Heavy soils are those that contain more clay particles relative to sand and silt components. The high clay content causes the soils to be less friable, making them “sticky,” and more dense, thus making them hard to work with. They often remain cold and wet in spring and need grit or coarse organic material to admit air and help roots remain healthy. Because of their tendency to be cold and wet for longer periods than lighter soils, there is an increased tendency for Phytophthora spp. to cause problems on apple rootstocks planted in heavy soils. Phytophthora spp. cause root, crown, and collar rot of apple and root rot of other tree fruit and small fruit species. Also, Phytophthora has a swimming zoospore stage that is favored by cool, wet soil conditions, allowing for more tree-to-tree spread in the orchard. Due to the tendency of heavy soils to be cold, tree development on heavy soils can be delayed in the spring relative to those on lighter soils. When bloom is delayed later into the spring season, higher air temperatures during bloom may lead to higher incidence of fire blight compared to similar trees on lighter soil that likely bloomed earlier. This is often why we observe pockets of fire blight in orchards with areas of heavy soil.
Answer provided by Alan R. Biggs, West Virginia University.