Healing the Land with Trees
There are a number of ways trees (and shrubs ) used in agroforestry systems cam improve soil fertility and help heal degraded land.
First, many trees species used in agroforestry systems perform biological nitrogen fixation (through symbiotic associations with microbes in thier roots). This nitrogen, thus converted in organic form, is made available to other plants/crops through litterfall and nutrient cycling processes.
Trees are also deep-rooted plants. As such they can access nutrients (such as nitrogen, calcium, magnesium and phosphorus) in the subsoil – these nutrients out of the reach of more shallow rooted agronomic plants. Again, these nutrients are made available to these agronomic plants through litterfall and nutrient cycling processes (this is referred to as “nutrient pumping” in the literature).
The large amount of litterfall produced by trees (both due leaves and root turnover) improves soil organic matter content. This combined with the possitive effect of root growth on soil porosity means trees tend to improve soil water retention and infiltration.
The shade provided by trees also produces a micro-climate that promotes more diverse insect and microbial communities, such systems tending to have less incidence of pests detrimental to crops.
 Shrubs are really just small trees, both are woody plants. In addition to being generally smaller, shrubs tend to have a rounded shape and several main stems growing from ground level, rather than a single trunk. We use the term “tree” in reference to all woody plants we propogate at ACRAA, although some might more correclty be classified as “shrubs.”