Downward movement of nitrate and ammonium nitrogen in a flatland ultisol

Three field trials were conducted in the early and late rainy season on a Piarco Series soil (Aquoxic TvapuduEts) in Central Trinidad to monitor downward movement of NH4+-N and NO3--N under flat-tilled and ridge-tilled conditions as affected by mulch application. The first experiment was carried out in the early rainy season under bare-fallowed conditions, while okra (Abelmoschus esculentus) was the test crop used in the two remaining trials, which were conducted during the early and late rainy season periods. The plots were fertilized with urea seven days after crop emergence and 14 days after land preparation for the cropped and bare-fallowed experiments respectively. Soil samples were collected weekly after fertilizer application and analyzed for NH4+-N, NO3--N and soil water. Urea application increased NH4+-N levels in the soil and NH4+-N was the dominant inorganic N form observed for the first four weeks after fertilization. Mulch application decreased NH4+-N and NO3--N soil levels. Ridging the soil increased downward movement of NH4+-N and NO3--N. Under bare-fallowed conditions, downward movement of NH4+-N was noted in the 30 to 45 cm soil layer at seven days after fertilization, while under cropped conditions its movement was restricted to the 15 to 30 cm layer within the same period. In bare-fallowed soil, increased NO3--N and its downward movement was noted after four weeks of fertilization. In the cropped soil, downward movement of NO3--N was observed one week after fertilization in the early rainy season and three weeks after fertilization in the late rainy season.