Is soil drainage or rainfall more important?
These two factors interact in helping to determine whether trees survive. For example, trees can cope with less rain when they are: • growing in a gully or the bottom of a valley; • along the edge of a stream or river; • near a spring. And they need more rainfall when they grow: • on ridges; • on other very freely drained soils; • on windy sites; • in areas sometimes subject to periods with drier air. How much does soil aeration matter? It is very important in determining which trees can grow. Reduced soil aeration often occurs through poor soil drainage, because of: • low-lying or flat topography; • small soil particles; • soil compaction. It may also tend to occur because much of the available oxygen has been used up by roots, soil micro-organisms and larger animals living in the soil. What governs the acidity of the soil? • The kind of parent rock from which the soil originally came; • Soil drainage and aeration; • The kinds of leaves falling on it (D 13); • Additions from the atmos
