The effect of two commercial fertilizers on some wood properties of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) has been determined at macro- and micro- levels. Three trees, approximately 30 years of age, representing two treatments, (NH₄)₂SO₄ and NPK (N-NO₃), and an untreated control, were available. Selected trees had been fertilized first in the year 1957 at 21 to 23 years of age, and subsequently in the year 1959, with 400 lb of fertilizer per acre each time. Sections from five vertical levels of each tree were examined. The wood investigated represented each growth increment formed after 1956 and some selected ones before 1956. Response to treatment was found in the year following fertilization, with an accelerated growth rate, and a general decrease in specific gravity, per cent latewood and mean tracheid length. Maximum growth response occurred the year of re-fertilization or the year following. The greatest reduction of per cent latewood and specific gravity accompanied re-fertilization. For both fertilizers, a 4.4 per cent reduction in specific gravity followed the two separate applications, but differences in specific gravity before and after treatment were not statistically significant. Tracheid length reduction subsequent to NPK treatment was 14.5 per cent compared to less than one per cent with (NH₄)₂SO₄, but again the differences before and after treatment were not statistically significant. The tree with initially longer tracheids responded more adversely than that with shorter tracheids. Relationships between several wood characteristics indicated that macro-specific gravity was highly influenced by per cent latewood, diameter at breast height and increment width. These three variables together accounted for 57.6 per cent of the total variability in specific gravity. Macro-specific gravity was highest at the base and lowest at the top of the tree, and at any one height it was lower near the pith and higher towards the bark. Six consecutive growth increments, representing three age segments (before treatment, including and after first and second treatments, respectively), were studied at micro-level from breast height sections of each tree. Six positions within each increment were examined. Results indicated a 20 per cent reduction in tensile strength due to NPK treatment that was highly significant statistically. No apparent influence was found for (NH₄)₂SO₄; tensile strength values were, however, lower following re-fertilization as compared to untreated control. Micro-specific gravity and tracheid length followed trends similar to those of tensile strength. Strong correlation (r=0.92) was observed between micro-specific gravity and micro-tensile strength. It is proposed that N in the form of (NH₄)₂SO₄, when compared to NPK, was a better fertilizer for the two treated trees examined. It was concluded that weak residual effects of fertilization on these Douglas fir trees persisted four to five years after re-fertilization. For both treatments, increased specific gravity and growth rate resulted whereas cell length failed to demonstrate normal increase with age. / Forestry, Faculty of / Graduate