Most soils have adequate nutrients to support seedlings the first year. It is usually best not to fertilize seedlings unless definite deficiency symptoms are present. Evergreen trees are not very demanding and normally do not need fertilizer at any age. If there is too little iron in the soil, your trees may suffer iron chlorosis. Too much nitrogen on pines will kill small roots and, on new plantings, may result in lower survival. On poorer soils the deciduous plants may respond to nitrogen. Apply 1/4 cup of a low analysis fertilizer such as 12-12-12 to a 2 feet diameter area around the seedling. If the plants are yellowish, a soil sample is needed to determine the precise need. For more information, reach out to your local forester.