Dan, my point is, put a lignin-containing cell in there, it's possible the nutrients from that cell do not biodegrade and are not available. But it's still referring to the cell wall constituents of that cell.

But even in that scenario it's not making other nutrients (outside, in other materials or cells) unavailable, just not contributing more until that biodegradation occurs.

I'm sorry, but as per my emphasis in caps, the experts are saying that lignins interfere with bioavailability in OTHER cells. Not "that cell" that refers to the lignin cell. I have not looked at Haug to try to make sense of this and, indeed, it sounds kind of strange. But I'm just reporting what is written by the experts.

It sounds as if the lignins (which are very hydrophobic) protect the non-lignin cells by surrounding them, making the surface area of those other cells less accessible. By keeping water away from those other cells, the digestive enzymes that are responsible for the degradation, which are suspended in that water, are less likely to attack those other cells. Of course, the bacteria that degrade the organics do so by releasing those water soluble enzymes.

The amount of lignin in chips is highest in chips from the center of the truck. So if you can get chips from a crew trimming branches growing over the roads you'd be better than if your chips came from large logs. We recently had a pine tree removed. It was about a foot in diameter at the bottom and 8-10" where it broke about 8 feet up. They chipped the whole tree. My neighbor had 8 pines removed in the last two weeks. They were up to 18" in diameter at the bottom and 50 to 60 feet tall. Those trees were also totally chipped.

It's my opinion that if you use chips to improve soil you can do this by adding manure. If you can think of a farm that allows folks to board their horses for a fee, that would be your best source of even possibly free horse manure. Both the manure and the chips will improve soil, the chips for a longer time period and the manure will provide the nitrogen that the chips will use.

To give you some perspective the carbon to nitrogen ratio of wood chips is 400 to one. Manure is 25:1. I would suggest you try one fourth manure either mixed or manure under the chips.

This year I plan on a new , expanded garden and am digging manure into my clay. I've dug some of it twice each time with about 2 inches of manure. I also dug in some leaves that I blew in with my mower. I have a truck load of pine chips that I'll spread in the paths. If I have enough I plan to spread manure before applying the chips. In the future the paths will be dug up as I rotate crops.

As an argument to raised beds the patch I've dug is about 8-10" above the lawn so I'm not worried about drainage.

I think the question was about composted wood chips. If you're going to add fresh wood chips to soil, indeed you'd better add some nitrogen. If you're going to add composted wood chips, it shouldn't make much of a difference.

I dump unchopped leaves in my gardens all winter long. I have a couple pin oaks which don't drop leaves until after leaf cleanup is done. The leaves blow against my fences, so I rake them up and dump over the fences. Then this process gets repeated. In the spring those leaves get dug into the gardens along with kitchen scraps and manure if I get any.

I try to use well composted manure. The last load I got was "many years old". My wood chips are from late last summer and I don't consider them composted, they don't melt the snow on top of the pile.

I can use horse manure with wood chips in them, but I prefer manure where the bedding was straw. Straw mixed with manure will make it heat up whether in a compost pile or under a hot frame.

In summation I use unchopped or chopped leafs after sitting all winter. But I'm very careful about wood chips. I use them more as a mulch, not near any plantings, than as a soil improver.

If you want to quick-compost organics, add fresh manure to them, not composted manure. Fresh manure had loads of nitrogen. But if you want to amend a garden bed, composted anything is best.

I agree with previous comments - nutritionally, tree parts are tree parts. Chopped leaves will decompose faster, but if you have shredded trees then use that. I do have something to add about cardboard - it is a nutritional zero and it tends to dissolve into sludge. No reason you can't use it, but it ain't trees.

The great thing about using tree parts (as opposed to shrubs or plant scraps) is that trees send their roots deep into the soil and will tap minerals that are not available close to the surface. This is nature's way of bringing these trace minerals into the environment on the forest floor and enrich the soil. A lot of people spend money on rock dust and other mineral supplements - when you can get the same result by using shredded leaves or other tree scraps and letting it decompose.

That's interesting about trees being more (at least mineralogically) nutritious than smaller plants. Is there some reference for that? Not completely obvious that there are minerals down deep that are less abundant at the surface though. Soil is basically ground up regolith. But to the extent that soluble minerals are in the water table, roots that reach down to that depth could get them. That is, to the extent that dissolved minerals leach into the water table, there's more down there than there is up here.

Ash from burning trees used to be one of the main sources of potash (potassium), and I believe potassium accumulation in soil that's heavily composted especially with woody material over long periods is a common effect. That said, I don't know if that's because there's substantially more potassium in woody materials compared to other plants (or that they get the potassium from deep roots); it may just be that potassium remains in wood ash (doesn't burn off as other components may) and is a higher percentage of the ash as a result. (That is, if one were to burn/concentrate equal amounts of some other plant by dry weight, would you get the same amount of potassium?)

I believe wood ash also contains some phosphorus as do composts, not sure if this can be attributed to sourcing either this or potassium from some deeper mineral subsoils.

I guess that's true that minerals are concentrated in wood ash, because the water, and much of the carbon departs in the fire. Unfortunately for me, I have alkaline soil, and wood ash is very alkaline (carbonates), so when I'm thinking about application to a garden bed I consider the stuff toxic waste!

Of course, if you compost tree leaves, as I do, and which was half of the question from the OP, the nutrients contained ought to be pretty much the same as the tree they came from. So with respect to mineral content, leaves and wood chips ought to be the same, and have higher mineral content than grass or manure compost.

>>That is, to the extent that dissolved minerals leach into the water table, there's more down there than there is up here.

I can believe trees and other deep rooted plants can pull out things they need, selectively, and if there is more of a certain mineral at depth because it has been depleted in the surface soil, then sure. Not so sure about this as a general statement though. Active topsoil should have a relatively rich mixture of solubles in pore water. Soil (and porous rock) is generally thought of as a filter for stuff that is in surface water. I would drink water from a deep well before I'd drink leachate from my garden beds, for example. :-] But maybe that wasn't your point. And it probably depends on the nature of the topsoil, subsoil and rock, plus the water that is leaching through it (rainwater vs. surface water for example). Lots of variables.

Has anyone tried using the remains of burned charcoal for carbon additive? Just wondering if that is the same thing you guys are speaking of.

Plants don't get carbon out of the soil. We're talking about minerals. In principle, you can grow stuff in pure sand, with no soil organics/carbon.

I think harold was asking about the original question in the thread, i.e. can charcoal be used as a carbon source for composting or as a soil amendment (to support soil organic matter).

It's not really compostable so it will not help a compost pile. There is some evidence it can be a good soil amendment particularly for heavy clay soils that need to be fluffed, since it's very porous and spongy. If you're interested search 'terra preta' or 'biochar'.

I have two woodstoves and I sift the charcoal out of the ash, resift it for size, and use the finer stuff (<1/4") as a soil amendment. Medium size (up to chicken wire mesh size) goes back into the stoves as fuel. Larger chunks (mostly from brush burn piles) I use in the charcoal grill. Nothing wasted!

You mean burned charcoal ash as a soil structure amendment? Doubtful. You moisten the stuff, press it, and dry it, and it's a brick. In fact, fly-ash brick from coal is a common building material, though that stuff has some cement added as an extra binder. It's the calcium hydroxides in the ash that make it unappealing for alkaline soils, and also what makes it harden. Wood-ash brick (without added cement) is an archaic building material. What makes carbon a good soil amendment is when it is in the form of botantical lignins, that really fluff up soil. It may be that *chunks* of charcoal, rather than wood ash, is a useful soil amendment, though. That stuff certainly would add fluff.

I should add that the OP was talking about nutrition, not soil structure. That's what I was referring to when I said that plants don't get their carbon out of the soil. They get it out of the air. Adding carbon to the soil adds zero chemical nutrition. If you have acid soil, raising the pH could be nutritionally advantageous, however. The nutrition in soil carbon sources isn't the carbon.

That's interesting about trees being more (at least mineralogically) nutritious than smaller plants. Is there some reference for that?

Yes, Stephen from Alberta Urban Garden sent leaves and two brands of rock dust to a lab for analysis. You can see the lab report here. He also did a video on the results, though I like the one below that I think explains and applies them better for the gardener.

Well, that's an interesting study, but what it seems to be saying is that leaves can be just as good a source of minerals as rock dust. That's useful to know. But that wasn't the question. The questions were whether tree leaves have more nutrients than other small plants do because their roots go deeper, and whether leaves had more nutrients than wood.

Stephen's study is a little suspicious though. He seems to assume that the trees that the leaves came from are growing in the same soil that the rock dust came from, and that they are successful in pulling those minerals out. But it could be that those trees were just growing in a place where the soil was much richer in minerals, and ANYTHING growing there would come up with a high mineral content. The lesson could be to get your rock dust from where those trees were growing!

I would prefer leaves over wood "chips" for adding to my compost piles. The closest I come to adding woody material is adding wood "shaving" which come mixed with the chicken manure collected from my coop. Shavings are generally paper thin and break down more quickly, more so than chips or thicker woody materials. I collect bagged leaves in the fall when neighbors place them on the curb for collection, but I do avoid collecting bags from yards that contain gumball trees as the balls also take more time to compost than just typical leaves.

I was following this interesting conversation because I had read a bit about carbon added to soil but had not researched it very much. So I tried this experiment at my own risk but it worked out okay. I work in a glass factory which uses carbon to colorize brown beer bottles. I took home about one pound of it and sprinkled it over my soil as I was tilling it to replant my daylilies and Asiatic lilies. They grew just fine with beautiful saturated colors. I wondered if anyone else had tried that type of carbon.

daninthedirt wrote:

You mean burned charcoal ash as a soil structure amendment?

No, I meant the CHARCOAL not the ash! We're talking about carbon I thought. Hence the process of using the larger pieces as fuel in my grill and stove, and putting the fine charcoal in the soil.

@harold100: What kind of carbon is that? Coke, activated carbon, ? What mesh size?

Well, charcoal is pretty much just wood, but without the water and some volatiles. So it would make a marvelous soil structure additive. But charcoal is lots more than carbon. Lots of hydrocarbons. Which is why it burns well. Carbon doesn't burn easily. You don't want to load up your grill with carbon. I believe Harold was talking about burned charcoal. That's ash. Mostly carbon, and some calcium carbonate.

But wood ash also has potassium, and if you can stand the alkalinity of the stuff, it's a good source of that. Nutritionally, that's pretty much what comes out of ash.

I should add that I believe that glass colorization is done with CARBON - not charcoal. I think I've heard it makes for an amber glass color. When you think of carbon, don't think of charcoal. Think of ground up graphite. If you add charcoal to melted glass, geez, it's gonna burn! But again, carbon doesn't do anything for soil, nutrients or otherwise.

"But again, carbon doesn't do anything for soil, nutrients or otherwise."

Precisely!! And 'carbon' when referring to plants usually means carbon dioxide, which plants pull in from the air, sequestering the carbon and releasing the oxygen during photosynthesis and transpiration.

btw, carbon will also do nothing to change or intensify flower colors, either.

I think that terms are getting confused here.

Charcoal - the result of burning wood slowly with minimal oxygen. It is black and contains a very high amount of carbon and no nutrition for plants - but it has excellent properties for binding and holding on to nutrients for plants and keeping them available in the soil. Google the term "Bio-char" for more discussions on this.

Note #1 - charcoal is not charcoal briquettes used for outdoor grills. Briquettes are soaked in oil byproducts and are not suitable for use in gardens.

Note #2 - Charcoal is a good addition to compost piles. Not because it adds anything - it does not. But because the nutrients released from the composting process will bond to the carbon in the charcoal which will hold it in the soil (once the compost arrives in your garden). Less will leach out down into the water table. On the other hand, if you just add the charcoal to your soil, it will do you no good until it has picked up nutrients released by microbes involved in the decomposition process. So, adding it to compost gives it a head start.

Carbon - an element, crucial for life and biology. Carbon in and of itself, though, is not particularly useful for gardeners. It is essential due to the way it combines with other elements to form vital bio-chemical compounds.

Ash - is the result of burning wood with lots of available oxygen. The fire burns fast and the result is grey ash. Ash is high in Potassium (the "K" of NPK ratings) which might be good for your garden. BUT it is also highly alkaline and will change the pH of the soil - so only use it if that is your intention. So, an important distinction here is that while charcoal is neutral and nutritionless, ash is not. It is best confined to situations where you have had your soil tested and you know you need it's qualities.

Good summary, but no confusion here. Charcoal is an OK addition to gardens, because it is just blackened vegetative lignins. Doesn't add much nutritionally, but it should work fine for enhancing soil structure. Just like compost does. But why add black lignins, when you can add fresh ones? That's a good point about briquettes not being good for gardens. Nasty stuff.

Also useful to add to the list is coal. Coal is pretty much rock. Mostly carbon, but often with some sulfur, chlorine and heavy metals. Sure won't help a garden bed. NOT coal ash, which is just as bad as wood ash. High sulfur coal will make for acidic soil.

"Carbon" is a catch-all term. We can also talk about whether it is good to add hydrogen or oxygen, which are present in a vast number of forms.

I'm thinking that bio char is longer lasting than new brown lignins. If so, that would be a big plus...a long lasting nutrient sump.

Good point. I don't know much about bio-char, but it says online that it can exist in soil for thousands of years. That must mean it is mostly carbon, but with a porous structure. So it's like perlite, but made of carbon instead of volcanic glass. So the lignin structure is preserved, but the lignins are gone. Bio-char is basically small-lump charcoal. NOT briquettes. You can even burn bio-char, much as you would charcoal. Nutritionally, it is zilch, but is nice stuff for soil structure.

I made more potash this past week. Norway Spruce potash. It's good because, as already said, It's one of the components of fertilizer. And it reduces the acidity of the clay I have here. I also get some bio-char because I pick out the larger unburned pieces of wood and leave the smaller. We used to call them clinkers.

I don't much care to know anymore. I get rid of the pile of logs and it improves my garden soil. I used to have so much I just threw it up in the air and let the wind blow it across the lawn.

If you burn wood save the ashes. I put it in an old food service tray and covered it with a sheet of metal with a couple concrete block pieces to keep the ash, potash, from getting wet. An old metal refrigerator bin works good. Plastic could burn if the ashes are still hot.

I have alkaline soil and, as I said, I consider wood ash to be toxic waste because of that. It never gets anywhere near my garden beds. So if you're tempted to use ash in your garden, be smart about it. In fact, with regard to pH and potassium, just get a soil test done. More potassium is not necessarily better.

I agree fresh lignins are just fine without purposefully burning, eventually they will become permanent soil organic matter (or a small fraction anyway). I only use fine charcoal because I can recover it from woodstove ash so it's already here.

Lots of knowledge being shared in this post. I just go for what is easy - year before last I wrote 3 lines in an e-mail asking the public works director to have his work crews that vacuum the leaves off curbs anyway, to drop off a few at my house. They brought me 8 truckloads which I figured to be 200-250 cubic yards of leaves. That was mulch 8-12 inches deep all over my gardens and enough leaf mold to repeat the process this year.

It's just simpler for me to use what will decompose fast and more exercise too.

Wow. That's a lotta leaves! Were they shredded? Whole leaves will take ages to decompose, and probably blow away before doing so. I use shredded leaves as mulch (and dig it in when composted). Marvelous stuff. Fairly low in NPK, but a great soil structure amendation.

The vacuuming process pretty well shreds them and they get compressed in the trucks. When they tilt the trucks for the leaves to slide out, they look like a hay bale with the strings cut - slides out in segments.

What is left has been rained and snowed on and a hurricane wouldn't move 'em. Well on the way to becoming leaf mold. This is NC and they vacuum (weather and time permitting) from from November thru January. There is a farm near here (100+ acres) where they take what us hobby gardeners can't use and they pile 'em deep, let them sit for a year and grow tomatoes the following year.

Public works mean cleaning city streets, right? What about all that toxic dust and oils and gasoline and etc. leeks from ground will go to your vegetable beds?

That's an interesting point. But any "toxic dust" will be carried by the wind all over the place. No way to avoid that. It's all over your garden right now. As to leaks from automotive fluids, gasoline will evaporate. It won't hang around. I think the same is true for antifreeze (ethylene glycol). I've heard that even organic farmers mix diesel and molasses and us that as a herbicide. Automotive lubricating oil isn't good stuff, however, but it's hard to believe you're getting very much of it from leaves on the street. The oil part of motor oil will degrade bacterially, but all the stuff they add to it - chlorinated compounds, lead and other heavy metals, won't.

Trees on city streets absorb all gases from cars, so those leaves as well as grass near the roads are toxic. And, as I understand , trees and bushes are planted along streets and roads in order to catch dust, gas, noise from traffic.

Gases from automobile exhaust aren't going to be stopped by leaves and grass. Those gases get well mixed in the atmosphere, and you're probably breathing plenty of them right now. Yes, plantings are made near roads to cut down on noise, but I have a hard time believing that they catch much dust and gas. Never seen piles of captured dust leaning on the sides of trees near a road.

I should add that since we breathe, and our lungs filter out what is in the air, we absorb lots of stuff that comes out of cars, so by that definition I guess we're all pretty toxic as well.

The volatile pollutants that come out of car exhaust end up in the air, and not deposited on surfaces like leaves. The non-volatile pollutants are largely particulates - mostly carbon. Not good to breathe in lots of particulates. Once mixed in soil, however, they won't be breathed in.

"Trees on city streets absorb all gases from cars, so those leaves as well as grass near the roads are toxic. And, as I understand , trees and bushes are planted along streets and roads in order to catch dust, gas, noise from traffic."

I find this entire statement suspect :-) 'Toxic' to what? Other plants? Hardly!! And unless you plan to eat them yourself, not much chance of them being toxic to you, either. Comments about what you are exposed to with just breathing the air or taking daily walks exposing you to more toxic particulates than you would ever encounter with urban leaf collection are completely valid!

And I have no idea where you live, but what lives along roadway verges here is naturally growing natives and weeds. In residential properties that abut well traveled roads, trees, shrubs and hedges may have barrier or edge plantings but not with the intention of absorbing or being a barrier against roadway dust and gas. And plants have virtually no impact on noise abatement.

To be blunt, what comes out of cars is largely toxic in the gases and suspended particulates, and not in the solids that end up on the ground. You're more likely to kill yourself by breathing the exhaust that comes out of a car than by licking the pavement on which it was driving. Those toxic gases aren't absorbed by plants. Carbon monoxide? Rapidly converted to CO2 that is benign, and absorbed by plants. Nitrogen and sulfur dioxides? Toxic to plants, but isn't absorbed by them. Benzene and formaldehyde? Can be absorbed by living plants, but the plants rapidly break them down.

I will argue with gardengal48 about noise abatement. Plantings around roadways definitely absorb road noise. That's well established. https://www.trees.org.uk/Trees.org.uk/files/8c/8c69f212-a82e-424b-96d1-c8ff6dc02403.pdf.

Sorry but I do this for a living :-) It is my business to know these things. Sound waves are only deflected or abated by a solid surface. Since plants are not solid, they have no effect on sound other than some slight filtering. Even a very thick, dense planting of trees and shrubs will have minimal impact. That is why they construct tall and thick solid masonry walls along freeway verges.....to deflect road noise.

"Unfortunately, simply planting trees or thick bushes won’t be enough to block the sound. To get an idea of the effectiveness of landscaping, point a spray hose at it. If water can pass through, sound waves can too. Instead of blocking noise with your yard, add a fountain or waterfall that generates its own noise. These sources of more-pleasant white noise reduce the impact of the road noise. This may sound silly to some of you, but you would be amazed at how creative, relatively inexpensive and pleasant this option can be.

To block the sound from your house and yard, you need a strong wall that is relatively tall. In a perfect world, you would erect an 8-foot tall fence of masonry with no holes. An 8-foot wall will give up to 10 decibels of sound reduction, equivalent to the difference between an alarm clock and a dishwasher. Shorter walls provide less sound proofing, but are still valuable since much of the noise created by cars is tire noise which occurs right at ground level." Sonic Shield Soundproofing

"Vegetation, if it is high enough, wide enough, and dense enough that it cannot be seen through, can decrease highway traffic noise. A 61-meter width of dense vegetation can reduce noise by 10 decibels, which cuts in half the loudness of traffic noise. It is usually impossible, however, to plant enough vegetation along a road to achieve such reductions.

Roadside vegetation can be planted to create a psychological relief, if not an actual lessening of traffic noise levels. Since a substantial noise reduction cannot be obtained for an extended period of time, the FHWA does not consider the planting of vegetation to be a noise abatement measure. The planting of trees and shrubs provides only psychological benefits and may be provided for visual, privacy, or aesthetic treatment, not noise abatement." USDOT Federal Highway Administration

I think we're in basic agreement. Foliage suppresses noise, but you need a lot of it to do it well. BTW, if I have a stand of foliage a few feet thick, and I point a spray hose at it, little of the water will reach the other side. Done that with a stand of grapes. So that's a poor example. Yes, planting a line of trees sure won't do much for sound suppression, but a thick hedge will. Here is another good article ...

https://articles.extension.org/pages/62185/tips-for-using-plants-to-reduce-noise-in-the-landscape

... but like you say, it takes a decent planting depth to make a difference. This reference does say that "Plants and other soft surfaces absorb and scatter soundwaves." They certainly don't do it as well as a thick wall does, but they make a difference. Noise reduction landscaping is a major business.

Let me add to what I said about foliage planted along roads. One important function is absorption of water that runs off the road and erosion control. Deep roots make the soil highly permeable, so you end up with wet soil and not running water.

Would you grow tomatoes or cabbage or greens in 3ft distance of a road?

Those "city leaves" are ok for a lawn or a flower garden; but I wouldn't amend the vegetable beds with them.

I'm growing beets and collards right next to the curb. In the summer, my melons are there as well. No worries. I will say that lead levels in that bed are a bit elevated, but not dangerously high. No question where that lead came from.

You can do what you want with your city leaves, but your arguments aren't credible. In fact, you haven't really given any arguments about why you shouldn't use city leaves. Just vague opinions. Please be specific. What substance are you trying desperately to avoid? I know *exactly* what is in auto exhaust, and it's not hard to find out what happens to it. I called out five auto exhaust constituents above.

I guess you're saying that those ag fields I drive by every day are polluted and toxic. And the diesel tractors that drive right on them! Oh my.

I'm not arguing, I'm just telling that "city leaves" are not safe for veggie beds amendment.

One thing, when you pick leaves yourself from a property, which you could see what is going there; and another thing , when public city workers collect everything what laying everywhere on city ground mixed with any possible polluted elements, lawn treatments, gasoline/diesel leeks , carwash chemicals and etc.

p.s. It may sounds like paranoias, and it is a bit off topic, but do you know what happened with your beets and collards when you not look at them. How many dogs pee on them, for example, or what kind of chemicals were dusted/sprayed accidentally or intentionally by strangers walking on that curb?

That's right. You're not arguing. You're telling. Just like those folks down at the overpass with their card table and color poster are telling me that the world will end next week.

Lawn treatments don't end up on fallen leaves. They end up in the soil under the lawn. Who treats their lawn in the winter after leaves fall?? As I said, gas leaks evaporate. I hardly ever see puddles of gas. And when it's gone, it's gone. After that, I can worry about breathing gasoline. Carwash chemicals? Uh, like what? Detergent?

Oh goodness. Are we worried abut dogs peeing on things? Like maybe also birds peeing, and insects peeing? You don't have any of THAT stuff on your plants, do you? And I don't worry about toxic chemicals being sprayed on fallen leaves. Those leaves spend most of their lives up in the air, and they get picked up after they fall. You have every right to voice opinions, but I hope people exercise a little common sense before believing them. Yes, it does sound a bit like paranoia. You brought that up. That being the case, I think it's time to move on.

In addition to chemical pollution of those city curb leaves, Up to now, it is not recommended to use manure from livestock that was fed by hay with herbicides, cause their traces are affecting veggies and flowers.

Back to residential or commercial city lawns treated with herbicides during summer. Cut grass lays on those lawns and becomes hay, and with heavy wind or rain, which happen year around, all that treated hay washed to curbs, mixed with fallen leaves , collected by public workers and delivered to your garden to be decomposed. Pretty much same product as composted manure from livestock .

It is toxic to any plants but to grass for hay production.

Kind of off-topic, I know, but on the sound issue: I think one of the advantages of trees, bushes, etc., indeed most plants, is the pleasant white noise gardengal's quoted piece refers to from fountains. Yes, it's inconsistent (fewer leaves in winter, wind), but it can be quite noticeable. Even low levels of plants rustling, insects, etc makes a big difference.

I often stay at a place outside the city often that's about a kilometer from a highway - most of the time, it is not noticeable, only in very still weather/specific climate. Trees swaying even slightly dominates most of the time. (Well, the blasted power tools in summer are the other exception)

A little late to the discussion but I have to agree that city street leaves should not be a big toxic risk. Among all the other reasons stated, there is one major difference between soil along roadways and leaves collected from the roadway: that is the amount of time they sit there. When gasoline was leaded, soil along the streets ended up higher in lead content because it got tiny amounts added to it all day every day. If you live on an older street you probably have elevated lead by the curb. Nowadays the problem is limited to PAHs (soot hydrocarbons) which are carcinogens and break down only slowly in soil. Leaves left in the street for a few weeks don't have time to absorb much of that. Gases are a non-issue. Gasoline, oil and anitfreeze? Again, short exposure time, low dose, and most of that is biodegradable in a compost pile. It actually takes quite a bit of any of these contaminants to significantly raise the levels in garden soil, if you do the math.

I would not grow vegetables along the street but I would have no problem using leaves from the street. Of course, everyone has to do what they're comfortable with. YMMV.

I pretty much agree with dan and tox. The worst thing about city leaf compost was the 9% small rocks in it.

My suggestion: use leaves when they are available, and cardboard and wood chips when they are available (and usually also may be when leaves are not readily available). Basically, one of the things why compost is theoretically rich in nutrients, is that so many things can go into it.