Invisible Fire

Fire. Essential and dreaded. Fire has historical tendrils back to primeval times when people first discovered how to make fire when needed. Fire also includes the combustion and flames of a gas stove and oven, campfires, propane-fueled lanterns, and barbecue briquettes. All welcomed, desired.

Every garden is always on “fire”—without flames. Combustion is the oxidation of carbon-based materials which usually produces a flame. Garden “fires” are usually invisible, slow, and important. A single teaspoon of soil contains over one million of the soil’s beneficial bacteria alone This does not include: nitrogen-fixing soil algae, microbes, beneficial fungi and many other useful life. Each consumes various life-forms or inert elements and releases very minute amounts of heat into the soil. Like an aerobic workout, soil heats up—only in extremely tiny amounts.

Combustion in the compost bin also is at a lower temperature than a visible fire. Microorganisms feed on the carbohydrates; the carbon. The warmth of a well-made compost pile is tangible garden low-grade combustion. When the first nips of frost arrive, a compost made of fallen leaves, green lawn clippings, manure and kitchen scraps (I send all my kitchen compostables to an earthworm bin.) sends its wisps of warm fog to rise above its mass.

Only the fanatic composter runs the risk of spontaneous combustion. The pile has to be very large. And it rarely bursts into flames, but usually smolders like a coal mine "fire." Tim Dundon aka Zeke the Sheik (the compost king), of Los Angeles County, accumulated a compost pile over 17 years which reached the gargantuan size of 40 feet tall and up to 200 feet long. Twice in one month the pile spontaneously burst into flames and smoke, and the fire department arrived to douse the flames. While Zeke defends composting as "the key of energy that will eventually set mankind free from misery and gravity," I'd say his pile shows what happens when a zealous intellect elevates a natural process to unnatural proportions. He turned a beneficial process into a hazard.

The image above is a manipulated Polaroid. See my web site (www.robertkourik.com) for more examples of "Impressionistic Photography".

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Roots Grow Up!

Surprisingly, many of a tree’s feeding (not “structural” roots) roots grow up, not down. In a paper published in The Landscape Below Ground (International Society of Arboriculture, 1994. pg. 3), Professor Thomas O. Perry states “Most tree roots range in diameter [from that of] of a lead pencil to the size of a hair. These smaller roots…grow upward into the surface inches of soil and the litter layer.”

The significance of the importance of the most aerobic layers of the soil and duff can be seen in a simple study using potted tree seedlings. You can see the dramatic difference in the illustration of growth between the three soil depths. The seedlings in the pot on the left are growing in soil collected from the top two inches. The middle pot has soil only from the 2-4 inch depth of a forest floor. The pot on the right is trying to grow in subsoil.

The top two inches is so vital to a tree’s health, whether it’s native or ornamental. This is also the favorite two inches for all the plants in a humid climate with periodic rains. Take away that top two inches by planting it to lawn, raking the duff up for “a cleaner look” or allowing so much foot traffic that the roots are exposed, and you have a disaster in the making. This is perhaps the most important illustration in Roots Demystified because it so graphically reveals where tree and shrub roots prefer to grow and feed.

The photo is of a cross-section of three feet of soil beneath a vineyard. Most of the roots are in the top foot or so of the soil. Below that level it gets increasingly more like clay subsoil. (If you click on the image to enlarge it, you will see a few roots below the first foot and even a few that found the lens of gravel.)

The aerobic-loving soil life – where you find the most soluble nutrients - needs to breathe. The deeper you go, the less aerobic you get, and the number of “good guys” (beneficial soil flora) will rapidly diminish. Studies done with agricultural plants provide a lot of useful information. One example is alfalfa, it can grow roots much deeper than peach trees can, yet both get most of their moisture (along with nutrients) in the top one to two feet of the soil.

But trees still need “dirt”. Perry puts it quite succinctly when he says that trees on soils as little as five inches thick produce only poor tree and shrub growth. (one can then imagine how far the roots must grow laterally in the shallow soils to gather sufficient moisture and nutrients.) You can get fair growth with a ten-inch depth, good growth at 16 inches and excellent growth with 20-30 inches of topsoil. Most remarkably, according to Perry, the tree vigor is likely to gradually decrease with soil deeper than 30 inches. (The Landscape Below Ground, International Society of Arboriculture, 1994. pg. 9)

A PRACTICAL TIP FOR GARDENERS
As I’ve often said: mulch, mulch, mulch. Replicate the duff that forms in a natural forest. Establish as many permanent pathways as possible. Try to let the pathways “breathe”—allowing the air to flow into the roots and the carbon monoxide to be expelled. You can use chipped bark, chipped tree trimmings, gravel or whatever local material suits you.


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NOTE: The comments section at the bottom of the post has disappeared. Click on the "___ Comments" button or the title under the "Blog Archives". Thanks, Robert