Determination of Application Rates of Nitrogen Fertilizer for ‘Shiranuhi’ Mandarin in Plastic Film Houses Using Soil Electrical Conductivity

Abstract

In this study, we developed an equation to determine nitrogen fertilizer rates for ‘Shiranuhi’ mandarin [(Citrus unshiu × C. sinensis) × C. reticulata] growing soils in plastic film houses to prevent salt accumulation and maintain optimum nutrient levels. We analyzed the amount of applied fertilizers, soil chemical properties, and yields from 45 farms that vary in salt concentrations on Jeju Island, Korea. The results revealed that the average soil chemical property values, such as electrical conductivity (EC), available P2O5, and exchangeable cations (K+, Ca2+, and Mg2+), were higher than the acceptable range, except for pH and organic matter. The results showed that higher amounts of nitrogen and phosphorus were applied through fertilizer application rates (N 327 kg ha-1, P2O5 294 kg ha-1, and K2O 236 kg ha-1) than the standard amount and rate of fertilizer for the cultivation of ‘Shiranuhi’ mandarin. The EC values and NO3-N concentration in the soils had a close correlation (r = 0.96, p < 0.001). The yields did not differ with the amounts of the applied fertilizers while there was a significant negative correlation with EC values (r = -0.455, p < 0.01) and the concentration of NO3-N (r = -0.476, p < 0.01) in the soils. Our results demonstrated that the NO3-N concentration in the soil of plastic film houses has a substantial effect on the soil EC value. Soil EC levels may determine the recommended nitrogen fertilizer rates depending on each tree’s age.Recommended application rates of nitrogen (N) fertilizer according to the soil electrical conductivity (EC) levels depending on ‘Shiranuhi’ mandarin tree ages in the cultivated soils of plastic film houses on Jeju Island. Tree ages (years) N recommendation equation† EC (dS m-1) 0.5 1.0 2.0 4.0 Recommended rates of N fertilizer (kg ha-1) 1 - 5 Y = - 1.6273X + 18.065 17.3 16.4 14.8 11.6 6 - 10 Y = - 8.1363X + 90.323 86.3 82.2 74.1 57.8 11 - 15 Y = - 16.273X + 180.65 173 164 148 116 16 - 20 Y = - 24.409X + 270.97 259 247 222 173 21≤ Y = - 32.545X + 361.29 345 329 296 231 †Y: Recommended rates of N fertilizer (kg ha-1); X: Soil EC value.