MAGNESIUM-DEFICIENCY IN DOUGLAS-FIR AND GRAND FIR GROWING ON A SANDY OUTWASH SOIL AMENDED WITH SEWAGE-SLUDGE

Soil and plant samples were collected from chlorotic plantations of Gr and fir (Abies grandis) and Douglas-fir (Pseudotsuga menziesii) in Win ter, 1989. The soils had been amended in 1981 with an average of 300 d ry Mg ha-1 of municipal sewage sludge. The sludge amendment resulted i n an N application rate of approximately 8000 kg ha-1. Foliage analysi s indicated a severe Mg deficiency (0.25 g kg-1 in sludge-treated vs. 0.93 g kg-1 in untreated area) might be the cause of chlorosis. No oth er nutrient showed concentrations in the deficient or toxic ranges. Tr ace metal levels in foliage were elevated significantly for Ni, Cd and Cr in sludge-treated sites, but not toxic levels. Soil samples taken to a depth of 1.4 m indicated the potential for soil acidification (up to 0.9 pH unit) in soil surface horizons. In addition, exchangeable C a, Mg and K may have been depleted in surface horizons. Exchangeable A l and Fe were greater in the surface of sludge-treated sites. These ob servations and the loss of much of the nitrogen added during the sludg e amendment indicated that nitrification and cation leaching was likel y the mechanism for acidification and depletion of exchangeable cation s. Fertilization of the plantation with MgSO4 or dolomitic limestone w as carried out in Spring 1990. New foliage collected in June, 1990 was non-chlorotic and significantly higher in Mg concentration than unfer tilized foliage (1.1. vs. 0.7 g kg-1, respectively). Results of this s tudy indicate that it is important to assess the potential for initiat ing a nutrient deficiency due to secondary effects of sludge applicati on in forest systems.