Leaf Mold

Leaf mold, leaf mould, duff, humus.  Whatever you want to call it, I've been giving it much attention lately. Online information is scarce. It seems to be a terribly overlooked, under appreciated, and especially useful material. I've been meaning to post some notes about leaf mold but it seems like the days are not as long as they used to be, even with setting the clocks.

Leaf mold, quite simply, is rotten leaves. To produce leaf mold, pile up a bunch of leaves and leave them alone. Just leaves, nothing else. It may take a year, maybe 2 if its cold or dry, but eventually they will decay. What you end up with is stable humus. The properties and benefits of using leaf mold are plentiful. I view leaf mold as being as important as compost and mulch. Being free, the price is right, and fits in my budget. It can take time to gather a pile of leaves, but there are plenty of people out there gathering up leaves for me, bagging them up and placing them beside the road in a neat stack-this really saves me a lot of work!

Leaf mold is not the same as compost. Compost is produced by bacterial decomposition. Leaf mold is produced by fungal decomposition. Compost is hot, aerobic, and quick. Leaf mold is cool, slow, and can be produced with little oxygen. This means you don't have to turn it. Where compost needs a variety of ingredients to attain the right carbon to nitrogen ration to feed the bacteria, leaf mold needs only the one ingredient-leaves. Leaves have a CN ratio ranging from 80:1 to 200:1. There is some nitrogen available, but not enough to allow the bacteria population to explode.

Down here in Florida, the state does not recognize the distinction between bacterial and fungal decay. The state's definition of compost includes any and all decomposed organic matter, and it must be sterilized in order to be sold. This is part of the reason why leaf mold is not available commercially. Another factor is the time it takes to produce the stuff. It's a darn shame. The fungus in the leaf mold, when added to the soil, serves as a nutrient superhighway. Sterilizing leaf mold would destroy the fungus. If the state had any idea what is going on, they would make the distinction, and create an entire industry overnight. I've looked high and low for sources of commercial leaf mold, found one in Texas. Where compost prices range from $20 to $35 per cubic yard, the dealer in Texas listed $100 per cuyd. That's 10¢/pound! For leaves! With the drought in Texas, the horse and cow people are buying up hay, driving up prices all over the south. An 800 pound bale used to be $20, now it runs me $40-that's 5¢/pound. The price of hay is through the roof, yet leaf mold is still twice the price.

Soil Conditioner
Leaf mold serves as a soil conditioner rather than a natural fertilizer. It primarily changes the structure of the soil rather than serving nutrient needs. Its the fungus. All the little hairs of the fungus grabbing onto soil particles help to bind loose soil, while at the same time the hyphae helps to break up compact soil. The natural growth habit of the fungus will move from the leaf mold to the surrounding soil in all dimensions. Start with a small area of leaf mold, end up with a greater volume of better soil. Leaf mold will continue to break down until the only thing left is stable humus which will remain in the soil for decades to centuries, taking a fire to destroy it. Until then, the leaf mold is rich in organic components: humic acids, carbohydrates, lipids, and stuff I never heard of. It is complex and impossible to manufacture. As the foundation of the soil ecosystem, there is nothing better.

Water Retention
I've read claims that leaf mold will hold several times it's weight in water. So I checked into it some... I've been raking up the massive volumes of leaves around here. Some I ran over with a mower to chew into small bits. I sifted some of this material through 1/4" mesh, filled up a trash barrel so I'd have something to experiment with. It's real fine stuff let me tell you. I weighed out 4 ounces of this dry material, enough to fill a 16 ounce drinking cup. This went into a bucket. Weighed the bucket, then added water. The next day I drained off the excess water, weighed it again. The 4 ounces of leaves held 18 ounces of water. It's not a big enough data sampling to be greatly accurate, but it held 4 1/2 times its weight in water. Impressive.

NPK values of leaf mold are nothing to write home about. I've seen it listed with low values of 0.6 - 0.2 - .04 and high values of 2.2 - .8 - 1.6, depending on the species. What leaf mold brings to the table are minerals. The roots of trees accumulate nutrients from deep in the ground, sending plenty to the leaves. While the nutrients are drawn back into the tree before the leaves are shed, most of the minerals remain as they are part of the leaf structure.

Www.composterconnection.com wrote:

Pound for pound, the leaves of most tress contain twice the mineral content of manure...And they provide the perfect nutrition for beneficial microbes. In short, they make soil come alive.

Peat Moss Substitute
Leaf mold is said to be an excellent substitute for peat moss in potting mix. I used to buy peat moss, about 10 bucks for a compacted bale 1'x1'x2' or so, uncompacted was 3 cubic feet. Finely shredded leaves is also a pretty good substitute, and the price is right.

It's all about the fungus
Most tree leaves have a considerable amount of lignin as opposed to grass which has an abundance of cellulose. Bacteria can break down cellulose, but do not produce the enzymes which break down lignin. That's where the fungi come in, in particular, White Rot fungi. From Wikipedia:

White-rot fungi break down the lignin in wood, leaving the lighter-colored cellulose behind; some of them break down both lignin and cellulose. Because white-rot fungi are able to produce enzymes, such as laccase, needed to break down lignin and other complex organic molecules, they have been investigated for use in mycoremediation applications. Honey mushroom (Armillaria ssp.) is a white-rot fungus notorious for attacking living trees. Pleurotus ostreatus and other oyster mushrooms are commonly cultivated white-rot fungi, but P. ostreatus is not parasitic and will not grow on a living tree, unless it is already dying from other causes. Other white-rot fungi include the turkey tail, artist's conch, and tinder fungus.

The best video I've found on making leaf mold is by Lee Reich, author of The Pruning Book. It was available on Youtube.  I'm trying to find it.
Pile up the leaves, walk away.  Nature will take care of the rest. 
It doesn't get any easier. Lots of videos out there with ways to speed up the process, but it's a long slow process. What's your hurry?

Let's look at some of these speedy ideas:

Running over the leaves with a lawn mower will certainly reduce the size of the leaves. This will knock a few weeks to a few months off the decomposition time.  It's still going to take a year or two. All that effort, plus the fuel, plus the noise and air pollution does not offer significant gain. Wait for it, the leaves will rot if you don't do a thing other than pile them up.

Without question, a moist pile will decompose faster. However, the volume of water needed to moisten a pile of leaves can be considerable. For every pound of leaves, another half pound (half pint) of water adds up pretty quick when you have a ton of leaves piled up. If you want to run a pump and stand there with a hose for a couple of hours, thats up to you. The rain will get them wet, and they will hold on to much of that water for quite some time. Letting nature do the work will get the job done in good time. No need to check on the moisture in the pile, it will get there when its ready.

You can turn the leaves, but this step is not needed. A pile of leaves that is never turned or aerated will decompose just fine. Time saved is questionable. Leave em be.

Adding Greens
This does not speed up the process, it CHANGES the process. Instead of leaf mold, you will have compost. There is some cellulose in the leaves which will compost, but by adding greens you are creating conditions to promote the bacteria. The fungi will break it down if you let it, and will produce more humus out of the same amount of cellulose than the bacteria will. If you want to make compost, go ahead, but it will be compost, not leaf mold.
Just leaves, nothing else.

This works. A pile of leaves kept in the sun will dry out faster than the same pile kept in the shade. Moisture is your friend.

Erecting a fenced bin certainly helps keep the leaves in one place, and may be worth the effort, particularly in densely settled residential areas. If you've got some extra fence around, go for it. If you go with a fence, consider having the center of the heap lower than the sides to promote water collection. Instead of a domed heap, think donut topped heap.

In my opinion, no additional work needs to go into making leaf mold. If you want to put in more effort, I think it would be best spent gathering more leaves. With the fall leaf season about to come upon us in the northern hemisphere, now is a good time to think about adding a leaf mold project to your endless list of chores. All you have to do is pile them up in an out of the way spot and walk away. Check it from time to time, get a feel for how the process works. When the time is right, put it to use. Spread it on your lawn, rake it in, Mulch your plants. Use it in your potting soil. Dig a 2-3" layer into part of your garden beds and observe the results. Chances are you'll be wishing you had gathered a whole lot more of those leaves.

Try it. What have you go to lose? Don't cost nothing. For this sandbar of a state I live in, there's not much in the soil. I'll take all the help I can get. Leaves are abundant. With the drought of the last couple of years, the leaves out back are 6 inches deep in some places. The woods around here are a mineral warehouse.


Permies.com Forum discussion on Leaf Mold from which this article was born. 

Humus - the essential ingredient: Graeme Sait at TEDxNoosa

Read online at OpenLibrary.org:
Leaf-mold for soil improvement in home gardens by David E. Hill
Published 1978 by Connecticut Agricultural Experiment Station in New Haven.