Fertigation of apples with nitrate or ammonium nitrogen under drip irrigation. II. Nutrient distribution in the soil

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Abstract
Nutrient distribution in the soil under drip irrigation was monitored by soil sampling and analysis of soil solution collected by vacuum suction through ceramic cups buried in the soil near the drippers. Concentration of ammonium and nitrate nitrogen, phosphate and potassium, and the pH were monitored in three fertigation treatments with N fertilizers containing 0, 50 or 100% NH4 +. All treatments received equal annual doses of nitrogen (128 kg/ha) and potassium (356 kg/ha). The 50% and 100% NH4 + treatments received also P (185 kg/ha) in the form of orthophosphate and polyphosphate, respectively. Only traces of NH4 + (1–2 ppm) could be detected in the soil solution. The NO3 concentration in the soil solution was proportional to the quantity of nitrate in the fertilizer, and was built up in the soil gradually every year as fertigation commenced. Nitrate concentration in the soil solution varied widely between years. Phosphate concentration was raised from 0–4 ppm to 10–20 ppm in the soil solution following application of either polyphosphate or orthophosphate. Potassium concentration of the soil solution was also raised significantly during fertigation. When fertigation ceased, close to harvest and during winter, the soil solution was depleted of nitrate, and the P and K concentrations declined close to their pre‐fertigation level. Phosphate mobility was detected to a depth of 90 cm close to the drippers. Nitrate and potassium were leached to the lower soil layer following excess irrigation, close to harvest. Soil sampling showed that the concentrations of N‐NOg, P and K were highest under the drippers and decreased gradually with distance from it, in all three directions. The decreasing gradient of N‐NO‐j was thought to be the result of a significant interception and uptake by the root system of a low nitrogen dose fertigated by an efficient irrigation practice. Fertigation with ammonium polyphosphate decreased soil pH under the dripper.