| Abstrakt: |
The growth and nitrogen uptake of a maize crop (Zea mays) were studied under furrow irrigation on a red-brown earth soil. Plots receiving 120 kg N/ha as urea at sowing were compared with unfertilized plots and with plots receiving the same amount of nitrogen, as urea or aqua ammonia, added to water during three irrigations: in the first or last 2 h of three 6-h irrigations (at 20 kg N/ha.h), or continuously during three 6-h irrigations (6.7 kg N1ha.h). Dry matter accumulation and nitrogen uptake measurements made 21,36,49,84 and 150 d after emergence all revealed a consistent pattern of results. The same pattern was also evident in grain yield. Urea-fertilized plots gave higher yields (mean of the three application methods = 3.2 t/ha) than ammonia-fertilized plots (1 .8 t/ha). Application early in the irrigation was superior to application throughout irrigation (3 .2 and 2 .5 t/ha, respectively), while plots receiving nitrogen late in the irrigation gave the lowest yield (1 .8 t/ha). There was no significant (P< 0.05) difference between yield from plots receiving urea early in irrigation and that from those receiving urea at sowing. Soil nitrate-nitrogen concentrations below the hills were higher for urea treatments, suggesting greater penetration of the urea solution. In the ammonia treatments, a much higher concentration of ammonium-nitrogen was found immediately under the furrow than under the furrows of urea-fertilized plots, confirming the relatively restricted movement of ammonia into the soil. Ammonia concentration of the irrigation water fell by up to 32% over a 90 m length of furrow. This loss was ascribed to volatilization. Urea was superior to ammonia largely because it minimized mineral nitrogen retention near the soil surface and because it was not susceptible to volatilization losses. Fertilizer application early in the irrigation, during the period of high infiltration, resulted in lower retention of mineral nitrogen close to the furrow surface and away from the root zone. It also reduced the proportion of nutrient solution lost to the drainage system. |
