Fruits rarely need extra phosphorus, but they will occasionally need potassium and other nutrients. If growth is slow or leaves and fruit look unhealthy, check with your nursery or your County Cooperative Extension agent to find out what should be added. As a general rule, fruit trees should be fed equal amounts of fertilizer in early spring and again in mid-summer. Keep fertilizers away from the trunks of trees and shrubs and water very deeply after feeding. An easy way to feed your fruit trees is with tree fertilizer spikes.