Pound for pound, the leaves of most trees contain twice as many minerals as manure. For example, the mineral content of a sugar maple leaf is over five percent, while even common pine needles have 2.5 percent of their weight in calcium, magnesium, nitrogen and phosphorus, plus other trace elements.
Since most trees are deep-rooted, they absorb minerals from deep in the soil and a good portion of these minerals go into the leaves.
Actually, these multi-colored gifts from above are most valuable for the large amounts of fibrous organic matter they supply. Their humus-building qualities mean improved structure for all soil types. They aerate heavy clay soils, prevent sandy soils from drying out too fast, soak up rain and check evaporation.