One Western state insists that even natural manures must carry fertilizer analysis tags (these are usually only required on mixtures of chemical fertilizers). One "manufacturer" of a worm-castings product tried to show the nitrogen reading in parts-per-million to make it sound more impressive. He was, however, compelled to mark his package as containing .005 per cent nitrogen, less than the nitrogen content of some drainage waters.
This low percentage of nitrogen in worm castings is not surprising when the feeding habits of the two common worm species are studied. They eat by dragging long strips of leaves or grass down into their burrows. In forests, their diet consists of fallen leaves, even during the summer months. Instead of seeking out other plants on which to feed, they prefer to work on and grind down the leaf fall of the previous autumn.
They are amazingly selective in their food, with a strong preference for leaves of certain kinds of plants. In brushing over the leaf cover in a mixed grove of maples and oaks, I was surprised to find that the only remaining leaves in spring were from oaks, while maple leaves had been consumed except for the tough leaf stems. In another grove, where oak, ash, dogwood and hickory were growing, leaves of oaks again were left behind. Leaves from hickories were eaten first; only then were the others touched.
In another forest, basswood, sugar maples, red maples, aspen and white pines were growing together in a mixed planting. The pine needles were untouched for two years, while basswood and aspen leaves were eaten completely. Maple leaves were attacked only after basswood and aspen leaves were stripped down to the last petiole. Herein lies a possible explanation for the slow breakdown of oak leaves in forest litter, as well as for the durability of pine needles when used as a mulch. Also, in both cases, bacteria and fungi have a hard time breaking through the outer cells of the foliage, so decay is delayed.
The important fact here, however, is that the preferred leafy diet of earthworms is relatively low in nutrients. If you analyze the mineral content of fallen leaves you will immediately realize that they are indeed very poor fare (as explained in the reference to leaves for composting in Chapter Eight). In order to stay alive, an earthworm must exist at a slow pace, yet consume enormous amounts of low-nutrient foods. It is interesting to note that when worms live in lawns, they feed only on grasses, never on nitrogen-rich clovers.