Effects of irrigation management on nitrogen availability in high density apple orchards

Direct application of fertilizers in irrigation water (fertigation) is considered to be an efficient method of fertilizing fruit trees. However, more information is needed on the relationship between irrigation and N inputs in order to target nutrient applications to meet plant demands. Soil solution was collected from permanently installed suction lysimeters and NO3-N concentration was measured over the growing season in a loamy sand soil and a silt loam soil in response to fertilizer applied either 1) at fixed rates (50g N tree(-1) year(-1) in 1991 or 75g N tree(-1) year(-1) in 1995 when water was supplied at either 4 or 8L water day(-1) in 1994 or according to evaporative demand estimated by an atmometer in 1995: or 2) at a concentration 75 or 150mg NO3-N L-1 when water was applied at SL water day(-1) in 1994 or according to evaporative demand estimated by an atmometer in 1995. Trials were randomised and replicated and were located in high density apple plantings of 'Gala' on M.9 roots-tock. Nitrate-N concentration in the soil solution measured at 30cm depth, beneath the drip emitter, increased rapidly with the onset of fertigation and decreased rapidly when fertigation ended. Concentrations remained relatively constant during the intervening period and were directly proportional to either the amount of N or the amount of irrigation water added. A model for predicting soil solution N concentration at 30cm beneath the emitter is proposed based on the mixing of the fertigating solution with the pre-existing soil solution. A better fit to the data was found for the silt loam soil than for the loamy sand soil. Nitrate-N concentration was measured in soil solution below the root zone (75cm depth) in loamy sand plots receiving 4 or 8L watts daily and was also proportional to the volume of water added, with the lower volume only delaying movement of applied N. In the silt loam soil, considerably lower concentrations of NO3-N were found beneath the root zone for both volumes of irrigation water and there was insufficient water moving to 75cm to provide samples after mid-July. Nitrate-N movement beneath the root zone was lower in the loamy sand soil in 1995, when irrigation was based on evaporative demand.