We
know more about the movement of celestial bodies than about the soil
underfoot. Leonardo da Vinci
Some
of you will have questions about your own situation – and that’s
how it should be. Many of these questions will be excellent for
demonstrating a concept or principle to the class, however, some of
you live in simply impossible conditions that conform to no known
laws of man and what you need is not an answer in a class room
setting but a consultation with someone first hand with your
situation as the sole focus – or perhaps a miracle, I don’t know
– in those cases, I will defer your question and ask you approach
me out of the classroom setting in order to answer you completely.
Let’s be fair to your fellow students, this is their time too.
In
teaching soils through the years, I have found that we will move in
and out of three different scientific discipline: 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. So this 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 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
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 – see the
chart on page 32. How does this happen?
- More small particles can fit into the same area.
- The shape of clay particles.
This could be
illustrated by coffee beans, ground and whole – if I could think
of a container to show this. I.e. a test tube.
Soil Texture
Is
determined through the proportion of these differing components found
in a given soil.
Ref.
Chart on 33
An
ideal soil is a mix of all these different components. 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.
Do
the texture triangle procedure
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
Symptoms
of deficiency of N vs. Fe
PH
chart on p. 54 and the differences of pH on nutrient uptake…
Fertilizers
NPK
– a ‘complete’ fertilizer and what do the numbers mean..
Nitrogen;
Phosphorous; Potassium
The
difference between fertilizers and amendment.
Discuss
lead uptake in soils…
Bibliography
Hillel,
D. J. 1992, Out of the Earth: Civilization and the Life of the Soil,
Reprint Edition, Berkeley, CA; University of California Press
Logsdon,
Gene, 1975; The Gardener’s Guide to Better Soil, Emmaus, PN; Rodale
Press
Gershuny,
Grace, 1986; The Soul of Soil; A Guide to Ecological Soil
Management, 2nd Edition, St. Johnsbury, VT; Gaia Services
Kohnke,
Helmut, 1995; Soil Science Simplified, 4th Edition,
Prospect Heights, IL Waveland Press
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