Tree fertilizer spikes

Invisible garden maintenance . . . we pushed fertilizer spikes into the drip lines of the trees and bushes in our outside beds, first time in at least twelve years that they've been fed. The soil in our garden is primarily builders rubble (was excavated when the building was constructed in 1937), with a thin layer of self composted vegetation that has accumulated over time, so not rich at all for plants that send roots deep.