili pepper in 1993 and Chinese cabbage '94 on high beds outyielded those on flat beds by 58 %, 240 %, and 161 % for the standard N appli cation rates. Plant available N concentrations in soil (N-min) followe d a seasonal pattern of fixation and/or immobilization of ammonium N f ertilizer during the rainy summer season, when water saturation inhibi ted nitrification most of the time. During the winter season this fixe d nitrogen reappeared as soils dried out. This was particularly true f or the more flood-prone flat beds. The accelerated mineralization pote ntial during the dry season led to N-min-concentrations obviating the need for N fertilization for carrot and vegetable soybean crops. No yi eld reductions were observed compared to the standard N application ra te. Although soil N supply was more limited during the rainy season, o nly yields of Chinese cabbage during 1994 on high beds were significan tly reduced by the N-min-method. A total of 360 kg N/ha (42 %) was sav ed by the N-min-method during 14 months and 5 vegetable crops. The com bination of hyperbolic relations of plant NO3-N as a function of soil nitrate and yield as a function of plant NO3 resulted in a good estima tion of yield=f(soil NO3) for vegetable soybean. Small differences bet ween measured sap NO3-levels and calculated upper limits demonstrated that soil N was not a limiting growth factor. Permanent high bed culti vation in combination with use of the N-min fertilization method was t he most resource-efficient combination for out-of-season summer vegeta ble production.