There are several reasons for planting seedlings. Many trees send up suckers, which can be dug up and planted elsewhere to add more trees to the landscaping. Nurseries often sell seedlings in addition to larger container trees and bare-root trees. If you have a large area where you would like to plant more trees, starting with seedlings is cheaper than buying larger trees for planting.
Clear the planting area of all weeds, sticks, stones and any other debris, including roots from old trees that have been cut down.
Test the soil for its pH and nutrient levels. If the pH is incorrect for the type of tree you are planting, amend the soil accordingly. If the soil is too acidic, add lime to the soil. If the pH is too neutral, mix compost into the soil. If the soil is missing nutrients, add nutrient-specific fertilizer to the soil. If you need to add pH and nutrient amendments, these can be mixed, then spread and worked into the soil. Whatever amendments you use, they should be tilled 6 inches into the soil.
Dig a planting hole as deep as the root ball and twice its size. Center the seedling in the hole, then backfill with soil. Water the seedling with at least 1 inch of water.
Mulch the seedling with 1 inch of compost or pulverized bark. If you're planting larger seedlings, use up to 3 inches of compost or pulverized bark. The compost protects the root ball from extreme temperature changes and helps to hold moisture in the soil.
Fertilize the seedlings after the first year, and once the roots become established. Use shrub and tree fertilizer, either chemical or all-natural.