Fertilization Effects on Soil Solution Chemistry in Three Eucalypt Plantations

Concentrations of nutrients in soil solution may by important indicators of nutrient availability for forest plantations, but we were unaware of the extent to which these concentrations varied temporally and were affected by fertilizer treatments at various sites. The effect of fertilizing eucalypt plantations on the concentrations of NH 4 , NO 3 , and inorganic phosphate (Pi), and on electrical conductivity (EC) and pH in paste extracts was assessed at three sites in Tasmania, Australia. Nitrogen and P fertilizers were broadcast at elemental rates in the range 0 to 600 kg ha −1 as (NH 4 ) 2 SO 4 and triple super‐phosphate within 2 mo of planting, and again at 26 mo at two of the sites. Nutrient concentrations in undiluted soil solution were inferred by adjusting for the effects of dilution and buffering. Both fertilization events decreased pH by up to one‐half a unit, and increased conductivity and the concentrations of NH 4 and P i by up to 10‐fold in the 0‐ to 60‐cm depth at all sites. Even at the highest rate of application, values of all parameters generally returned to unfertilized values before the second application of fertilizer. Rates of decrease in concentrations of NH 4 and P i after fertilization were unaffected by the age of the crop. When broadcast, applications of fertilizers may be needed at 2‐ to 4‐yr intervals to maintain high nutrient availability. A single application of (NH 4 ) 2 SO 4 to these soils at a typical rate of 200 kg N ha −1 will have minimal effect on pH, but multiple applications could decrease pH and adversely affect Ca, K, and Mg availability.