Autumn leaves are one of my favorite free, local, sustainable resources for the garden.
I like them for a number of reasons,
but today I’ll talk about why I prefer them over rock dust as a source of minerals for the garden.
Trees mine minerals from deep in the soil and return them to the soil surface when leaves fall.
Unfortunately, I could never find data on the mineral content of leaves.
So, I was delighted when Stephen from Alberta Urban Garden sent a variety of local leaves
- birch, poplar, apple, and Russian olive -
to a lab for analysis.
He also sent samples of two leading brands of horticultural rock dust
to serve as a point of comparison in assessing leaves as a mineral source.
I’m sure many of you have seen Stephen’s video on the results,
but I’d like to provide my perspective as well.
In addition to looking at the lab results,
I’ll compare leaves to rock dust from the perspective of an environmentally conscious consumer
who is looking for a quality product that is environmentally friendly,
renewable, and reasonably priced.
To help me evaluate leaves and rock dust as competing products,
I’ve prepared a report card.
I’m looking for a product that is organic, contains a full complement of essential and beneficial elements,
and has minimal levels of harmful elements.
Though I don’t expect the minerals to be immediately plant available,
I’d rather wait months than years for them to become so.
I’d prefer the product to be local and from a renewable resource.
Minimal environmental impact and fossil fuel usage are very important to me as well.
Finally, I’d like the product to be reasonably priced.
I’ll grade on a scale from A, which is excellent, to F, which is failing.
Admittedly, the grading will be subjective,
but I’ll do my best to be fair.
The first criterion is easy to grade.
Both rock dust brands are certified organic
and leaves are organic, so everybody gets an A.
Now let’s compare the essential elements in the leaves
to those in the first brand of rock dust,
which we’ll call Rock Dust A.
Stephen has already done a statistical analysis of the numbers, so I’ll be brief.
Green shading indicates that leaves have a significantly higher level of an element.
Red means Rock Dust A has a significantly higher level.
No highlight means that the elements are in a similar range or the amounts are statistically insignificant in both samples.
All numbers represent milligrams per kilogram.
The nitrate and nitrite numbers represent only the available amount,
but all other numbers are for total amounts - available and unavailable.
At first glance, it’s clear that the autumn leaves fare very well compared to rock dust A.
Leaves have significantly more nitrates and nitrites, phosphorus, sulfur, boron, and zinc.
Though in a statistically similar range, the leaves also have more potassium, calcium, and nickel.
Iron is the only mineral that is significantly higher in the rock dust.
Though in similar ranges statistically, rock dust A has more magnesium and manganese.
Copper and molybdenum are at statistically insignificant levels in both the rock dust and leaves.
Based on the essential elements, I’d give leaves the higher grade.
Now let’s look at beneficial elements.
Both cobalt and selenium are at statistically insignificant levels.
Sodium is significantly higher in the rock dust,
but that doesn’t change my preference for the leaves.
I have no reason to believe that our crops suffer from a sodium deficiency.
Now let’s compare the essential elements in leaves versus those in Rock Dust B.
Clearly rock dust B compares more favorably to leaves than did rock dust A.
Rock dust B has significantly more magnesium, iron, manganese, zinc, copper, and nickel.
Leaves have significantly more nitrate and nitrite, potassium, sulfur, and boron.
Though in a statistically similar range, leaves also have more phosphorus and calcium.
The molybdenum levels are not statistically relevant.
This is a tough call. Leaves have higher levels of some very critical elements,
but rock dust b has higher mineral levels over all,
so I’ll give the advantage to rock dust B.
Looking at the beneficial elements, we see that the rock dust has much more sodium, but this won’t impact my assessment.
So, let’s put the grades in the report card for the essential and beneficial elements.
I’ll give leaves an A-,
Rock Dust A a B,
and Rock Dust B an A.
Now let’s move on to the trace minerals that have no known beneficial role in plant growth and health.
In fact, many of them are quite toxic to plants and humans in sufficient concentrations.
Looking at these minerals in rock dust A and leaves, we see that most are in similar ranges or at statistically insignificant levels.
However, Rock dust A contains significantly more aluminum, barium, chromium, lead, and vanadium.
Though these elements are toxic in sufficient concentrations, I’m not suggesting these levels pose a threat.
That said, I see no reason to add more of these elements to the soil,
and I definitely prefer the lower levels found in the leaves.
The picture looks similar with Rock dust B,
which has significantly higher levels of aluminum, antimony, arsenic,
barium, chromium, lead, tin, and vanadium.
Again, many of these are quite toxic.
Though the levels here may not be high enough to be concerned about,
they are also definitely not a strong selling point,
so I much prefer the leaves.
Returning to the report card, I’ll give leaves an A for “minimal levels of toxic elements”.
Rock dust gets a C.
While we’re here in the report card, we can quickly add a few more grades.
Let’s start with plant availability.
The minerals in both leaves and rock dust are not plant available.
However, because the minerals in leaves have already been taken up by a plant,
they will be available sooner than those found in rock dust.
I’ll give the leaves an A and rock dust a B.
Another advantage of leaves is that they’re local,
while the 2 rock dust products have to be shipped from more than 1,000 miles away.
Leaves get an A and rock dust gets an F.
Leaves are also a renewable resource, while rock dust is not.
Leaves get an A. Rock dust gets an F.
Now let’s look at both environmental impact and fossil fuel usage.
Returning leaves to the soil is definitely a plus to the environment,
especially when the alternative is sending them to a landfill.
And collecting them requires little to no fossil fuel, at least in our case.
Most of the leaves are collected within a few city blocks from the house.
When we do use the car to pick them up,
we only do so when we’re already driving the car for another purpose
and we happen to come across some bagged leaves.
So, leaves get an A when it comes to environmental impact and fossil fuel usage.
Mined rock dust is a very different story.
Both of the rock dusts under consideration here are mined specifically for rock dust.
In other words, they're not simply waste products from another mining process.
As a results, all of the environmental damage and fossil fuel usage
incurred in the production and distribution of these products
can be attributed directly to the products themselves.
There are many environmental problems associated with mining, including air pollution,
ground water contamination, excessive water and fossil fuel consumption,
noise pollution, and destruction of landscapes and wildlife.
Turning a mountain of rock into a fine powder comes at a cost.
Most mining operations use the explosive ANFO -
ammonium nitrate/fuel oil - to blast rocks.
The ammonia in ANFO is produced using the Haber-Bosch process,
which is the same energy intensive process used to produce ammonia for synthetic fertilizers.
Drilling, crushing, and moving the rock requires large amounts of fossil fuels.
And a great deal of water is required in the operation of crushers, to lubricate and cool drill bits and to suppress dust.
Finally, shipping rock dust across country and around the world
burns even more fossil fuels.
So, while leaves get an A for environmental impact and fossil fuel usage,
rock dust gets an F.
Finally, let’s look at product cost.
Free autumn leaves definitely get an A.
The best price I’ve found for rock dust is 37 lbs for $44 including shipping.
At that price, I could easily pay anywhere between $100 and $600 per year,
depending on the application rate.
That’s definitely more than I want to pay, especially when there’s a free alternative.
I’ll give rock dust a C for cost.
So, from the perspective of an environmentally conscious consumer
who is looking for a quality product
that is environmentally friendly, renewable, and reasonably priced,
I believe leaves are the best choice.
When I average the grades on the report card, leaves get an A
and both rock dust brands get a C-.
Fortunately, nature has already devised a way to recycle nutrients from deep in the soil
back to the soil surface where shallow rooted plants need them.
So, why rely on explosives, drills, crushers, and fossil fuels
when you can rely on tap roots and leaves?
Even if you don’t have deciduous trees in your area, keep in mind that
all organic matter contains minerals that are released as organic matter decays.
Compost made from a wide variety of inputs
should provide more than enough minerals for most soils.
As I mentioned earlier, the grades I assigned are admittedly subjective.
If you would have graded anything differently, please let me know in a comment below.
Before closing, I’d like to thank Stephen from Alberta Urban Garden for sharing his lab results.
As we close, I’ll provide a link to Stephen’s video summary of the results.
There’s also a link to the video in the description, along with a link to lab results
Well, that's all for now.
Thank you very much for watching,
and until next time remember you can change the world one yard at a time.