(This, in general, describes the
Week 3 lecture. As usual, many directions were taken inside this
framework in response to questions. Get notes from other students if
you were not able to attend.)
We know more about the movement of
celestial bodies than about the soil underfoot. Leonardo da Vinci
We will
move in and out of three different scientific disciplines: physics,
chemistry and biology. We can only talk about one subject
at a time, because that is how we learn, but I want to stress to you
from the very beginning that these are interconnected in a very
intricate dance. Whatever you do to one will affect the other two as
sure as cutting up beets will give you red fingers. Remember that
and you will go a long way towards mastering the soils you garden
with.
Air and
water share spaces in the soil. After a rain, an event that has
happened here once in a while, as much as 100% of the soil pore space
may be filled with water; this same pore space may be 100% filled
with air in the event of an extended drought – in which case, all
the plants in that soil would be dead. Therefore the percentage
allotted to water and air is always in flux. Approximately half of
the volume of soil is pore space and can be taken up with water or
air depending on the current weather conditions.
Except for
a precious small number of you, most of you will garden in soils that
have 5% organic matter. Maybe even less. These figures represent an
‘average’ soil. There are variations from place to place, but
this representation is close enough for an average number through
out.
Soil
Formation
Soil forms over thousands of years and is an ongoing
process. Soils in California are relatively young soils and haven’t,
for the most part, developed any great depth.
The
following factors inform the process:
Climate
– including temperature and rainfall
Organisms
– from the itty bitty (microscopic) to the biggies (macroscopic)
Topography
– (the book calls relief) – land surface
Parent
material – the original rock
Time
– the factor that weathers us all.
Climate
Soil forms
from the parent material. Climate participates in this process in
many guises:
Wind
Rainfall
Freezing
A mild
climate forms soil more slowly than a non-forgiving climate
Organisms
Organisms
from lichen growing on a rock to a tree that sends its root hairs
down into crevices of the rock and fissure it.
Topography
Soil forms
more easily on a level surface. Look at the sheer face of a cliff
and you’ll see the extreme proof of what I’m saying.
Parent
Material
The rock
underneath your garden. Granite becomes soil less rapidly than
sandstone.
Time
Because
time ages everything.
Soil
Composition
Sand/Silt/Clay
– the physical sizes
Sand –
from 2mm to 5 hundredths of a mm
Silt –
from 5 hundredths of a mm to 2 thousandths of a mm
Clay –
smaller than 2 thousandths of a mm
Characteristics
of Soil Components
|
Property/Behavior
|
Sand
|
Silt
|
Clay
|
Water
holding
|
Low
|
Medium
+
|
High
|
|
Aeration
|
Good
|
Medium
|
Poor
|
|
Drainage
rate
|
High
|
Medium
|
Slow/Very
slow
|
|
Soil
organic matter
|
Low
|
Medium
+
|
High
|
|
Decomposition
of organic matter
|
Rapid
|
Medium
|
Slow
|
|
Speed
of warming
|
Rapid
|
Medium
|
Slow
|
|
Compactability
|
Low
|
Medium
|
High
|
|
Storage
of nutrients
|
Low
|
Medium
|
High
|
|
Resistance
to pH change
|
Low
|
Medium
|
High
|
Notes on
Clay Soil
Clay
particles, though tiny, have a much larger surface area – clay
particles are hollow with an interior that looks very much like
Marina del Rey from the air,
jetty and boat slip like interior contours creating a much larger
surface area than would seem possible.
More
particles fit into the same area (less pore space between them)
Clay
particles are electronically charged and bind water (and therefore
nutrients) to the particle while such water (nutrients) are washed
away easily in a sandy.
Soil
Texture
Is
determined through the proportion of these differing components found
in a given soil.
An
ideal soil is a mix of all these different components, sand, silt and
clay. While it is possible to have a soil that is composed of one or
the other component, the likelihood is that it will be a combination
of all three. The proportion of one to the next determines how you
call your soil.
Textural
Triangle Exercise
Activity:
Using the Textural Triangle
Sand,
silt, and clay are the three particle sizes of mineral material found
in soils. The percent of each of these in a given soil is called the
"particle size distribution" and the way they feel is
called the "soil texture". Soil Scientists have created
classes which break these textures into 12 categories. The textural
triangle is a diagram which shows how each of these textures are
classified based on how much sand, silt, and clay is in each.
To
get the sample you will use for this exercise, choose a spot in the
area you wish to plant. If it is a large area, you may wish to take
several samples to work with in different jars or you may take
several samples and combine them together in one jar. In the first
case, the soil might have several diffrerent textures that you wish
to account for while the second case, the soil would be rather
homogenous and consistent throughout the planting area. In this
class, we will only work with one sample.
Using
a pint jar, add soil to fill the jar about 3/4's full. Add water,
leaving about ½ to one inch of headspace. If you have Calgone Bath
Beads, or alum at home, add one teaspoon to your jar of soil and
water; if not, wait until class where will have alum on hand. Shake
the mixture up as thoroughly as you can. Allow to settle. The ideal
amount of time the mixture should settle out for the most clarity is
about 24 hours. We will not have that luxury in class.
Once
it has settled, observe the layers. There will be three distinct
layers of soil. The first will be sand, as it is the heaviest. The
second will be silt and finally, on top, clay. Sometimes the
difference from one to the next will be color. Other times, it is
all the same color, but the texture is different. There can be a
certain amount of art to finding which is what. Assign percentages
to each layer, based on your best guess. Remember that the total of
your three percentages MUST EQUAL 100. The soil in the water is the
100%.
The
following directions assume you start with the sand. You do not have
to; use the one you feel most comfortable with, always remembering to
total to 100.
1.
Place the edge of a ruler at the point along the base of the
triangle that represents the percent of sand in your sample. Position
the ruler on or parallel to the lines which slant toward the base of
the triangle.
2.
Place the edge of a second ruler at the point along the right side of
the triangle that represents the percent of silt in your sample.
Position the ruler on or parallel to the lines which slant toward the
base of the triangle.
3.
Place the point of a pencil or pen at the point where the two rulers
meet. Place the top edge of one of the rulers on the mark, and hold
the ruler parallel to the horizontal lines. The number on the left
should be the percent of clay in the sample.
5.
The descriptive name of the soil sample is written in the shaded area
where the mark is located. If the mark should fall directly on a line
between two descriptions, record both names.
Feel
the texture of a moist soil sample in your classroom. Sand will feel
"gritty", while silt will feel like powder or flour. Clay
will feel "sticky" and hard to squeeze, and will probably
stick to your hand. Looking at the textural triangle, try to estimate
how much sand, silt, or clay is in the sample. Find the name of the
texture that this soil corresponds to.
Practice
Exercises:
Use
the following numbers to determine the soil texture name using the
textural triangle. When a number is missing, fill in the blanks
(note: the sum of %sand, silt and clay should always add up to 100%):
|
%
SAND
|
%SILT
|
%CLAY
|
TEXTURE
NAME
|
|
75
|
10
|
15
|
sandy
loam
|
|
10
|
83
|
7
|
|
|
42
|
|
37
|
|
|
|
52
|
21
|
|
|
|
35
|
50
|
|
|
30
|
55
|
|
|
|
37
|
|
21
|
|
|
5
|
70
|
|
|
|
55
|
|
40
|
|
|
|
45
|
10
|
|
We
will do this in class, together, next week. BRING YOUR SOIL SAMPLE &
TRIANGLE TO CLASS.
Carbon Sequestration
Wetland
soil is an important carbon sink; 14.5% of the world’s soil carbon
is found in wetlands, while only 6% of the world’s land is composed
of wetlands.
Agriculture
Globally,
soils are estimated to contain approximately 1,500 gigatons of
organic carbon, more than the amount in vegetation and the
atmosphere.
Modification
of agricultural practices is a recognized method of carbon
sequestration as soil can act as an effective carbon sink offsetting
as much as 20% of 2010 carbon dioxide emissions annually.
Carbon
emission reduction methods in agriculture can be grouped into two
categories: reducing and/or displacing emissions and enhancing carbon
removal. Some of these reductions involve increasing the efficiency
of farm operations (i.e. more fuel-efficient equipment) while some
involve interruptions in the natural carbon cycle. Also, some
effective techniques (such as the elimination of stubble burning) can
negatively impact other environmental concerns.
In
practice, most farming operations that incorporate post-harvest crop
residues, wastes and byproducts back into the soil provide a carbon
storage benefit. This is particularly the case for practices
such as field burning of stubble - rather than releasing almost all
of the stored CO2 to the atmosphere, tillage incorporates
the biomass back into the soil where it can be absorbed and a portion
of it stored permanently.
Enhancing
carbon removal
All crops
absorb CO2 during growth and release it after harvest. The
goal of agricultural carbon removal is to use the crop and its
relation to the carbon cycle to permanently sequester carbon within
the soil. This is done by selecting farming methods that return
biomass to the soil and enhance the conditions in which the carbon
within the plants will be reduced to its elemental nature and stored
in a stable state. Methods for accomplishing this include:
Use cover
crops such as grasses and weeds as temporary cover between
planting seasons
Concentrate
on perennial food production vs. annual production
Cover
bare soil with hay or dead vegetation, protecting soil from the sun
and incorporating much more compost into the soil so it holds more
water and is more attractive to carbon-capturing microbes.
Restore
degraded agricultural land, slowing carbon release while returning
the land to agriculture or other use.
Agricultural
sequestration practices may have positive effects on soil, air,
and water quality, be beneficial to wildlife, and expand food
production.
Typically
after 15 to 30 years of sequestration, soil becomes saturated
and ceases to absorb additional carbon, implying the extistence of a
global limit to the amount of carbon that soil can hold.
Governments
such as Australia and New Zealand are considering allowing farmers to
sell carbon credits once they document that they have sufficiently
increased soil carbon content
Organic
Matter
The end
process of compost is: humus
Humus is a
complex organic substance resulting from the breakdown of plant
material in a process called humification. This process can occur
naturally in soil, or in the production of compost. Humus is
extremely important to the fertility of soils in both a physical and
chemical sense (see below). Physically it helps the soil retain
moisture and encourages the formation of good soil structure.
Chemically, it has many active molecules that can bind to plant
nutrients, making them more available. It is difficult to define
humus in precise terms because it is a highly complex substance, the
full nature of which is still not fully understood. Physically humus
can be differentiated from organic matter in that the latter is rough
looking material, with coarse plant remains still visible, while once
fully humified it become more uniform in appearance (a dark, spongy,
jelly-like substance) and unstructured in structure; which is to say,
it has no determinate shape, structure or character, it is not
square, round or triangular.
Plant
remains (including those that have passed through an animal and are
excreted as manure) contain organic compounds: sugars, starches,
proteins, carbohydrates and organic acids. The process of organic
matter decay in the soil begins with the decomposition of sugars and
starches from carbohydrates which break down easily as detritivores
initially invade the dead plant, whilst the remaining cellulose
breaks down more slowly. Proteins decompose into amino acids at a
rate depending on Carbon: Nitrogen ratios. The humus that is the end
product of this process is a mixture of compounds and complex life
chemicals of plant, animal or microbial origin which has many
functions and benefits in the soil as outlined below;
The
process that converts raw organic matter to the relatively stable
substance that is humus feeds the soil population of micro-organisms
and other creatures which helps in maintaining high and healthy
levels of soil life.
Effective
and stable humus are further sources of nutrients to microbes, the
former providing a readily available supply whilst the latter acts as
a more long term storage reservoir.
Humification
of dead plant material causes complex organic compounds to break down
into simpler forms which are then made available to growing plants
for uptake through their root systems.
Humus can
hold the equivalent of 80-90% of its weight in moisture, thus
increases the soil's capacity to withstand drought conditions.
The
biochemical structure of humus enables it to moderate- or buffer-
excessive acid or alkaline soil conditions.
During the
humification process microbes secrete sticky gums- these contribute
to the structure of the soil by holding particles together, allowing
greater aeration of the soil. Toxic substances such as heavy metals,
as well as excess nutrients, can be bound to the complex organic
molecules of humus and prevented from entering the wider ecosystem.
The dark
color of humus (usually black or dark brown) helps to warm up cold
soils in the spring.
Humus
which is also capable of further decomposition is referred to as
effective or active humus. It is principally derived from sugars,
starches and proteins and consists of simple organic acids. It is an
excellent source of plant nutrients, but of little value regarding
long term soil structure and tilth. Stable humus consisting of humic
acids on the other hand, are so highly insoluble (or tightly bound to
clay particles that they cannot be penetrated by microbes) that they
are greatly resistant to further decomposition. They add few readily
available nutrients to the soil, but play an essential part in
providing it's physical structure. Some very stable humus complexes
have survived for thousands of years.
Humus
should not be thought of as 'dead'- rather it is the 'raw matter' of
life- the transition stage between one life form and another. It is a
part of a constant process of change and organic cycling, thus must
be constantly replenished- for when we are removing prunings and
crops for the kitchen we are depriving nature's cycle of potential
humus. This is why we need to substitute compost and other sources of
organic matter to maintain the fertility of our productive land.
Organic
matter placed on the soil is called: mulch.
Organic
matter dug into the soil is called amendment.
Some of
either can be called humus, but not all.
Organic
matter in the soil mitigates any negatives of that soil:
Too much
clay is opened up by adding OM.
Too much
sand is cohered by adding OM.
Micro and
macro organisms live on OM.
There are
several different schools of thought on how to get OM in to and used
by the soil: from double digging, to using a tiller to sheet
composting.
Soil
Water
Nutrients
enter a plant via soil solution.
Water
coheres to itself (describe the miniscus).
Roots take
up water one molecule at a time. Water molecules cohere throughout
the plant – form the water column. That water molecule
pulled into the plant root will pull along one behind it and one
behind it.
Discuss
water pulled across a moist soil and watering away from the plant’s
base. Water not making across differing soil types.
Each
shovel of soil holds more living things than all the human beings
ever born.
Nutrients
Nutrients
Available (via atmosphere or water)
Carbon
Hydrogen
Oxygen
Primary
Nutrients
Nitrogen
Potassium
Phosphorus
Secondary
Nutrients
Calcium
Magnesium
Sulfur
Micronutrients
Boron
Chlorine
Copper
Iron
Manganese
Molybdenum
Nickel
Zinc
Fertilizers
NPK
– a ‘complete’ fertilizer and what do the numbers mean..
Nitrogen;
Phosphorous; Potassium
The
difference between fertilizers and amendment.
A
Soils Bibliography
|
Out
of the Earth: Civilization and the Life of the Soil; ©1992
University of California Press , Hillel, Daniel. Hillel has
written one of the most beautiful books on soil that has ever been
published. This book introduces a little of soil science to the
reader, but more than that, it fosters a love of the soil and an
understanding about the magnitude and gravity of misuse and
degradation; civilizations have paid little heed to the soil
underfoot and it has cost them dearly. A delightful read!
|
|
Soils
and Men, Yearbook of Agriculture 1938, ©
1938, United States Department of Agriculture, The Committee on
Soils. A government publication, I challenge you to read from
beginning to end! It is referenced here because it clearly shows
the US government knew about the soil food web and chose to ignore
that information in favor of more commerce in chemical based
fertilizers. We are at a point where ignoring the soil food web
is too costly to continue.
|
Teaming
with Microbes: The Organic Gardener's Guide to the Soil Food Web,
Revised Edition, ©
2010 Timber Press, Lowenfels, Jeff and Lewis, Wayne. This is the
second edition of the book that blew my eyes open on the biology
of the soil and how we cannot ignore that biology plays at least
as big a part of soil fertility as chemistry. We ignore biology
to our own detriment and destroy our soils.
|
|
The
Rodale Book of Composting, ©1992
Rodale Press, Martin, Deborah and Gershuny, Grace Editors. This
is the only book to read on composting. Everything else is
compostable. Only.
|
|
The
Soul of Soil; A Guide to Ecological Soil Management, 2nd
Edition, ©1986; Gaia
Services, Gershuny, Grace. This fabulous and passionate book is
injured by being targeted to farmers (only) and therefore all
recommendations are written in “pound per acre,” when we need
ounces per 100 square feet. When I used this book, I wrote up a
formula in Excel to convert all these into a usable figure.
|
The Worst
Hard Time, The Untold Story of Those Who Survived The Great
American Dustbowl ©
2006; Mariner Reprint Edition, Egan, Timothy. Not strictly a soils
book, but a real eye opener that shows how we are repeating many
of the same mistakes today as what lead to the disaster we call
the Dustbowl. This book is gripping reading and is not fiction.
It really happened and it happened on a scale unprecedented in
modern times. We can do it again if we fail to heed these words.
A VERY good read on soils and man's relationship to them.
Also note, not included in the lecture, because I didn't see it until the day following, look at this article on perennial grains and the work currently being done on them.
|
david
,
Schumacher, E. F., ©1973 Blond & Briggs. This classic inspired much
of the current thinking in sustainability. Although it does not relate
directly to gardening, it explains the basis of the idea of natural
capital. E.F Schumacher Society website:
,
Jeavons, John and Cox, Carol, ©1992 Ten Speed Press. A more accessible
book than Jeavons’ “How to Grow More Vegetables: etc.” Written by
pioneers in the U.S. of Biodynamic French-Intensive gardening, it tells
how to create fertility on a closed-system basis, that is without
inputs, by growing both calorie crops for humans and carbon crops for
compost.
,
ibid. How to garden (and farm) with the least effort and inputs
possible, by someone who has been farming all his life, and fighting big
ag at the same time. All of his books are excellent. Gene has a blog at 
,
Nabhan, Gary Paul, © 2002, W.W. Norton. One of the original books on
eating local which inspired many others, including Barbara Kingsolver.
All of his books are worth reading.
, Sandbeck,
Ellen, © 2003 Broadway Books. This small book tells how to actually
accomplish the act of gardening, including how to use tools, how to move
big rocks, and why herbicide doesn’t work on concrete.
, Dell, Owen, © 2009 For Dummies. Dell has been telling the truth about the impacts and inputs of gardening for a long time. 
,
Astyk, Sharon and Newton, Aaron, © 2009, New Society Publishers.
Explores the world food crisis and why big conventional ag can't solve it.
, Ludwig, Art, ©1989 Oasis Design, 
, Holmgren, David, © 2002, Holmgren Design Services. A more theoretical approach to sustainable design concepts.
Mollison, Bill, © 1988 Tagari Publications. This comprehensive book is
the textbook for the Permaculture Design class. A reference for those
who have already been introduced to the principles, as well as a dual
duty doorstop and blunt instrument :-). It is on this list for the sake
of comprehensiveness.
,
McHarg, Ian, © 1982 John Wiley and Sons, Inc. This book is more a
lanscape-level and regional-planning book, but has beautiful graphics
and exposition about where to site projects. This book addresses the
too-rarely asked question, “where is this projec
