too much nitrogen?

[Frandon]

things dont grow under fir trees is it too much nitrogen

[Ethel]

See how very dry your soil is beneath and around the tree - it sucks all the water up making it difficult for other plants to thrive

[Clanad]

In addition to the previous... unless the fir trees are trimmed of branches quite a ways up the trunk, the shade produced is detrimental to anything growing underneath. Firs, at least here in the western U.S. are also termed allelopathic ... they give of chemicals that are harmful to many plants. The shallow root system extending well outside of the drip line contributes to the problem already mentioned...