Precipitation is ample all year, but is substantially greater during the summer. Needleleaf and mixed needleleaf-deciduous forest grows throughout the colder northern parts of the humid continental climate zone, extending into the mountain regions of the Adirondacks and northern New England. Here soils are Spodosols with a low supply of bases and a horizon in which organic matter, iron, and aluminum have accumulated. They are strongly leached, but have an upper layer of humus. Cool temperatures inhibit bacterial activity that would destroy this organic matter in tropical regions.
Soils are deficient in calcium, potassium, and magnesium, and are generally acid.