Answer: In forests, needles, leaves and other plant debris accumulate on the ground, decompose and release nutrients into the soil that are reused by trees and other plants. In a typical home landscape, plant debris is often raked up and removed, taking nutrients with it. In addition, the soil that landscape evergreens must survive in is often much different than native forest and prairie soils. Many times, these urban landscape soils are deficient in certain nutrients, and are clay soils that have been severely compacted, poorly-drained and very alkaline. In these situations plants may eventually become stressed to the point that they are no longer attributes to the landscape, but eyesores or worse yet, dead.
Maintaining healthy evergreens in the home landscape is a combination of thoroughly understanding the characteristics of the planting site, selecting the right plant for the particular site, purchasing healthy plants, preparing the soil prior to planting, and maintaining the health of the plant on a regular basis. One of the regular maintenance tasks that may be necessary for the health of the plants is fertilizing.
An annual application of well-rotted manure or seasoned compost forms a moisture conserving mulch that provides a small amount of fertilizer to the trees. Spread a layer of manure (preferably heat-treated to kill weed seeds) or compost two to three inches thick over an area at least equal to the branch spread. Keep all mulches at least two inches from the trunk to prevent trunk rot or other problems. Do not do this with poorly-drained (water-logged) soils, since it will only make a bad situation worse.
Water soluble fertilizers applied either through root feeders or to the soil surface provide some nutrients. However, water soluble fertilizers may leach through the soil quickly with sandier soils and may need to be reapplied regularly through the spring and early summer. Do not use root feeders on poorly-drained soils, especially heavy clays. Root feeders may be difficult to use in heavily compacted soils, but they do provide an effective and environmentally sound method of tree fertilization. Fertilizer "spikes" last longer in the soil and also provide an effective form of fertilizer if applied in adequate quantity. Water soluble fertilizers, spikes and "briquettes" are comparatively expensive.
Apply fertilizer any time from early spring until midsummer (July 15). If conditions are very dry, make sure the trees are well-watered. Never fertilize drought stressed plants. Applications of fertilizer at normal rates in late summer may stimulate growth late in the season. Late growth may not have time to harden off before cold temperatures arrive and the trees may be more likely to suffer winter injury and dieback.
There are specially formulated fertilizers for conifers, or you can use an all-purpose fertilizer such as a 10-10-10. I think the jury is out on whether or not your evergreens need an acidified fertilizer such as Miracid. It's an attempt at replacing the acidity that would be leaching into the soil if you allowed the needles to decompose beneath the tree, but just how much would leach into the soil is anyone's guess. I'd keep things simple (and inexpensive) by using a balanced, general purpose garden fertilizer for your evergreens.
Best wishes with your landscape!
Q&A Library Searching Tips