ALL plant material will compost if you grind it fine and mix in enough
manure, but will the result leave "humus", and who has a huge quantity
of manure to mix with pine needles? Apparently much less humus results
from hot, bacterial composting than with slower fungal methods of
composting.
There are three common types of lignin (and many other more specific
ones). The people at LaVal U. seem to think that dry grass, hardwood
chips, and softwood chips break down differently (i. e., they tested
countless samples of soil mixed with various types of sheet composting
materials and analyzed the evolution of both the material left behind
and the soil leachates). They found that grass lignin breaks down
quickly to form humus, resulting in a brown soil, but that the humus
doesn't persist for very long in soil thereafter. Conifer lignin takes
forever to break down, but when it does it produces compounds toxic to
most plants, and little persisting humus. Hardwood lignin breaks down
into healthy soil at a moderate rate, if I recall, but the abundant
humic compounds that result are purported to have half-lives of 1000
years in the soil!
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