Clay soil differs from loam or sand in that it is composed of fine particles of soil that are so closely spaced together that it will not allow drainage. Clay soil is slippery in spring when it rains and rock-hard during the dry summer months. When trees are planted in clay, the soil characteristics can prevent the roots of trees from developing. Trees that sit in poorly-drained clay soil often will suffer from root rot. Improve clay soil by incorporating amendments to improve drainage.
Till the soil to a depth of 12 inches.
Spread a 4-inch thick layer of organic, loamy amendments such as straw, compost, peat moss and well-rotted manure over the soil.
Mix these amendments into the soil with a tiller.
Dig planting holes for trees that are wider than the rootball but no deeper. Trees prefer to be planted at the same level as they were grown.
Remove the tree's burlap covering or unpot the tree from its container by turning it on it's side. Support the trunk with one hand and gently tap the pot with the other to release it.
Place the tree in the planting hole. Cover the hole with soil. Water the tree to eliminate air pockets.
Mulch around the tree to prevent grass from growing over the roots and competing for nutrients or water.