CONSIDERATION OF THE SOIL N-MIN CONTENT IN N FERTILIZATION OF VEGETABLES IN INTENSIVE, YEAR-ROUND PRODUCTION IN A TROPICAL, RICE-BASED LOWLAND

From 1993 to 1995, reduction of traditional N application rates by soi l N-min was evaluated in an intensive rotation of four vegetable speci es in the tropical, rice based lowland environment of southern Taiwan. When crops were grown on traditional flat beds, no yield reductions d ue to this method were observed. This could be attributed to accumulat ion of soil nitrate in the dry season, and to overwet soil conditions in the rainy season. High soil moisture induced water stress and shall ow root systems in vegetables, preventing them from effectively absorb ing available nitrate. Consequently, more nitrate leached below the ro ot zone. Permanent high beds successfully alleviated water stress in v egetables during the rainy season and crops developed profound root sy stems. Soil nitrate was efficiently absorbed, and less nitrate was lea ched below the root zone when more N was applied. Better yields could not be sustained with the N-min reduced method, and even the recommend ed fertilizer rates should presumably be adjusted to the greater veget able biomass-production potential on permanent high beds.