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Effects of Phosphorus on Growth and Mycorrhizal Development of Douglas-fir in Greenhouse Pots¹

ABSTRACT

Results of this study confirmed that P deficiency is a cause of poor growth and poor mycorrhizal development in greenhouse grown Douglas-fir seedlings [Pseudotsuga menziesii (Mirb). Franco] in certain horizons of two pedons of an Andic Haplumbrept from western hemlock stands (Tsuga heterophylla) of the coastal area of Washington. The phosphorus levels in foliage of 9-month old seedlings without supplemental P were as low as 0.04%. Phosphorus treatment at the rate of 300 kg P/ha increased foliar P levels to an average of 0.19% P compared with an average of 0.08% P in untreated seedlings. In contrast, foliar N levels were reduced by P fertilization. Highest response to P treatment occurred on one of the Al horizons where seedling dry weight at 9 months was increased from an average of 0.31 to 1.89 g. In addition, P fertilization significantly increased development of mycorrhizal roots on seedlings.

In the absence of field results confirming P deficiency in these soils, the practical implications of this study are tentative. However, the results of this experiment indicate need for field study of P fertilization of forests in the coastal area of Washington and perhaps also in Oregon and British Columbia.

¹ Contribution from the College of Agriculture Research Center, Pullman, WA 99164. Scientific paper no. 5152, Project 1971.

² Forest Scientist and Research Technologist, respectively, Western Washington Research and Extension Center, Puyallup, WA 98371.

Received for publication August 17, 1978. Accepted for publication September 6, 1979.