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When the Leaves Begin to Turn: September: Old-Field

For example, in what are known as the "pine barrens" in the South -- wretched soil, too poor for ordinary crops -- what is known as "Old-Field Pine" will grow of its own accord. Then, after a while, the trees can be cleared out and fine crops can be raised by drawing on the accumulation of enriched soil made by the decay of the "needles," the leaves of the Pine. This lasts for three or four years. Then, if the farmer fails to add any fertilizer himself, the land goes back to barenness. As I said in The Adventures of a Grain of Dust, what can such a farmer expect, always drawing on his bank and never putting anything in?

HOW THE TREES PAY THEIR BOARD -- AND MORE The fallen leaves, decaying, make what is called "humus," and this humus is a very important thing to have in the soil. The roots of trees continually bring up mineral salts from the subsoil, and a certain amount of these valuable salts are left behind in the leaves when they fall. The tree, having closed business in preparation for the winter, has no further use for these salts, which are merely the raw material for its thousands of little food factories, the leaves. Then, as the fallen leaves decay, these salts are carried into the surface soil by the rains and left just where they will be wanted by growing things the following year. It is for this reason that certain kinds of trees, hardy pioneers like the Old-Field Pines, can be profitably grown on soil far too poor to raise ordinary crops.

The acids produced by the decay of the leaves also help to perform one of the miracles of nature -- the changing of rocks into humming-birds and trees and squirrels and wood flowers and fresh milk. These acids of decay dissolve rock; and rock, as you no doubt know, is the original substance out of which soil is made. So the trees, from year to year, are constantly "converting a liability into an asset," as a business man would say. The stones of the field, which are only a nuisance to the farmer in their raw state, are of great benefit to him when their hidden riches are unlocked -- when, by the wonderful alchemy of Nature, they are converted into soil; for the rocks contain sulphur, phosphorus, potash soda, iron, silica -- it's the silica that helps put a good backbone into the wheat blades and the corn -- and these things are converted into wood and fruit and nuts and vegetables.

To this faculty which dead leaves and other dead vegetation possess of helping convert rock into soil is due the curious fact, recorded in The Adventures of a Grain of Dust, that pebbles, under certain circumstances, are good for cows. In the counties of Wisconsin where there are plenty of pebbles, the production of butter and cheese is something like 50 per cent greater than in regions where there are comparatively few pebbles. And if you examine a pebble taken from among the roots of grasses or the finer roots of trees you will see the "finger-prints" of these little roots, where their acids have eaten into the stone.

II. WHERE THE LEAVES GET THEIR AUTUMN COLORING It isn't known, for sure, where the Robins and other birds get the coloring for their eggs -- nor, indeed, just why they color them, although there are some interesting theories about it1 (A Year in the Wonderland of Birds.) -- but it is known where the gorgeous coloring of the leaves in the fall comes from; and if you believe, as I do, that one purpose of Nature is to make life beautiful, we can see why she makes her annual art display in the autumn woods.

Here, as everywhere, in the Grand Plan of things, use and beauty go together; that is to say, it is in the very act of doing something useful to the life of the tree that the leaves sing their glorious color song of farewell, when the year and their work are drawing to an end. The leaves are, among other things, the food factories of the trees. (This is one of the things I meant when I said, at the beginning of this chapter, that "strictly speaking, trees grow on leaves.") In the autumn when all these little factories close down for the season, most of the manufactured product in the leaves at the time -- starches, sugars, oils, and so on -- is transferred into the buds and twigs for winter storage, but some of it is left behind and the sun changes it into the autumn colors -- the red and yellow and orange of the Maples, the scarlet flame of the Sumachs, the bronze of the Oaks, the riot of color in the Sweet-Gums -- purplish blue, crimson, lavender and pink, burnt sienna, indigo, cobalt, ochre.

The greenness of the leaves, which is so conspicuous in the growing season, is really made up of four kinds of pigments (coloring matter), two of which are bright green, of different shades, and two of them yellow; the same yellow coloring matter that Nature uses in flowers and in yellow fruits -- the orange, for example. As you know, a young leaf is not so deep a green as an older one; it has much more of a yellow tinge also. This is because it has a relatively small amount of the green coloring matter (chlorophyl) and a relatively large amount of the yellow pigment. In the sunlight the green coloring increases very fast and reaches its highest point about the time the leaf reaches full size.

In the natural course of events, when the leaf's life-work is over in the fall, the green color disappears, while the yellow tints remain behind. This accounts for the yellow and gold in fall leaves in their various shades and combinations. The reds are due to the kind of pigment that gives it distinctive color to the Copper Beech and to many buds and shoots. You can see it in the spring foliage of the Cherry.

The purple and mauve and orange tints come from the different degrees of acidity in the sap which is combined with the red and yellow coloring materials. The particular shades of these colors also depends on the character of the previous summer. If it has been rainy, keeping the veins full of sap and the leaf skin thin and smooth, the colors are brilliant and show through; but, if it has been dry, the colors are less brilliant and are made to look still duller through the thicker and more wrinkled skin. Much the same difference, by the way, to be seen between the pink cheeks of a young girl and the wrinkled and weathered face of the Old Salt who has long faced sun and wind!