Mycorrhiza development of loblolly pine seedlings in relation to soil reaction and the supply of nitrate

Summary A study was made of the relative influence of nitrate and soil pH on mycorrhiza development in seedlings of loblolly pine (Pinus taeda L.). The investigation was conducted in the greenhouse, using as the growth medium topsoil collected from a young pine plantation.Lime-induced Fe-deficiency occurred at pH 7.5, resulting in chlorotic seedlings with few mycorrhizas. Chlorosis was corrected, and normal mycorrhiza development restored, by adding Fe-EDTA without altering soil pH.Application of 18 1/2, 37, and 74 pounds of N per acre as NaNO₃ reduced mycorrhiza development at age 21 weeks, but had no effect at age 45 weeks. At 21 weeks, the degree of infection varied inversely as the percentage total N in seedling roots. The effect of NaNO₃ was due to the nitrate ion, since Na₂CO₃ did not reduce mycorrhiza development even though it raised soil pH.Alkalinityper se did not affect mycorrhiza formation in loblolly pine seedlings, but only indirectly through its influence on host nutrition. Normal mycorrhiza development was possible at pH values of 7.2 and 7.5, provided Fe deficiency was corrected, and soil nitrate level was kept low. The results may be interpreted in terms of the carbohydrate — nitrogen balance in the root tissues.