Soil pH, extractable phosphorus, and exchangeable cations as affected by rates of fertiliser nitrogen, phosphorus, and potassium applied over several years to Valencia orange trees

Abstract

The long-term, annual application of nitrogen (N), phosphorus (P), and potassium (K) to the drip area of Valencia orange trees growing in a Tiltao sandy loam soil in the Sunraysia district of the lower Murray was investigated for its effect on soil pH, Bray 1 extractable soil P, and exchangeable cations calcium (Ca), magnesium (Mg), K, sodium (Na), and aluminium (Al) at 5 soil depths.Nitrogen (applied as ammonium nitrate) generally lowered soil pH to a depth of 30 cm, the effect increasing with rate of applied N. At 20 cm depth, the highest rate of applied N (450 kg/ha) reduced soil pH from 7.6 to 3.9, while 150 kg N/ha resulted in a soil pH of 5.0. Consequently, levels of exchangeable Ca, Mg, K, and Na in the soil were lower. There was a strong negative correlation between soil A1 and soil pH. At soil depths of 10-30 cm, the highest rate of applied N resulted in high levels (>20%) of exchangeable A1 as a proportion of the effective cation exchange capacity (CEC). Available soil P increased with rate of applied P, but the effect decreased with depth. Significantly (P<0.05) increased levels of available P were found to 30 cm depth when P was applied at 30 and 90 kg/ha, and to 60 cm depth at 180 kg P/ha. At 180 kg P/ha, Ca from superphosphate decreased Mg to 60 cm depth. Soil K levels were higher with increased rate of applied K and increased soil depth. The highest rate of applied K (360 kg/ha) depressed available P to 90 cm depth. Exchangeable Mg, K and Na percentages of the CEC in most treatments and at most soil depths were above the desirable range. Soil Na levels for some treatments and depths approached the level for sodic soils.