Don’t Bag Your Leaves: An Analysis of Nutrient Loss and Soil Depletion for Leaf Removal

This is the time of year when the leaves all drop in their delightfully whimsical fashion.  And yet, it seems that fall is not an enjoyable time for many, especially if those leaves end up on the lawn. I’ve discussed problems with the practice of keeping a lawn before, and today, I’d like to look at our relationship to leaves and typical practices for dealing with leaves.  People spend quite a bit of their own energy and fossil fuel energy removing leaves from their lawns (raking, blowing, mowing, and so on).  These leaves seem to end up in three places for a typical American household in the sprawling urban and suburban areas:

1. In bags on the side of the roads for “compost” (most common in my area by far)
2. Blown onto the side of the road and left there and/or picked up by the city (more typical in urban settings)
3. In a burn pile smoldering away (this also happens a lot where I live).

Some of the more radical folk, like permies and guerrilla gardeners the like, might end up composting their leaves in our gardens or otherwise keeping the nutrients on our properties; but this is a rather rare approach and one not practiced by the bulk of the American populace. Many of us who are radical gardeners go out seeking the bags of leaves left on the curb to bring additional nutrients to our properties (I do this every year).  However, not all of us have the benefit of composting or keeping our leaves–not raking, mulching, or composting leaves is illegal in some township or city mandates and also in many subdivisions throughout the US. And leaves aren’t often the only things found in the leaf compost bags–usually when I go out to collect, I find a good deal of other matter and live plants.  This year, at about 10% of the houses where I picked up leaves, I also found the remains of someone’s backyard garden (tomatoes) and at another 10% I found other kinds of food from trees just thrown away (walnuts, apples, acorns, pears, etc).

And so, today, I’d like to explore this practice a bit and its long-term impacts on soil health…and what we might do instead.

What’s Written on the Bag: Linguistic Framing of Leaves

When people talk about the leaves that drop from the trees in the fall, they often call it “leaf litter” or “yard waste.”  Looking at the brown paper bags that people buy to store their leaves in for pickup, they say “Lawn refuse” or “Yard Waste” or “Refuse” on the sides. We label the bags and then frame what we put in them as waste or refuse; something to be discarded.  So we are already creating a framework for seeing materials that come from our yards as “waste” rather than seeing the nutrients in the leaves as a valuable thing.

What’s in the Leaves: Nutrients Contained in Fallen Leaves

The other issue I’ve been pondering for some time is the nutritional content of the leaves, and how many nutrients leave a typical American lawn in suburbia each year when they are discarded.  I found a reasonable analysis of the nutrient content of leaves here; their calculations are based on one ton of leaves (which, granted, is a lot of leaves).  This section presents a look at the nutrient content in leaves and how much is lost when we put them on the curb (or how much is gained by picking up someone else’s leaves).

According to the analysis above, 1 ton (2000 lbs) of leaves has the following:

• 940 lbs of carbon
• 20 lbs of nitrogen
• 2 lbs phosphorus
• 2 lbs potassium
• 32.8 lbs calcium
• 4.8 lbs magnesium
• 2.2 lbs sulfur
• Plus other nutrients and a great deal of organic matter (organic matter not calculated)

Before I get into my analysis of how much we lose by putting leaves on the corner, I have a few caveats.  First, this kind of analysis requires quite a bit of assumptions and estimations; I’ve done my best, but what I have found might be specific to my local area in South-East Michigan.  Second, I’m certain that leaf nutrition varies pretty widely based on the  soil profile of the region, the nutrients present or absent in the soil already, the types of trees, the weather for the year, and so on.  With that said, I do think even a rudimentary analysis, like the one I’m presenting, gives us some starting points.

Methods: On my trips out to get leaf bags from the side of the road this year,  I did some counting in my area to estimate the amounts and weights typical bags of leaves had.  I went out for three weeks and brought back 46 bags of leaves; I went out twice each week to different areas within a 3 mile radius of where I lived. I counted how many leaves people left on the curb and when I got back, I weighed them to get their average weight.  In a fourth week it rained, and while I collected bags, I did not weigh them because the water weight would have impacted my findings. I then did some simple math using the numbers above.

Amount of leaves per bag and per yard: Most leaf bags I picked up ranged from 8lbs – 22lbs, depending on whether or not they were shredded. Shredded leaf bags typically account for about 20% of the bags I pick up.  Even unshredded bags are often compacted and pretty heavy, especially if the leaves fell relatively flat or were slightly wet when they went in the bag.    The average for the bags was just above 10 lbs.  People in the suburban area where I go leaf hunting put out an average of 4.5 bags of leaves on collection day; some had many more, and most had been putting them out for a period of weeks. My estimate is that most families in my area put out at least 7 bags of leaves in a fall season assuming they have a few trees in their yard dropping leaves; this amounts to about 98 lbs of leaves (I’m going to round this to 100 lbs to make the math simple).

So for each 10 lb bag of leaves put on the curb, it has:

• 4.7 lbs of carbon
• .1 lbs of nitrogen
• .01 lbs of phosphorus
• .01 lbs of potassium
• .16 lbs of lbs of calcium
• .02 lbs of magnesium
• .01 lbs of sulfur
• And Iron, Zinc, and much, more.

That’s a lot of nutrients.  And that’s just the leaves–I don’t have any way of figuring out what whole tomato plants and other plants ripped up from the soil are also contributing, but I suspect these numbers are much higher.  And then, if homeowners are also removing their grass clippings all season long, that’s a whole other calculation to include.

What’s Left in the Soil:  Long-Term Removal of Nutrients and Inadequacy of Fertilization

Now one bag of leaves makes an impact, but perhaps a small one.  The problem is that one bag of leaves isn’t what’s being removed–this is about the long-term systematic removal of nutrients from our landscape.  As I was driving through a bunch of Detroit communities last weekend, what really struck me was the age of the houses.  Many of the houses I was looking at literally were 75 years, 100 years, or older.  In my research on dandelions, I discovered that typical lawn practices prevalent today really rose to popularity after World War II, which suggests that people haven’t been bagging up their leaves forever, but likely they have been doing it for some time.  So let’s assume that for the last 50 years, most citizens of Metro Detroit (or any other city or suburban area) have been removing their leaves from their property.  So if we take the numbers above and assume 100 lbs of leaves being removed each year, we end up with nutrient loss something like this:

These numbers are striking, but what is even more striking is the fact that I haven’t accounted for any other kinds of “yard waste” such as grass clippings (put out on the curb weekly or bi-monthly by many) or other kinds of yard compost that people don’t want.  I might do a case study of this at some point, but for now, we can see the potential for soil nutrient health decline.

I’d like to offer one other piece of evidence as well. My house was built in 1945, so my yard likely suffered the same fate for most of the last 70 years as well.  When I got at my homestead a few years ago, prior to putting in the garden, I ran soil tests.   These are exactly where the leaves (and lawn clippings) would have been taken from (and I knew from taking to my neighbors that the people who owned this house did such practices).  I found that the soil where I wanted to put in my garden was pretty much sand.  It had little organic matter and a P.H. of 8.2 (our PH is high in this area, but the lack of organic matter made it higher).  It had only 8 PPM of Phosphorus (considered extremely low), 22 parts per million of potassium (low) and magnesium of only 27 parts per million (extremely low).  To get my soil prepared for growing crops…what did I do? Went around and picked up everyone’s leaves, imported massive amounts of organic matter in the form of manure, compost, and this year, even a pile of seaweed from a local pond.  And now my soil tests are beautiful and my soil is rich and healthy (and much lower in PH thanks to the organic matter all of the leaves helped create).

What’s going back into the soil?  Not enough.

In a natural process, assuming no removal of nutrients, it takes 500 years or more to produce one inch of topsoil.  This gives you a sense, I think, of the enormity of the challenges that face us in regenerating landscapes with these destructive lawn practices.  But, you say, aren’t people putting things back into the soil?  Yes, and we’ll take a look at that next.

1) Plants can pull certain kinds of nutrients from the air; typically this is nitrogen and oxygen. So certainly, some nitrogen theoretically could be being brought back into the soil, although typical lawn ornamentals and grass don’t do this well.

2) Most nutrients are in the soil, and those nutrients are cycled through the dropping and decay of organic matter.  Trees and other plants (like Burdock, Comfrey, and Dandelion), penetrate deep into the ground and pull up nutrients to the surface, which they then deposit as leaves or dead plant matter.  But how many nutrients are down there for the taking?  If nutrients are systematically removed from the same spot over a 25, 50, or 100-year period, I suspect that at some point, the ground underneath will simply run out of those nutrients. I also have a theory (untested) that this is part of why we have so many plant and tree diseases.  We keep removing organic matter and nutrients from our landscapes in this form, through logging, etc, and those aren’t going back into our system.

3) People also often add fertilizers; but most commercial fertilizers focus only on the three macronutrients: nitrogen, phosphorus, and potassium.  All of the micronutrients (some of which are critical to plant and human health, such as magnesium or calcium) are not added.  Fertilizers also come in a water-soluble form (nitrate) and cause substantial problems for our waterways (I am not going to spend the time today to describe soil ecology, but a good introduction to it is here).

4) Mulch is sometimes also added to select areas.  But wood mulches are very high in carbon and little else.

5) Some people do buy compost and add it in select areas (say, a garden) but this is in select areas; I don’t see people dumping compost all over their lawns to make up for the loss of nutrients.  More on this in my next post.

So even with the inputs, it seems likely that a substantial net loss in nutrients in the soil immediately surrounds most of our homes and communities due to the “composting” practices of leaves and grass clippings.

Concluding Thoughts

What concerns me about this analysis, if its in the ballpark range at all (and I have reason to believe it is) is that each year, the soil surrounding our homes and in our communities gets more and more depleted as we continue to remove nutrients from our soils systematically through fall leaves and grass clippings.  This leaves me with a few serious concerns:

1) More and more people are becoming interested in urban gardening, homesteading, and the like, and yet, if they want to dig up their backyard, they are facing soils with literally no nutrients, organic matter, or life in them (as I faced when I started my homestead).  If we all wanted to do this, where would all the nutrients come from?

2) We aren’t looking at a few places of depletion, but probably we are facing it with nearly every suburban and urban home in the US.  When we combine this with modern industrial farming practices that kill soil life and strip the soil bare, its a very worrying issue.

3) The systematic loss of particular kinds of nutrients is also concerning; in my herbalism course, we have spent a great deal of time talking about magnesium deficiency (which the bulk of Americans are suffering from) and deficiency in other micro-nutrients; interestingly, many of these showed up in the list of what was in fall leaves and being removed.  Even if you were to grow some veggies in your soil that you added some organic matter to, would there be enough magnesium and other essential nutrients for your own body’s needs?

4) Perhaps most worrying of all: if history has anything to teach us, the widespread degradation and depletion of soil and other resources leads to a civilization’s collapse.  Combining this with many other ecological challenges suggests the widespread and systematic need for radical change.

I’ll elaborate on these concerns and what we might begin to do about this in my next blog post, which will discuss the concept of restoration agriculture and further discuss permaculture design.