Soil Erosion: A Historical Perspective


Getting started with our
conservation webinar, I’m Holli Kuykendall,
National Technology specialist for NRCS’s East National
Technology support center. I’m pleased to turn the
webinar over to David Lamm. David is the National
Soil Health team leader for NRCS’s
Soil Health Division. David, you may now begin. OK, and thank you Holli for
that wonderful introduction. And I want to thank everybody
for joining in today. I think it’s going to be a
very exciting presentation. I had a chance to hear Dr.
Montgomery’s presentation several years ago at the Salt
Water Conservation Society meeting. And also, I had a
chance to meet him last summer in person here in
Greensboro, North Carolina. Dr. Montgomery is a
professor of Space Sciences at the University of
Washington, located in Seattle. And he has a BS in
Geology from Stanford. And his PhD is in Geomorphology
from the University of California. He’s an author of numerous
books, one being Dirt– the Erosion of Civilization. And the second one, he’s going
to introduce this afternoon is called The Hidden
Half of Nature– the Microbial Roots
of Life and Health. And it’s interesting
that Ray Archuleta always says that soil without
biology is only geology. So we have a geologist
who’s going to be talking about the life of the soil. So with that, I’ll turn it
over to you, Dr. Montgomery. Thank you, David. And thank you all
for listening in. It’s a privilege to
talk to you all today. I have been a major fan
of the NRCS and the work that it does back from the
Soil Conservation Service days when I read Topsoil and
Civilization as an undergrad. And it got me thinking
about the role of soil in human societies. So I’m going to give you
a bit of an overview tour through these two books, Dirt
and The Hidden Half of Nature, today. If you’re interested
in this kind of stuff, I obviously recommend
the books to you. If you want to follow
this stuff, my wife and I– my co-author
on the second one– our Twitter handle is
up there on the screen. But I was asked about
a couple weeks ago to give a talk at a
conference in San Francisco and to open with the big picture
of why soil conservation is important. And what I decided to do is
open with this slide that shows Earth and Mars and ask
the simple question, which planet would you rather
live on– the one with soil or the one known as Mars. And soil, we can
look at, as you all know, as one of
the great resources that we have on this planet. Water and soil makes
this place habitable. But if we look at
the way that we’ve been treating soil over the last
few centuries, in particular, there’s great concern about the
damage that we’ve been doing. David Pimental’s famous
paper back from 1995 argued that some $430 million
hectares of arable land has been lost to
soil degradation since the Second World War. And that’s an area
equivalent to about a third of all the crop land that
we have on the planet. And this is a
realization that I think is central to your
guys’ mission, as is, I think, the idea
of soil degradation that Rattan Lal back
in ’99 commented on, that the world’s agricultural
soils have already lost 66 to 90 billion tons of
carbon, mostly due to pillage. And these issues
and these changes to soil on the
surface of the earth is why somebody like
myself, a geologist, would get involved in thinking
about the long-term influence of soil on human civilizations,
the idea of looking at what kind of lessons
can we learn from the past in terms of the importance of
soil and soil erosion and soil degradation, in particular,
on maintaining civilizations. And that’s the approach that
I took in the Dirt book, is trying to go back and read
the archaeological literature, try and integrate
what had happened and what was the influence of
soil erosion and degradation on ancient societies. And the first thing
you run into when you start researching
that perspective is the argument that it was
deforestation that helped drive the soil erosion
that contributed to the demise of societies
around the world, all those societies over on the
right-hand side of the screen being a few of the
places where people have made those connections. But I’m a geologist who grew up
in the steep lands of Northern California and did my PhD work
in the Oregon coast range, places where you might
expect large scale soil erosion to have happened
from forest clearing, if it simply was deforestation
or timber harvesting that was driving erosion
that contributed to the demise of
ancient societies. And you notice pretty quickly
that the trees grow back, long before the soil disappears
on any wholesale level. Sure, clear cutting
steep slopes increases the pace of landsliding
but just doesn’t pencil out anywhere near in terms
of the kind of soil loss that’s been documented in
the archaeological record in ancient societies. And that got me starting
to think that, well, could it have been agricultural
soil erosion and degradation that contributed to limiting
the lifespan of civilizations. And obviously if I’d come
up with the answer of no, I probably would have
written a different book. So just titling like Dirt
book the way we have, we kind of telegraphed
the answer. And a lot of the details
are in that book. But when you go
and actually work through the
archaeological studies, they show pretty clearly
that erosion played a role in the demise of
ancient civilizations, ranging from the Bronze Age
societies of Neolithic Europe to classical Greece,
Rome, the southern United States, Central America. The list goes on. There are societies in
Asia, Pacific Islands. And I tried to catalog
as many as I could and weave the commonalities
out through the Dirt book. And the most
fundamental commonality I can really see
in that story is that the invention of
the plow was far more the culprit in terms
of ancient soil erosion than was the forester’s axe. And that the
invention of the plow fundamentally altered
the balance between soil production and soil erosion
on our agricultural lands and dramatically increased
the pace of soil erosion to the point where, if you
think about that balance, if you’re removing
something faster than you’re replacing it, you’re
actually burning through and running out of it. What kind of a time
frame was actually involved in those processes? And could agricultural
soil erosion proceed at a pace for
which we can actually tie it to the longevity
of human societies? That’s the case that I
investigated and tried to try to pull together. I’m not going to talk about
lots of societies today. There’s a similar
story, though, that comes out in culture
after culture, in period of history
after a period of history, that classical Greece
illustrates pretty well. And in classical Greece, cycles
of erosion and soil formation really began when,
in the Bronze Age, the plow arrived from points
in Asia Minor to the east. And this scenario that
I’m going to walk through in terms of the
geoarchaeological evidence that’s been put together
for classical Greece is mirrored in other societies. But I like using
classical Greece because both the geology
and the archaeology were done in concert
with one another in integrated studies that tried
to reconstruct what happened to the landscape of Greece
after the last Ice Age through the present day. And Tjeerd van Andel
and Curtis Runnels, a geologist and an
archaeologist back in the ’80s, put together this story where
the next couple slides will show you cross
sections of hillsides that they put together
to show the transition in the Greek landscape
that accompanied the agriculturalization–
if I can perhaps invent a word– of that landscape. And so this first slide
shows the open oak woodland of the uplands in
Classical Greece, where you’ve got the trees on there. And then the little
vertical bars that are brown show you
the soil on the hillsides. There’s about a
foot to three foot thick bedrock
underneath, and then an alluvium down in
the valley bottoms. Well, when agriculture–
plow-based grain cultivation– spread up from the valley
bottoms up onto the hillsides– I’ll see if I can make
the pointer work here– so it spread from the valley
bottoms up onto the hillsides. It started off a
cycle of erosion that stripped the soils
off of the hillsides and piled it all up down
at the valley bottoms. And you can still find
agricultural implements from the Bronze Age up
here on the rocky slopes with just a scrubby maquis
vegetation today in places where we know the crops were
grown in Classical times. And if you think
about the problem of growing food on a landscape,
the spatial distribution of the soil matters, if you take
the total volume of soil that’s spread in a thin
layer on a landscape, blanketing the hillsides,
and you pile it all up down at the
valley bottoms, you’re going to be able
to grow food on a smaller portion of the landscape. And Classical Greece really
illustrates well this tension between how farming started down
in the valley bottoms, spread up onto the hillsides. But once the soil was
removed from the hillsides, they were not able to
support as a high population as they were when they were able
to farm the entire landscape. What did this do
to Greek society? One of the things I like about
the van Andel and Reynolds work is they went ahead and tried
to examine the human population density– so how
many people there were on the landscape in various
valleys of southern Greece, in this case, the
Southern Argolid. And they looked
at that population through time, from 6,000 BC
over there on the left up to the Modern Age on the right. And they see an interesting
pattern of a rising population in the Bronze Age
and then a crash through the Dark
Age between then and the age of
Classical Greece, where the population rose again. A second crash in the Dark
Age of the first Millennium, and then a rise
to the Modern Age. Now there’s two really
interesting aspects to this curve. And this is really
the figure that, when I found this researching Dirt,
the basic argument of the book started to gel in my
mind because there are two aspects to this curve,
one of which is fairly trivial. And that is, why does the
amplitude of these cycles increase with each cycle? And pretty clearly, that’s
due to the development of technology. I mean, we have far
better technology today in the Modern Age than they
did back in the Bronze Age. So it’s no mystery
that you could grow more food per
hectare of land to support a bigger population
now than you did then. But what sets the periodicity? Why this rise and crash,
rise and crash, and rise. And then, of course,
the question of what happens off the right
side of the graph. And is this an analog
for societies in general, and what might we expect
at a global level, as well? There’s very few places that
you can point to where there has been three agricultural
societies that occupied the same piece of land. And this 1,000 to 2,000
year sort of cyclicity is what got me really
thinking about what’s the relationship between the
way that people treat their land and how long the land
will be able to support human societies. Now it turns out, I’m
not the first person to have thought of this. Plato, back in the
third, fourth century BC, noticed the problem of
the Bronze Age soil erosion off the Greek landscape. And he wrote about in
one of those dialogues where he wrote that the
rich soft soil has all run away, leaving the land
nothing but skin and bone. But in those days, the
damage had not taken place. The hills had high crest. The rocky plain of Phelleus
was covered with rich soil and the mountains were covered
by thick woods, of which there are some traces today. What Plato was looking
at was evidence like things like oak
trees and all trees that were sitting up on soil
pedestals around plowed fields. And he put together the idea
that degradation of the soil had led to a reduction
in harvest, that meant that the population
of Greece was lower, which meant that they couldn’t
mount the armies they needed to protect themselves from
people coming from the east. He was in other words,
I think the first person who really put together the idea
that the way that people treat their land will shape
how long that land is able to support their society. He didn’t get a lot of credit
for this idea in the long-run, though. It was written in
one of his dialogues where he talked about
the story of Atlantis. It wasn’t viewed as the most
credible story from antiquity. But I think he was actually a
pretty good observer of nature and was really onto something
in noticing the Bronze Age soil erosion event in the days
before Classical Greece, when he was alive. So I’m going to skip over
a whole mess of societies that I talked about in terms
of Rome and North Africa. But I’ll just mention
that if we look at some of the societies
in the Middle East where the greatest
damage to their soil has been done for
the longest time, and that they haven’t recovered. We’re looking at places
like Syria and Libya, places that are not exactly
the stable points of prosperity today. But if we look at the
history of our own country and look at soil
erosion in the US, I think there’s some
very interesting ties to the way it has shaped the
development of our country. I go into it in the
book, but I’ll just share a little bit
with you here in terms of the magnitude of
historical erosion in the Piedmont region of
the American Southeast. So going from Virginia up here
all the way down to Alabama in looking at the hill
country of the Piedmont, that upland terrain where
soil erosion off the hillsides would be expected to start
when the soil is broken and it was first plowed,
and if agricultural soil erosion proceeded faster
than soil was formed, how much would actually
be lost, how fast? And Trimble and Meade put a
lot of this work together. And there’s been more recent
work in cosmogenic isotopes. It’s been very interesting
in this regard, too. But I like this one
graph because it shows the magnitude
of historical soil loss in the Piedmont region
since the colonial era. And you’ll notice that most
of it is four to 10 inches. And essentially the A
horizon has been stripped off the landscape. And I was out in North
Carolina not too long ago looking at some farms
with Ray Archuleta, and basically like, where
did the A horizon go? They’re farming the B horizon. There is widespread loss of
the top soil in this region. And four to 10 inches in
a couple hundred years across a landscape
of this extent starts to put into perspective
what could the Romans or the Greeks have done
with 1,000 year run at it, with much similar
technologies in terms of their plows. They’re a little different,
but not all that different. And here we’re starting
to get into the realm where we can actually
look at measurements and look at the pace of things. And stripping the A horizon
off of a pretty broad region of the country in
a few hundred years starts to make the case
that maybe this idea of widespread regional
soil loss really did have an impact on
ancient societies, let alone our own society. Well, George Washington
was one of the first people that I think really recognized
the long-term impact of the soil degradation
on the eastern seaboard, the original agricultural
powerhouse of the country. And he cared captured a
nicely in a 1796 letter to Alexander Hamilton
where he wrote that a few years more
of increased sterility will drive the inhabitants
of the Atlantic states westward for support. Whereas, if they were taught
how to improve the old instead of going in pursuit of
new and productive soils, they would make these
acres, which now scarcely yield them anything, turn
out beneficial to themselves. Well, Washington was
arguing, very explicitly– and this letter was actually
published about 100 years after his death but captured at
the end of the 18th century– he was arguing that
for American society to remain prosperous
with the way that we were farming colonial
times and degrading soil on the eastern
seaboard, we would have to spread across
the Appalachians to the fertile soils
of the Midwest. He was predicting,
in other words, our country’s westward
expansion a century before the idea of
Manifest Destiny was sort of thrown
around by historians. He was looking at it in
terms of a nation of farmers needing access to
undegraded fertile soils because of what we had done
on the eastern seaboard. And if you look at
the soils today, this basically shows you the
difference between a forest soil in North Carolina and
a conventional tobacco field that I’ve photographed
for a NOVA special that was on last November. I think it was the third
episode of Making North America. They realized they had left
soils out of the picture. And we ended up doing a
short segment for them. But essentially, if you
look at the soil on the left here is the analog
for something more like what the native soil was
like and the soil on the right being what happens after a few
decades of tobacco cultivation. The story is clear in
terms of soil degradation. Now I want to turn to picking on
my own home state of Washington for a few minutes here, in
part, because this photograph of the Palouse
from back in 1970– it’s the eastern part of the
state with beautiful loess soils. But this photograph
captures really well why a geologist like myself
would view agricultural soil erosion as a really
big long-term problem. All those little
channels across the hill there, those little
rills, you could just plow right across them. They’re easily wiped out
with a single pass of a plow. But they really add
up over time in terms of net movement of soil. And how much so? This picture from also
from the Palouse from 1961 taken by Verne Kaiser
shows you a cliff that developed in the
margin of the field around a fence that is enclosing
a water cistern that the farmer didn’t want to plow over. And so in 1911, when he first
broke the sod in this region, the soil was up to about here. By 1961, the base of the
cliff was down about here. And you’ll notice that this
little black thing running from there to there, and
then it picks up again there, this is actually a
stadia rod that shows you that this is five feet tall. The rod is basically
wiped out in the negative. But you can see a one foot
increment sitting about there. So that’s about a
five feet of soil loss on the edge of this
field, probably mostly due to tillage
erosion, but also somewhat to wind and rain. But this loss of five
feet of soil in 50 years translates into a loss of
about a foot a decade, which is about an inch a year. And if you look at the pace
at which soils form globally, there’s nowhere on earth
that soils are forming at a pace of an inch a year. So you should be sitting
there going, yeah, well, that’s an extreme example. And it is. That’s why I like to show it. But extreme examples
don’t make a solid case. So one of the things I wanted to
do in researching the Dirt book was to compile additional
data on both contemporary and long-term or
geological erosion rates, and on agricultural
erosion rates, in particular, to try
and ask the question of, is the pace of agricultural
soil loss enough that it could explain the
time scale of that periodicity in the graph from Classical
Greece and the magnitude of soil erosion in the
American Southeast. Does this all kind of
pencil out, in other words. And what I did is
something that has gotten very difficult
to convince students to do anymore, frankly. And that is, I went
to the library. I just went and I parked and
I gathered up all the data that I could find on what are
rates of agricultural soil loss around the world from
conventionally plowed fields. And what are rates of long
term geological erosion. Because if you’re
going to maintain soil on the landscape
over the long haul, those long-term
geological erosion rates give you an estimate
of the long-term rates of soil production because
when you have geological time to play with, if
those aren’t balanced, soil won’t be
maintained or it’ll be built up so thick that it
would be an incredible pile. But you look at the UN
global soil database, there’s on average one to three
feet of soil around the world. And we’ve got tens
of thousands of years since the last Ice
Age, so the argument that geological
erosion rates, and so if production rates should be
balanced, is a pretty good one. And I’ll show you
some data on that in a minute that confirms it. But the point here is that
I compiled about 1,400 and some odd
measurements of erosion from both agricultural
settings and from the long-term
background against which we could compare that to. And you’ll notice that I did
not use any universal soil loss equation-based model studies. I just wanted to actually
use real measurements, point measurements on the
ground, of erosion. And you’ll notice that
natural erosion rates span about seven orders of
magnitude from down here at a tenth of a
millimeter and fractions of a tenth of a millimeter
a year at the low end up to the Gorge of
the Tsangpo river over here at about two
centimeters a year, the most rapidly eroding place on earth. So it spans quite a range. And I’ve organized
the geological data into three categories–
cratons, soil mantle terrain, and alpine or glaciated terrain. Those cratons are the flat
bed parts of continents. They’re the areas that are
like most of Africa, Australia, the heart of the American
continent, the Midwest– places that tend to be low
relief, fairly flat, fairly earthquake free. And then those places erode
at rates up to about 1% of a millimeter a year. The soil mantle terrain
here in the middle are places that actually
are like the Piedmont, like the hills of
classical Greece, places where you have
significant topography up the steep slopes, but
they’re covered with soil. And those range up to
erosion rates on the order of a millimeter a year or so. So you could think of
long-term soil production rates in the areas
that we actually farm– the soil mantle
landscapes and cratons as places where the soil
is being produced at less than about a millimeter a year. If you look at places that have
erosion rates higher than that over the long-run, they’re the
alpine and glaciated terrain– real mountains, things like the
Cascades, the Alps in Europe, the Himalayas, the
Andes, places that don’t tend to have a lot of
soil on their surface and they tend to
erode fairly fast. Now look at the data
from agriculture. And what I mean by
agriculture here is a conventional
plow-based agriculture. And those erosion rates
range from a fraction of a millimeter a
year up to pushing a tenth of a meter a year. But if you play
the game of which of these natural
long-term erosion rates do the conventional
agricultural rates pencil out at, you basically
come to the conclusion that we’ve managed to convert
the places that we farm in this world, places like
Nebraska and Tennessee, into places that are eroding,
like the high Himalayas and the Andes. And this is through
the thick glasses of a geologist and a brute
force global data compilation. Results will vary by farm
and environment, obviously. But the basic story here
is that we’ve basically accelerated rates
of soil erosion in a way that would
be unsustainable. And this shows you
that same data viewed in a slightly different format. What I’ve got here
are percentile plots. And I’ve added a
couple other data sets. So we’ve got probability
distributions for geological erosion rates. That’s that black
line in through here. And then the
conventional agriculture dates– those black dots those. That’s the same data
that I showed you on the previous graph, but just
presented in a different way where you see each data point
in all these compilations. And you read the average
value is the 50th percentile up and over to get the rate. But the point that I
really want to make here is that when you add
data on soil production rates– the little circles,
the white circles– rates of erosion under
native vegetation– the little white triangles–
and rates of erosion under conservation agriculture–
the little white diamonds– they all plot pretty much on
top of that long-term geological soil erosion rate. In other words, this
leads to the conclusion that agricultural soil loss
is not because we farm, but it arises from how we farm. Conventional
plow-based agriculture is the outlier
sitting up here, well above long-term rates of soil
erosion, soil replacement, soil production, and well above
rates of soil erosion under conservation agriculture. And this I like to call the good
news slide because it suggests we have access to
agricultural practices that would not result in
long-term loss of the soil. The problem, of course,
is that those are not practices that are conventional
in the sense of widely adopted throughout our
agricultural settings. Now if we basically try and
take one more look at that data before I move on to
thinking about tying it back to the main thrust
of the Dirt book, we can look at the
different kinds of averages that we have in
distributions like this, in whether you like
the median or the mean, these are not Gaussian
distributions. So I’ll show you both
averages because it really doesn’t matter to the
point that I want to make. If we look at the
average rate of erosion under conventional plow-based
agriculture globally, it’s somewhere north
of a millimeter a year. Whether it’s a millimeter
and a half, four millimeters, let’s just call it north
of a millimeter a year, and compare that with
all these blue numbers– rates of erosion, the average
rate under conservation agriculture, which the data
was mostly no till agriculture, native vegetation
from around the world, rates of soil production,
and very long-term geological erosion rates. Those are all in
terms of fractions of a tenth of a
millimeter a year. So however you cut
this data, there’s a very big difference
between the long-term erosion rates under
conventional agriculture and the rates at
which soil is being produced for which
we could lose it under conservation agriculture. So anyway you cut this,
there’s more than a millimeter a year difference
between erosion rates under conventional agriculture
and rates of soil production. So if we play that out
and we look at, well, what does that mean for
the longevity of societies, we can do a very simple
calculation where we look at the net loss
of the soil of about a millimeter a year
or so– implies that erosion of a typical
half meter to one meter thick hill sloped soil, a
foot to three feet of soil on a hillside, would only
take about 500 to 1,000 years. And if you look back through
the archaeological record and you parse for
different regions, that’s about the lifespan
of most major civilizations with one really big
caveat– and that’s outside of major river floodplains. So it wouldn’t apply to
things like the Tigris and Euphrates, the Yellow River,
the alluvial rivers in China, the Indus, the
Brahmaputra, the Nile. The places where
human civilizations have lasted the longest as
contiguous civilizations are places where erosion of the
uplands from those environments feeds the floodplain
agriculture and refreshes soil fertility on an annual
basis in those environments. And some of them have
problems with salinity that I talk about in
the book, as well. But the key here to me
is that this periodicity that we saw in that Greek
landscape in terms of three societies occupying
the same landscape, that cycle of taking 500 to
1,000 years to erode the soil and then taking a couple
thousand years to build it back, the numbers kind of
pencil out at looking at that’s not a crazy hypothesis
to actually think about in terms of what’s controlling
the longevity of societies. Now this idea that
the way people treat soils affects the
longevity of societies, again, is not new. Plato talked about
it, but so did Franklin Delano
Roosevelt back in 1937 in the aftermath
of the Dust Bowl. He wrote in a letter to the
governors of the then all 48 states that a nation
that destroys its soil destroys itself. And I think that those
words are as true today as they were back then. But they have greater
pertinence on a global stage because we’ve made great
strides in this country at reducing the pace
of soil erosion. We still, I think, have
a ways to go in terms of reducing soil degradation. But your agency, in particular,
has been very effective since the Dust Bowl at
trying to crank down the rate at which American
fields are eroding. You go around the
world and it can be a bit of a different story. So this of course
motivates the question– is soil restoration possible. Could we reverse that
historical pattern, and could we do it
at a large scale? And that’s the
question we started to wrestle with in The
Hidden Half of Nature. This shows you my hands
holding a soil that are on our lot in North Seattle. This is what we
started with, this is what we have today
about 15 years later. You’ll notice there’s a
bit of a color difference between those soils. It was an experiment
in soil building that’ll describe a little
bit, but that has roots in previous efforts
because we could look back at some societies,
particularly the Dutch, that built the famous plaggen
soils out of marine sands. Whereby returning organic
matter to their soil, they built up some of the
most fertile land in Europe. And we could look at
the terra preta soils of the Amazon, where the native
soil is not terribly fertile, but through building
up carbon-rich soils, human activity actually
greatly accelerated the pace of soil building,
to the point where the best agricultural
soil today in the Amazon is located in the regions
that had the highest aboriginal population
density, where the best soils, in other words,
are where the most people were. So the idea that human societies
need to mine their soil to persist is
demonstrably wrong. There are societies
that have built fertile soil through their
agricultural activities. And this is something
that my wife and co-author on The Hidden Half of
Nature and I really learned for
ourselves in our yard when we bought a house
in North Seattle. And what this did
through the digression that I’ll share with you is
it brought to our attention the role of this
invisible world of nature that we call the
hidden half of nature, the microbial world,
the world that ranges down from the smallest. There are pieces of living
things, DNA, and viruses, all the way up to the red
blood cells, the giants of the microbial world. And you notice that
the range in sizes of things in the invisible
half of nature, things smaller than about a tenth of a
millimeter, that range in sizes is about five
orders of magnitude. That’s the same size
range as we have in the visible part of
nature, from amoebas all the way up to people. And the title of the book,
The Hidden Half of Nature, is not facetious. If we looked at the total mass
of microbial life in the world, it would rival the
total mass of the nature that we can know and see and
look and kick over and study. And the biologist–
I’m a geologist, we trained to study this
visible world of nature. But through the process of
restoring the soil in our yard, we really came to
appreciate the role that this invisible
hidden half plays in driving the dynamics that
support life above ground. This shows here our yard. When we peeled back the
lawn, we bought a 1918 house in North Seattle, I
should have dug a soil pit when we bought it. My wife wanted a garden. I didn’t think to do that. She didn’t think to do that. When we finally
peeled the lawn off to actually try and
put a garden in, we found that we
had glacial till. Now it’s essentially
nature’s concrete. It was sand, silt, and
gravel and boulders that were compressed beneath
a mile high wall of ice that overran North Seattle. It doesn’t look like the world’s
best soil to grow things in. Well, watch this roof line
of the house behind us. It’ll come back in a few slides. So we realized that we had
the geology part of soil. But we didn’t have
the biology part. And the thing that
David said about Ray talking about soil
without biology is just geology– well, that’s
what we had in our yard. And we basically decided we
needed to add the biology. So we started adding organic
matter in all the forms that we could get it and
painted a wheelbarrow up with racing stripes. And we started adding
oak leaves, wood chips, as much organic matter
as we could find. This slide shows you
her pruning shears. With the pit that we dug in
the soil, about six years in to doing very
intensive composting and mulching in the yard– and
you’ll notice that we’ve still got crappy till down here,
we’ve got all the organic matter we’ve added at the
surface up here– but notice we’ve got about two
inches of actual real soil that just was not there when
we pulled the lawn off. And you’ll notice that the
plant roots are getting down just about to the interface
between the soil and the till. We are able to build two inches
of soil in about six years. That’s almost four
inches a decade. I mean, this is off
the charts in terms of the pace at which
nature builds soil. And this was a real revelation
to me in terms of the degree to which how we treat
the surface of the earth, what we put on it and the role
of organic, that we could build soils, not just from the
bottom up by converting rocks into soil, but that we could
build them from the top down by adding organic matter
to integrate that with the soil. And to me, that was a big
change in perspective. And notice this is that same
neighbor’s roof line, again, about six to eight years
into our transformation of our garden. And fixing the
soil below ground, receding the microbial
life that was driving the nutrient cycling in the
below ground environment really fostered an explosion
of life above ground that led to a radical
transformation of how we actually see and use our yard. And a lot of that boiled
down to the effect of exudates in the rhizosphere. Now as a geologist,
looking at the rhizosphere and thinking about plant
roots as actually pumping carbon rich substances out into
the soil was a bit novel to me. This is not what I was taught
about soils and soil fertility. But it was fascinating to
learn about this rich zone of microbial life
that forms a living halo around the roots
of plants and the way the plants will push out
30% to 40%, in some cases, of the material that they
fix through photosynthesis out of their root systems
and into the soil. And it makes kind of
no sense about why a plant would do that, unless
there’s a purpose for it. And it turns out that
purpose is to feed that rich population
of microbial life in the rhizosphere. And plants wouldn’t be
bothering to do this if the microbial life didn’t
provide something in return. And as we looked
into this process and started reading the
microbiological literature in terms of the
relationship between plants, the roots, and the
microbes in the soil, we really learned the idea
that the plant roots are not just straws sucking up
material from the soil, but it’s a two way street across
which plant exudates are moving out of the root systems
and into the rhizosphere to feed the microbes in the
soil that basically consume all those carbon-rich goodies. And their metabolites,
their waste products, are taken back up by the plants
and help nourish the plants. What kind of our metabolites
are they producing? I was shocked when I
learned that the microbes in the rhizosphere are
producing things like plant growth promoting hormones. Why would a microbe produce a
plant growth promoting hormone? Well, it’s to basically help
nourish its sugar mama that’s capturing solar energy, pushing
sugars out into the soil, and helping to feed
those microbes. The microbes have and interest
in keeping the plant healthy. And the plant has an interest
in keeping the microbes healthy. It’s essentially a symbiosis. Fungal hyphae that actually
connect up to plant roots end up going off and
prospecting in the mineral soil to actually bring
nutrients to plant roots. They’re bringing
things like phosphorus, things like all
the micronutrients that plants need to grow, a lot
of things that simply aren’t in the fertilizers
that we’re so used to applying in our
agricultural environments. It was the mycorrhizal fungi
that were actually providing those to a lot of plants. Another thing that I was
very fascinated to learn in looking into
these connections was the idea that
when above ground pests– like this
little guy here– start chewing on the
leaves of plants, those plants can push exudates
out of their root system into the soil to try and grow
the populations of microbes in the rhizosphere whose
metabolites are taken back up by the plant
and that actually make the plant taste
bad to the pest and help repel those pests. In other words,
this different view of the relationship between
plants, the root systems, and the life in the soil,
the microbes growing around those roots, leads us to
characterize this relationship as plants have sort
of offshore, they’ve outsourced some other
basic chemical defense mechanisms to the microbes
in the rhizosphere. And their way of
actually harnessing that is by feeding the
appropriate microbes. In other words, there
are these partnerships in the soil that can help
explain some of the phenomena that we see. And in part, one
of those phenomena is how fertilization can affect
the growth of plant roots and the production
of plant exudates and how that, in turn,
influences microbial life in the rhizosphere. If we look at an
experiment done in Vermont with 100-day-old
tomato plants, this is actually about a two
feet section through here. With no fertilization,
the roots look like this. Conventionally fertilized,
you’ve got far less roots. Essentially, the plant
gets relatively lazy. If you’re providing it with
nitrogen and phosphorus, and potassium, it doesn’t
need to actually build this extensive root
system to try and capture those kinds of nutrients
through the interactions with the rhizosphere. A compost and manure system, you
get a much greater and denser growth of plant roots, in
part, because those nutrients in the compost and manure are
not immediately accessible. And the plant is growing
roots and putting out exudates to grow the microbes that will
then break down the compost and release the nutrients
to take back up. In other words, the
diet that a plant eats, what it’s provided with in the
soil, whether it’s a fertilizer rich diet or an organic
matter rich diet, will actually
influence the degree to which that plant will engage
with the microbial community. And that will affect
its ability to draw up other micronutrients. The last thing about the
experiment in our yard that I would like
to share with you all is an observation that
I made late in the game. And that is that life
came back to our yard in about the same
order as which it evolved on earth, without
of course the dinosaurs. But the microbial life
was the foundation that fueled an explosion
of life above ground. If we look at the timeline,
for about 540 million years here down to about 145
million years at the bottom, and we just look at how
bacteria arrived, then plants arrived, then
detritivores arrived, then herbivores, ferns, reptiles,
then if you substitute crows for dinosaurs, we’re on track
there, and then mammals, birds. That same order of life
coming back to the yard is the order which life
evolved on the continents. And I think what
that’s telling us is that there’s some very
fundamental relationships in which microbial life sets
the foundation for building the ecosystems in the
soil that in turn support ecosystems above ground. Now while this was all
happening, Ann and I, we’re pretty fascinated by
microbial life– archaea, bacteria, protists,
viruses, and fungi, the big players in
the microbial world. And at this point, we were
pretty fascinated with what they could do to
boost the fertility of the soil in our yard and
support life in our yard. But we got thrown a
curve ball in that while we were putting
the story together, Ann was diagnosed with a
microbially-caused cancer. And she’s about five
years post-surgery. She’s pretty much through
that phase of her life now, thankfully. But it made us start thinking
about and looking into what’s the role and influence of
microbial life on human health and our own health. And at this time,
the discoveries about the human
microbiome were exploding. And we didn’t
realize how much they would parallel what we were
learning about the soil microbiome. But if we look at
the human microbiome, and we can look at in terms
of number of our cells, there’s about as many
microbial cells in our bodies as there are human cells. We’re sort of half
human, half microbial, if you look at the total
number of cells in our bodies. If you look at the number
of genes in our bodies, we have about
23,000 human genes. If you look at all the
microbial genes that are doing things in
our bodies, there’s millions of them,
five or six million. We’re wildly outnumbered in
terms of genetic diversity within our own bodies. This let us into
starting to think about what are the
role of microbes in maintaining human health. We know about the role
of pests and pathogens. We all get colds
every now and then, but some of the discoveries
in the recent microbial world have looked at the
relationship between microbes and the human immune system
through a very different lens. And I’ll relate this back to
the soil in a couple minutes, but there’s some really
interesting and key parallels here that, frankly, I
found really surprising but I think are very
informative and illuminating. So this leads us to a
place that none of us really like to talk
about– the human colon. It’s sort of at the bottom
end of our digestive system. And if you take a look
at the human colon through a cross-section
and blow it up, you get something
like this down here. Why would we look
at the human colon? Something like 80%
of your immune tissue is wrapped around the colon,
almost like an envelope around the colon. And 80% or so of
your microbiome, the microbes native
to your own body, live within the human colon. And it turns out that
when you actually read the gastroenterology
literature, which we are subjected to in
researching this book, you find that there are these
cells called goblet cells that produce mucus that goes
out and lines the colon. And some of the terms that
the gastroenterologist use are very parallel to what was
being used in the soil world. They refer to these
as exudates that essentially line the colon. And there is bacteria that
live within the mucus layer. And they consume and
eat those exudates. There’s a parallel with
the root system there. And the parallels get
even more interesting. When we look at blowing
up a cross-section across the human colon– so
there’s the lining of the colon cells there– there’s these
particular immune cells called dendritic cells that
will stick a little arm up through the lining of the colon
and sample either the microbes in the mucus or actually
in the lumen, where the stuff is going through. And why are they doing this? They take those samples,
which are called antigens, and they bring them
back and share them, show them to their pals, these
immune cells called T cells. And this turns out to be
absolutely central to how your immune system works because
those samples, the antigen, that the dendritic cells
show to the T cells are the key to
activating the T cells. T cells just sit there
as inactive cells not doing anything
until they’re presented with the right antigen that
spurs them into activity. And they’ll either be triggered
as pro-inflammatory T17 cells. Or if they have
different antigen, they may be triggered
as anti-inflammatory T regulatory cells. In other words, it turns
out that which microbes are in your colon influence the
balance between inflammation and quelling inflammation
in your immune system. In other words, the information
in the microbiota in your colon is informing your immune
system as to whether to lead to inflammation and fire it
up, or to quell it and crank it down. And there’s a hypothesis that
the change in the human gut microbiota may have
actually influenced the rise of chronic
diseases in the second half of the 20th century. We’re all familiar
with how the incidence of infectious diseases
decrease in the 20th century, thanks to things
like antibiotics and better sanitation. Well, we haven’t had a
really good explanation for yet is why the
commensurate increase in autoimmune disorders
and chronic diseases. And it has been
hypothesized that that is due to changes in
the human microbiota through either antibiotics or
through changes in our diet. How does the diet
connection work? Well, we’ve got to take
a very brief detour through the human digestive
tract to illustrate that. But basically the
idea is that if you look at the human
digestive tract from the stomach, the small
intestine, and the colon, the number of microbes
in the stomach is very small because
it’s very acidic. There’s a few in
the small intestine. But our colon is
actually the center of are microbial diversity. And there’s hundreds of millions
of microbes per milliliter of fluid in the colon,
greatly outnumbering what’s in the stomach. And if you look at what happens
in these different organs, the stomach dissolves things. The small intestine
absorbs the things that were dissolved in the stomach. This is where the proteins and
the fats and the simple sugars get absorbed into your body. The colon, it turns out,
is a fermentation tank. All those plant
foods that are really hard to digest that
we don’t actually have the enzymes to break
down end up in our colons. And that’s where the
microbes in our colon, with their extra
several million genes, they have the
ability to break down those complex
carbohydrates, things that your doctor calls fiber. And what they do with
that fiber is actually something that is nothing short
of miraculous in terms of what it does for our own health. So if we look at fiber
coming into the human colon, there’s different words for
it, lots of different forms. It gets consumed by our
microbiota in our colon. Their metabolites,
their waste products, produce things like butyrate. Why am I going to
focus on butyrate? Well, it turns out
butyrate is what nourishes the cells that line the colon. Most of the cells in your
body are fed by your blood. Colon cells are different. They’re nourished
primarily by butyrate, which is produced
by microbes that are consuming and fermenting
the fiber that is in our diet. The other things
that butyrate does is that when those dendritic
cells sample it as antigen and show it to T
cells, it triggers the T regulatory cells, which
are the inflammation blocking cells. So in other words, if you eat
a diet that’s high in fiber, it will nourish
your colon lining and stand down
your immune system rather than firing it up in a
chronically inflamed situation. This is a totally
different way of looking at how the immune system
relates to microbes. I was always under
the impression that microbes were
disease-causing organisms and pests and pathogens. But it turns out that
having the right microbes in that environment actually
benefits the health of people, much like the way having
the right microbes in the roots of a plant
benefits the health of plants. Well, how might this have
influenced our health over the 20th century? Well, if we just look at the
total carbohydrate consumption from 1910 to 1997, we
ate about the same amount now in terms of total
carbohydrates as we did then. It’s just in a completely
different form. Early in the century, we
ate mostly unprocessed whole grains, high
carbohydrate, high fiber diet. We now eat a high
carbohydrate, low fiber diet, mostly processed grains
where the bran in the fiber part of the grains has been
separated from the rest of what goes into the diet. And there’s a whole
slew of maladies for which a connection has
been suggested due to changes in the human microbiome. Some of these have
been demonstrated to be causal at this point. Others, it’s correlative. The point we make
in The Hidden Half is that over the
next couple decades there’s going to be a lot of
revolutionary science in terms of how changes in
the human microbiome has affected public health. And what I really
wanted to get to was that at this point in
researching the book, Ann and I had this big
sort of aha in terms of, if we think about
the root and the gut as organs of plants and
animals, respectively, they’re kind of the same
thing inside out. If we look at
roots, the exudates are feeding the root microbiome. In the human gut,
it’s our own diet and the mucus exudates
from our colon lining are feeding our
internal microbiome. And those microbes,
in both cases, help with nutrient acquisition
and make metabolites that are critical to maintaining
the health of both plants and in people. And it’s based on making them
from organic matter in the soil and from our diet in our
gut– a different kind of organic matter. And if we look at the way
that microbes are influencing plant defense and
our immune system, there’s this
communication that’s going on, chemical communication
going on between the host organism and the microbiome. And this perspective leads
us to a different view, I think, of both the soil
in the outer world of nature and also in what Ann and
I call our inner soil– the lining of our own gut. And it’s suggested that
maybe we need to think about soil in a different way. And if we think about– back
in the plant world– in terms of what it is that makes for
a healthy diet for plant life, thinking about the role of
these beneficial microbial metabolites that are
produced by the microbes that are fed by the organic
matter in the soil, you get those into
a plant diet when you have a lot of organic
matter for those microbes to process and break down. A fertilizer diet, sort of
a conventional fertilizer, can provide a lot of
the macronutrients, but you may not
be getting as many of the micronutrients and
those beneficial microbial metabolites that we can get
by feeding a soil a soil life diet. We go into those
connections in terms of how those things work in the book. And we go a lot
farther into looking at what it means for the
human diet, what we should all be eating. But the basic message boils
down to thinking about mulch in your soil, inside or out. And the last thing that I
want to share with you all, since we’re closing in on
the end of the hour here, is that if you look at
how to actually apply that idea of trying to cultivate
the beneficial microbes in the soil with what we
call the soil life diet, how would you go
about operationalizing that in agriculture? And I spent this last year
from April till September visiting farms
around the world that were applying the principles
of conservation agriculture, as the FAO defines them, which
involve three basic principles. Minimal or no soil
disturbance– things like no till, the direct
planting of seeds. Maintaining a
permanent ground cover and retaining crop
residues, including cover crops in crop rotations. And then using diverse
rotations to help break up pathogen carryover
and maintain fertility. And I basically visited
people like Dwayne Beck in South Dakota,
Kofi Boa in Ghana, David Brand over here on the
right, Gabe Brown over here on the left, both in Ohio and
North Dakota, respectively. And I visited Rattan Lal at
Ohio State University, where he showed me his long-term
effects of applying conservation agriculture
principles to his soil trials, with the before over here on the
left with this light yellowish soil, the dark soil on the
right with 20 years of no till and organic amendments. Big difference in the soil. And I’m writing that
stuff up right now. We need to finish
the book by June to deliver it to my publisher. It’ll come out next spring. But the basic idea is that
these ideas of conservation agriculture really do
work to restore soil in intensive production
environments. And they’re scalable– from
small scale subsistence farms in Africa right on up to
big operations in the US. And I want to
emphasize that I think that we’re missing the
boat to some degree with a lot of the
arguments around GMO versus organic agriculture. I think the biggest room
for really rapid progress is prioritizing, thinking
about soil health in terms of our
framing of how to look at agricultural
practices and looking at conservation
agriculture as a way to do it, and reframe
the argument around not a question of a difference
between low tech organic versus high
tech GMO approaches, but to focus on the real
question, which I think is how to apply the
understanding of soil ecology and soil building to the
applied problem of profitably sustaining high crop yields
in the coming post cheap oil environment. I think there’s room for great
optimism and progress here. The people that I
was visiting, who were putting these
principles into practice, use different techniques
on their farms. They’re all very different. But they’ve greatly
reduced their reliance on agrochemicals. None of them–
well, one exception, I went to the Rodale Institute. They’re the only organic
producers that I visited. But all these places had
maintained their yields while greatly
reducing their inputs and thereby made their
farms more profitable. And so as a final point, I
think there’s some side benefits from restoring healthy soils. And it can actually help us
feed the world in the post oil environment, maintain
yields with fewer inputs. It can help sequester
carbon in the soil. There’s a whole another
talk we could do about that. But the potential is huge. And it could help us
conserve biodiversity on that quarter of
the continents that are agricultural lands,
which will be greatly important in the future. So I’ll leave you
there, since I think we’re running up right
into the top of the hour. And I’ve managed to
actually do this on time. I apologize for
talking fairly fast. But if you’re interested
in following this stuff that Ann and I are following
up on on the relationship between microbial
life and the soil and in the human gut and their
connections and parallels, we’re on Twitter at @dig2grow. Our website is up there, too. And obviously, I’d
encourage you all to check out and read
the books and stay tuned for the next one. I think if we have time,
I’d be happy to engage with any questions. OK, thank you, Dave. I’m hoping that everybody else
was as blown away as I am. That was a great connection
between healthy soils mean healthy food
mean healthy people. I think that’s just a great
connection you made there. And I’ve got a couple questions. And again, I encourage
folks to get the books. I haven’t read the second
one, but I have read Dirt and looked at it several times. But there was a question kind
of going back a little bit, too, when you redid your
garden, any idea about how much organic matter you were
talking about having to add to get those kind of changes? And give it to us in pounds for
acres, not wheelbarrows per– Wheelbarrows per season. I would love to be able to
give you numbers on that. But the honest answer is we
didn’t do a controlled science experiment on the yard. It was something that
Ann started and called it her organic matter crusade. We would put a lot of
organic matter into the yard. And I think that’s why we
were able to do it so fast, a big part of it. There’s a lot of coffee
shops in Seattle, they all put their
coffee grounds out behind their places. There’s a lot of trees
that get chipped up. So we had a lot of nitrogen
rich and carbon rich sources we were able to put
back into the soil. And we also used a
lot of our own worm compost out of our kitchen. So in terms of the rate at which
we were doing it, I’m not sure. We may be fairly fast. What I can tell you
about is how much we changed the carbon
content of our soil. Unfortunately, I can’t
tell you how much carbon we had to put on to do
it because we were not that controlled. But we did recently go through
and look at the carbon content in different parts of the soil. And the part of the yard that
we did not restore– the bit out behind the garage– had
the original carbon content about a percent and a half. Most of the planting beds
now are up to about 5% or 6%. Some are a little higher. And the vegetable
beds, which got the majority of
the worm compost, are up to about
11% or 12% carbon. So we’ve basically sequestered
a lot of carbon in the yard. But we had access to large
amounts of organic matter. And we applied a lot early
on, and then it tapered off. And we’re now at the
point where the yard itself generates
enough organic matter to mulch and cover the beds so
we’re not adding it any more. We’ve built it up. Sounds like you
had quite a variety of carbon [INAUDIBLE] ratios. Not all organic matter or
organic materials are the same. Yup. And we didn’t control for it. It was not much of
a science experiment in terms of the yard. It was an eye opener
for me in terms of the potential to do it. But what I think was the much
better controlled experiment were all these farmers
that I visited. I was incredibly impressed
with what they’re able to do. And the changes in the
carbon content of their soils mirrored what
happened in our yard. You brought up the idea of
having the change following an evolutionary pattern. And we talk about creating
habitat and it’ll come. Can you talk a little
bit more about that, what you noticed as far as
the change in the bacteria to the higher developed
organism, that kind of thing? Yeah, one of the things
that we did early on that I didn’t talk
about here is that Ann started applying soil
soup, spraying microbes in the garden. So she was trying to
rebuild and reseed the microbes at the start. And it took about three or
four years before we really started to see
changes in the soil, with starting to get darker. And we were growing that layer
beneath the organic matter. And I started
thinking about, well, how is this organic
matter breaking down. So that’s what’s
happening to it. Where is it going? And we actually
couldn’t obviously see the microbes that
came back at the start. But it was the
progression that we saw come in after we
reseeded that was pretty noticeable in terms of the
detritivores and the arachnids, and then the worms, and then
the larger things eating them, and then the birds
eating the worms, and then the eagles
eating the birds. That whole order
was pretty parallel. So I think you can think of it
in terms of– there’s a kind of diagram like a soil food web
that people like Elaine Ingham will show, where you’ve
got the microbes are feeding the next level up. Everybody is feeding
the next level up. So you can think
of microbial life as the foundation
that is actually the critical linchpin between
taking that organic matter and recycling it back into its
native components, if you will, the building blocks
for further life. The most broken down point
is when that stuff gets back into the microbes. And then the microarthropods
and the nematodes eat them and excrete the really
nutrient rich micro-manure that is the foundation,
then, for the soil food web. Well, listen, we appreciate
your time, David. It has been very informative. I think I’m going to cut
the questions off now. I encourage folks
to get the books. There are others out there. Farmacology with Dr. Daphne
Miller is another one. It talks more about
the interaction between food and human health. And I just really
think that 10 years from now we’re
going to be thinking that it’s going to be so
obvious, so why didn’t we think about this before. So anyway, with
that, I want to– I think you’re totally
right about that. Yeah, yeah. Hopefully we continue
to see this change. Because there’s one
thing I told David when I asked him to
do this– his book depressed me because there
was just constant civilization after civilization
falling because they didn’t learn from history. And hopefully we can
become wise enough to learn from our mistakes. And then with this new concept
of understanding the ecosystem that we’re all a part
of and building that into not only how we eat the
food but how we produce it and the impact it
has on our bodies, I think it’s going to lead
to a more optimistic future. At least I feel that way. So with that, I want
to thank you and just say good day there, buddy,
and have a pleasant evening. Thank you much, David. Thank you.

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