Profile distribution of nitrate-nitrogen in relation to fertilizer management in banana

The purpose of this study was to evaluate the profile distribution of NO3-N in relation to fertilizer management practices in banana fields of tehsil Hyderabad. Fifteen sites receiving variable rates of fertilizer N were sampled. Soil samples, drawn up to 90 cm depth were analyzed. Majority of banana plantations (80%) were of 2 to 4 years old with average age of 3.4 years. The area under banana ranged from 1 to 80 acres and averaged to 20.3 acres/growers. The amount of different fertilizers applied by the banana growers ranged from 259 to 1078 Kg N (av. 499 kg N), 57 to 852 kg P2O5 (av. 411 kg P2O5), and 0 to 432 kg K2O (av. 29 kg K2O) per hectare per annum. Urea and DAP were the most common nitrogenous acid phosphatic fertilizer used by the growers. Nitrophos was the third common fertilizer. The manure application was a common practice used by growers. It ranged from 0 to 23.7 t/ha and averaged to 6.7 t/ha. Assuming average composition of 0.5% N, 0.3% P2O5 and 0.5% K2O, mean application rate 6.7 t/ha will supply a maximum of 33.5 kg N, 20 kg P2O5 and 33.5 kg K2O/ha. Banana yield ranged from 24.7 to 59.2 t/ha and averaged to 41.3 t/ha. Level of management, virus attack, plant population, fertilization and manuring etc. were identified as major causes of variability in production levels. The soils supporting banana plantations were medium to heavy in texture (loam to clay) and non-saline (EC = 0.18 to 0.48 dS/m) with pH from 7.4 to 8.0. Organic matter content of top soil (0-15 cm) ranged from 0.88 to 1.45% (average 1.12%). Nitrate-N distribution in soil profiles of banana fields ranged from 1.4 to 18.9 mg/kg in top soil (0-15 cm) and 2.7 to 22.4 mg/kg in subsoil (15-30 cm), 6.3 to 20.2 mg/kg at 30-60 cm and 2.1 to 11.4 mg/kg at 60-90 cm. The mean values of four depths were 10.2, 10.5, 11.31 and 7.9 mg/kg respectively. CV values were the highest for surface 0-30 cm soil and showed a decline with depth at 60-90 cm. Highest conic of profile NO3-N was 22.4 mg/kg at 15-30 cm depth. Majority of the soil profiles contained up to around 10 mg/kg. Thus, average NO3-N contents ranged from 7.9 mg/kg at 60- 90 cm to 11.30 mg/kg at 30-60 cm, with the profile (0-90 cm) average content of 10 mg/kg. Olsen P content of the surface soil (0-15 cm) ranged from 5.6 to 44 mg/kg (average 23.8 mg P/kg) and NH4OAC-K ranged from 120 to 350 mg/kg (average 196 mg/kg) in top soil (0-15 cm). The coefficient of correlation between rate of N fertilizer, organic matter and NO3-N content of soil showed that none of the correlations were significant including that NO3-N content of the soil was not related to either organic matter or rate of fertilizer N applied. It is concluded from the results that conjunctive use of organic matter and mineral fertilizers by banana growers has not only improved crop productivity but also maintained the fertility and favourable physical and biological environment of soil for sustainable agriculture. However, no relationship could be established between the rate of N fertilized applied and the NO3-N contents of soil profile. It appears that NO3 contents of the soil profile were not high enough to warrant precautions for restricting the movement of NO3-N down the profile. As no ground water sample was collected of analyzed and because the NO3-N content of the 60-90 cm soil (lowest depth sampled) was the lowest (7.9 mg/kg), it seemed unlikely that nitrate movement through the soil was actually contaminating the underlying ground waters. However, excess use of fertilizer N for prolonged periods may lead to the nitrate pollution of ground water.