Fertile Soil Grows on Trees (and Bushes)

No matter where you live, any garden can benefit from more organic matter, and fall provides the best kind--leaves. Leaves are a fertility bonanza for the gardener. It's worthwhile to gather your neighbor's as well as your own. Crumbled or shredded leaves are even better. But whatever you do, don't burn them or throw them out!

Another good source of carbon is chipped branches. Leaves can be handled just like any other organic matter--use them for compost, for mulch, or for fermentation. Wood chips, on the other hand, do require slightly different treatment, so I'll go into them last.

Fallen leaves are my favorite mulch for vegetable beds, perennials, and bulbs. You can rake them up and pile them on your garden as a weed-suppressing mulch that enriches the soil. Or leave them where they fall, to fertilize lawns and flower beds.

Leaf compost has an excellent balance of nutrients, with lots of minerals, not just nitrogen. And because it feeds fungi as well as the usual bacteria, it promotes a wider array of microorganisms in your soil's food web.  It's considered outstanding for seed starting mixes, because it contains gibberellic acid, which stimulates seeds to sprout. 

Traditionally, professional gardeners gathered leaves and composted them separately to make a special soil mix for starting seeds.  Leaves often compost slowy, because it can be hard for air to penetrate once they mat down.  This can be an advantage when you use them for mulch, as they make a weed-suppressing blanket that has fewer gaps than most other mulches. This slower composting in a cooler pile means that leaf mold, as it's called, has more fungi rather than the bacteria that dominate hot compost.

Leaves also make a great addition to regular, hot compost. Just alternate leaves with other types of garden waste, and turn the pile to let in air.

Using Wood Chips

If you have a chipper, or can get chipped branches from a tree service, they are another great source of fertility. There are three important things to know about wood chips:

1. Chips from small branches (under 3" diameter) are different from more mature wood. They have more nitrogen and decay faster. They are the kind of chips to use as mulch in gardens. Mature wood chips are best for paths, animal bedding, or longer-term composting. Both are great for hotbeds, see below.
   
2. Chips will be best for garden use after sitting outdoors all winter, and especially if they are wet. By spring, breakdown will have started and fungal mycelium can use them to enrich your soil.
3. No wood product should be dug or turned into the soil. They should stay on the surface, just as they do in the forest. When sawdust or wood chips get buried in the soil, they can rob it of nitrogen temporarily,  stunting nearby plants. On the surface, there is plenty of oxygen, and the wood can slowly break down into plant food.
   

Wood Chips as a Natural Heat Source

One way to heat a greenhouse or provide bottom heat for seed-starting is with a wood-chip hotbed. Make a bin (pallets are perfect) three or four feet on a side. Fill it with thoroughly wetted chips. Within a month, it will heat up and provide even heat for a long time.  You can set seed-starting flats on top, nestled into the wood chips to add warmth for starting tender seedlings.  A couple of hoops and some clear plastic makes a heated mini-greenhouse, called a hot frame.

It is also possible to cover your hotbed with soil and grow plants in that. I have done this often with regular compost as the heat source. (Traditional hot beds were made with horse manure. The market gardeners who provided Paris with vegetables in the 1800's were able to grow 8 crops a year using this technique. And Paris is at the same latitude as Montreal!)
If I were using wood chip for this sort of hotbed, I would put cloth between the soil and the chips so they didn't mix. In a traditional bed, the compost is fully broken down by the time the crops are ready, but with wood chips, they wouldn't be ready to mix with soil. You would probably want to use them as mulch after the hotbed was done.

Another use of woodchip hotbeds is to produce hot water. A pallet bin inside a greenhouse with a coil of irrigation tubing buried inside can warm water which then circulates by convection to warm a growing bed. The amount of heat with these installations is variable, since the type of wood, the amount of moisture, the microorganisms available, and the ambient heat all play a part. This makes them great for adding a boost of heat to growing plants, but less appropriate for household hot water.

People have used large piles to heat water for household use, but it's experimental at this point, with no surefire formula available yet. If you are interested, try looking up the work of Jean Pain and his successors.