Case Study – 5 Year Garden Soil Transformation W/ Leaf Mulch

Even though I’ve documented and discussed my journey with leaf mulch over the years on YouTube, (see here, here, here, here, and here) I figured I should put it in written form as well. So this article will my attempt to document how I’ve transformed my garden soil over the last five years from orange/brown infertile soil to a black crumbly fertile medium for all plants.

The beginning

When I built my garden from scratch back in the Spring of 2017, it was a tough job. I was basically taking a piece of ground that had been a lawn for 20 years. Now, just prior to becoming lawn, this piece of ground (my whole yard) was new construction. And that means that the original top soil was likely removed, and then replaced with fill and compacted. This is not an ideal situation to build a garden….

So, I removed the sod using a tough, but efficient method. But that year I applied a layer of hardwood mulch to the whole garden. I added the mulch for three reasons –

• Retain water
• Suppress weeds
• Build soil

Now, these three reasons are logical and sound. The layer of mulch will shield the soil from the sun, which will reduce the water evaporation. The mulch itself will act as a physical barrier preventing or hindering weed growth. And finally, as the mulch breaks down it will build soil as the decomposed organic matter will improve soil structure, as well as decompose complex molecules, which makes nutrients available for plants.

And I did the hardwood thing for two years. And it did work, sort of. But in the end, the amount of mulch I added to the garden was not nearly enough. I only added a 1″ layer, because it wasn’t free (aka expensive), and I was short of time to apply it.

Now, I didn’t take any pictures of the soil back then, as we didn’t yet have a website or YouTube channel. But, recently I did some soil testing, and the image below is what the virgin soil looks like under the grass:

But by the end of 2019 I had a layer of ‘black soil’ on top of my garden – a whopping 1/4 (6 mm) layer! Not very fertile, and not much organic matter.

Discovering the power of leaf mulch

One day in a random friendly conversation with a neighbor, he mentioned how he bagged and threw out all of his leaves. And, he has a lot of mature sugar maple trees, which put down a LOT of leaves. So I asked if I could take them off his hands, and he was more than willing to let me have them. So, I got around 35 bags of leaves that first year and piled them up on my garden.

Building soil slowly

The following year after applying a leaf mulch, I decided to do a mason jar soil test. I did multiple samples, some within my garden, and some just outside. As I was doing these, I noticed just how much blacker my garden soil was than the soil from my lawn. And then realized that it was the leaf mulch I had applied the previous year – it had broken down and created a thick, 2″ layer of black crumbly soil.

As you can see in the above image, the soil sample from my vegetable garden is much darker than from my lawn. And, the color of soil^[1] can be an indication of organic matter content^[2], but you have to have some before/after or know the base soil. So, in absence of some ‘before’ pictures I am just using soil from my lawn that is adjacent to the garden.

But even before I did my soil testing, I knew I had done something great. In 2020, the first full year of leaf mulch had resulted in my tomato and pepper plants being absolutely huge, and we had huge yields to match. In previous years (2017-2019) I had ok yields, but something drastically changed in 2020, but I didn’t fully realize it until I was taking soil samples for my mason jar test. Then, it all became clear.

Nutrients from leaves

So, this is kind of an after-action report, but instead of a postmortem I was diagnosing my great success that I happily stumbled upon. But, if your soil has low organic matter content, then adding some will give you huge benefits in that exact growing season. And adding leaves as a mulch did exactly that, it is one of the big benefits of lea mulch.

You see, those leaves aren’t just carbon or something to decay. They contain trace amounts of macro and micro nutrients. And those ‘trace’ amounts will build up if you add enough leaves! I noticed this in the form of huge plants and vegetable yields that first year.

Reducing compaction

Adding organic matter will reduce the compaction of your soil. And this happens as the soil microbes, worms, and other organisms break it down into usable nutrients for your plants. It forms something known as hummus, which will allow soil particles to bond together and form clumps….also known as soil structure^[3]. This soil structure thereby reduces compaction, allowing your roots to more easily penetrate the soil and access nutrients and minerals. This in-turn allows water to infiltrate the soil better, and has a second-order effect of improving drainage^[4]. But the thing is, it is improved whether you have sandy soil or clay. Organic matter reduces drainage in sandy soil, but increases it in clay soil…..

Getting organic matter deep in soil without tilling

One effect of leaf mulch that I never expected was how this decomposed organic matter would penetrate deep into my soil via natural processes. The reduced compaction makes it easier for the decomposed organic matter/hummus to naturally penetrate down into my soil. And worms really accelerate this process, as they eat some decaying leaves, then poop them out (worm castings) deeper in the soil.

These effects were clearly visible as I took soil samples. Each year I could see that by piling up 6-12″ of uncompacted leaves on my garden, it was resulting in about 2″ of ‘black soil’ depth by the following Autumn. See the images below. And please forgive me, I didn’t take the best pictures back then, but nonetheless you can clearly see the organic matter penetrating down into my soil.

So, I’m at the point now where I can dig holes with my bare hands in my garden. And it has primarily come from leaf mulch. Although I have applied a top-dressing of compost to my entire garden for the last couple of years. The thing is, you don’t need to till organic matter into the soil. Let the microbes and worms do the work for you. What is kind of funny is that I never tilled mine in….not because I knew I didn’t have to, but because my soil is so rocky I figured I would damage any tiller!

Although I’ve documented all of this each year (links at beginning of article), here is my 2024 update that really shows the progression year over year.

Soil testing

So, after 5 years I figured it was time to test my soil’s fertility. And I did so in a comprehensive way, by taking six different soil samples. I broke them up by taking two from my ‘old garden’ that had 5 years of leaf mulch, two from my ‘new garden’ expansion that had two full years of leaf mulch, and finally my lawn, which doesn’t get much love from me.

In the above table, the ‘optimum range’ are from the soil testing company I used. For the nutrients, they are specifically saying what you would need in a typical growing season, for a typical vegetable garden. We remove these nutrients by harvesting our crops, so they need to be replenished each year.

I did my testing with a company called RX Soil. The results came back a few days after I sent my samples in, and they give you a report via a web-portal. I was pleased with them, and will use them in the future. If you wish to do a sample with them, you can get a 10% discount if you use the code growitbuildit at checkout (affiliate program).

The results are not totally surprising, as both the old and new garden have loads of nutrients more than the lawn does. Now, the thing is, it appears that I am accumulating some nutrients though – namely phosphorus and potassium. And having too much of these can be a problem^[5], as they can mineralize with other elements and make it so your plants can’t utilize them. This is a process known as nutrient lockout, and while it isn’t good, you generally need really high results to be in danger.

Now what I found very interesting about this was that the accumulation of these nutrients appears to happen really quickly, as the ‘new garden’ only had two years of leaf mulch and phosphorus and nutrients aren’t that much lower than the ‘old garden’.

How much organic matter/nutrients is too much?

Since it was apparent that I’m putting in more nutrients than what I’m taking out, I figured I should try to research what the upper limits were, as I want to keep using leaf mulch just for the water retention and ‘no-weeding’. And, well, I couldn’t really find an upper limit.

There isn’t a hard number (in ppm) that anyone can give you, because the nutrient lock-out phenomena^[6] is depending on several factors like soil texture and pH^[7]. And there are some case studies one can find of people who ‘burned’ their garden from too much organic matter or fertilizer, but they were around 4X higher than my levels.

Conclusion

So, over the course of five years I have transformed my soil from an inorganic, infertile medium into a crumbly black fertile heaven for plants by using leaf mulch. And I really didn’t need to wait five years to realize the benefits – they happened almost immediately the first year I applied leaf mulch. But, with my recent soil test I learned that I am accumulating excess phosphorus and potassium in my soil. And while Haven’t had any problems yet, that could change in the coming years. When would that happen? I don’t know. But I don’t want to find out.

I should note that I have received a hidden benefit from only using leaf mulch and plant-based compost (I make it myself), as that has a lower proportion of phosphorus. Had I used manure-based compost, I would have added a lot more phosphorus^[8] (proportionally).

Based on those soil test results, I will not apply compost this year (fall of 2024) to my old garden, and will retest the soil next year. I want to see if I keep accumulating nutrients by just using leaves.

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