Raking it in

Fall's colorful parade has reached its peak around here. There is incredible beauty whichever way we turn and, less obviously to the uninformed, just as much bounty as well.

Most gardeners know what that bounty is. It lies in the showers of red, orange, purple, and gold that spiral to the ground with every gust of wind. As soil-builders these autumn leaves are second to none. They enrich our garden soils in ways we still don't fully understand. Unlike grass and weeds, which do similarly good things for the soil, leaves do not bring weed seeds.

Apparently, after feeding the trees through photosynthesis for much of the season, leaves go into decline in the late summer en route to becoming the spectacularly colorful things we appreciate in the fall. In so doing they transfer many of their stored minerals, including much of their nitrogen, back into the tree for use early the following spring.

Even so, what remains is still significant. The dead leaves are particularly rich in calcium and potassium, which surface-feeding tree roots quickly take up the following year.

Under natural circumstances, the leaves decay where they fall. But if we gather up some of these leaves (either from the forest floor or neatly bagged from suburban streets), we can boost the productive capacity of our own garden's soil tremendously. We fatten our cabbages and boost our tomatoes at the expense of the trees, in other words.

Like all other organic materials, leaves don't feed the crops directly. Rather, they feed the soil organisms that, in turn, convert them into the nutrients and other substances upon which plants thrive. How best then can we get these nutrients to our garden plants? There are three basic ways:

Incorporation. Many very rich garden soils are made by spreading a 6- to 12- inch layer of leaves over the garden each year and tilling them in. If your garden is new and you are starting from scratch, you might be advised to use some other basic amendments, including lime, depending on a soil test (see your local extension agent about this).

Stick to natural ingredients (manures, rock powders, kelp meal, and the like) rather than chemical fertilizers, because the need is to boost the micro-organism populations within the soil as quickly as possible. A largely sterile soil, in fact, will process the leaves very slowly, and you may even end up, temporarily, with a fermenting mess in your soil. A rich soil, on the other hand, can take relatively large quantities of organic matter and convert it into a nutrient-rich humus within a matter of weeks once the warm temperatures have arrived.

If your soil is a heavy clay, it would be best to till in the leaves in the fall before the ground has become too soggy. Gardeners with light, sandy soils can postpone the operation until spring, in which case the leaves will provide a protective covering all winter long.

Mulching. A permanent or semipermanent mulch of leaves enriches the garden soil slowly just as it does in the forest by decaying steadily from the bottom up. It is best to shred the leaves (mowing them with a power mower is a pretty effective method) before applying them as a mulch. Otherwise, mix the leaves with hay or straw before use. Whole leaves on their own tend to mat down into an impervious layer that sheds water and cuts off the supply of air to the soil.

A mulch of shredded leaves can be applied in the fall, raked to one side during the spring planting, and pulled back around the plants once they are firmly established and the soil has had a chance to warm up.

I find that a 4-inch layer of shredded leaves applied in the fall will have declined to an almost paper-thin layer by late the following summer. This steady decay feeds the plants all season long even as it cuts back on weed growth and conserves soil moisture.

Composting. Fall leaves, composted with other garden residues or on their own with the aid of a little manure or commercial, biological compost-starter mix, make a rich top dressing for the garden by the following June in the North and a whole lot earlier than that in the South, where winter cold reduces biological activity for a much shorter period.

There are a number of ways to compost leaves - in a pit, a wire-fence enclosure, a bin, or simply in a large pile or windrow.

The pile method is perhaps the simplest: Spread a 12-inch layer of leaves in a circle 5 to 6 feet in diameter (or you can make a 5- to 6-foot-wide row as long as convenient) and cover it with an inch-thick layer of manure or garden soil. You also can sprinkle on some compost activator, if you wish. Water if the leaves are dry.

Keep repeating the process until the pile is about 5 feet high. Cover with a plastic sheet to keep out the excess rains and melting snows of winter.

You can leave the pile alone and it will still decay. On the other hand, if you get fresh supplies of oxygen into the pile every so often, you will speed up the decay. A simple way is to get a stout stick (a broom handle will do) with a point on one end. Once or twice a week push the stick into the pile and wiggle it around. Keep repeating the process all around the pile. This way you will loosen it enough for fresh air to penetrate.

Gathering and processing fall leaves may appear time-consuming, but the soil-building benefits that come from the practice make it time well spent. As organic gardeners have long known, there's much more gold in those autumn leaves than just the color.