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 e xtent 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 NH4, NO3, 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 bro adcast at elemental rates in the range 0 to 600 kg ha(-1) as (NH4)(2)SO4 an d 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 infe rred by adjusting for the effects of dilution and buffering. Both fertiliza tion events decreased pH by up to one-half a unit, and increased conductivi ty and the concentrations of NF4 and P-i by up to 10-fold in the 0- to 60 c m depth at all sites. Even at the highest rate of application, values of al l parameters generally returned to unfertilized values before the second ap plication of fertilizer, Rates of decrease in concentrations of NH4 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 main tain high nutrient availability. A single application of (NH4)(2)SO4 to the se soils at a typical rate of 200 kg N ha(-1) will have minimal effect on p H, but multiple applications could decrease pH and adversely affect Ca, K, and Mg availability.