If you have clay soil in your garden, you probably also have bald spots from pulling your hair out every time yet another plant smothers in the thick, airless soil. Clay soil is the bane of anyone with the misfortune to have to work it. Amendments can help change the structure of the soil, but it takes a lot of material and can get expensive. If you are tired of lugging wheelbarrows of heavy clay soil out of the yard, and not up to shouldering the expense of mounds of soil amendments, some fruit trees will do just fine in clay soil.
Apple trees are easy to grow, even in clay soil. The thing to keep in mind when planting your tree in clay is to plant it just a bit higher than it is in its current pot. This will allow the roots to remain closer to the surface so that air can get to them. Make sure that 2 inches of the root ball are above the surface of the soil and then pack soil around it. Planted in this manner, the tree will require water more frequently, so keep your eye on it.
Apple trees prefer acidic soil, so perform a soil pH test prior to planting. The ideal pH is 6.5. Make sure your apple tree gets full sun, especially in the morning.
Most pear trees have very deep roots, so try to find one that has been grafted onto quince stock. These have shallower roots and are more suited to clay soil. Plant your pear tree in the autumn in a sunny area. Pears, members of the same family as apples, have generally the same growing requirements as apples. You will need two pear trees for pollination and flowering to occur.
Avocado trees grow best in USDA zones 9, 10 and 11, where there's lots of sunshine and warm temperatures. These trees have shallow roots, so clay soil isn't too much of a problem for them, as long as the pH is between 6 to 6.5.
Plant avocado as you would the apple tree, only 1 inch deeper and after you mound the soil over the crown, add 1/3 cubic yard of coarse woody mulch. Spread the mulch 6 inches from the trunk, all the way around the tree. The best time to plant an avocado tree is in March or April, to give the roots time to get established prior to the heat of summer.