The relative value of ammonium and nitrate as N sources for Pinus radiata D. Don seedlings was determined under various conditions. Short‐term experiments varying from 3 weeks to 6 months' duration were conducted utilizing three growth media: solution, soil, and exchange resin beads. In the solution cultures N sources were compared at two levels of acidity (pH 3.6 and pH 5.9), three root‐zone temperatures (15, 20, and 25C), and three N levels (2.8, 11.2, and 22.4 ppm N). In the soil cultures N was added at five levels as NaNO 3 or NH 4 Cl with a nitrification inhibitor. Three levels each of ammonium and nitrate were used in a factorial arrangement in the resin cultures. Radiata pine seedlings grown either in soil, solution culture, or exchange resins exhibited greater growth and N uptake with an ammonium source than with nitrate. This was true in experiments at several levels of acidity, in all three growth media and regardless of the root zone temperature or N level maintained. Increases in root zone temperature in solution cultures increased growth and N uptake and increased the differential response to N source. Eight‐fold differences in N concentration in the solution cultures produced only small differences in growth and N uptake. N levels of approximately 2.0 ppm N in solutions proved satisfactory for normal growth. Picea glauca (Moench) Voss seedlings were tested concurrently in the solution cultures and exhibited the same favorable response to ammonium N source.
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McFee et al. (1968) studied this question.