A moist sample of a soil with a high content of the larger sand particles feels rough and gritty when pressed and rolled between the thumb and forefinger. A moist sample with a high content of the microscopic particles of clay feels smooth and almost oily.
Soil texture is important because it helps to determine how well a soil can pick up and retain air, water and nutrients. It also helps to determine drainage.
SOIL STRUCTURE: Sand, silt, and clay particles often become bonded together to form a single clod or multiple crumbs. Soil structure is the way these sand, silt, and clay particles are grouped together. The size and arrangement of these groupings influence drainage and nutrient availability.
These collective structures are called AGGREGATES or PEDS. The ideal garden top soil has an open structure with lots of pore space and the aggregates are the size and shape of crumbs from the morning's toast. Adding organic matter to the soil helps in the formation of HUMUS which, in turn, helps to form and stabilize soil aggregates.
SOIL PROPERTIES: Soil texture and soil structure determine, for the most part, how well air, water, and nutrients - along with feeder roots - can pass through the soil and how well the soil can pick up and hold the air, water and nutrients.
SAND particles are relatively large, boulder shaped, and have a diameter ranging from 0.05 mm up to 2.0 mm. This gives the overall structure an abundance of air spaces or pores. Sandy soils are often called "light soils" because of their light density.
Sandy soils take up air and water easily. They have sharp drainage. They are the soil of choice for plants that need an open soil structure. They amend easily with organic matter which slows the rapid drainage.
SILT particles are intermediate in size between the particles of sand and clay. Silt particles range in size from .002 mm in diameter up to 0.05 mm. They can be boulder shaped or flat. Silt particles may combine with clay particles to form soil aggregates. The air space or pores around the silt particles or the silt aggregates is intermediate between that of sand and clay as is the drainage.
Silt is an important component of loam.
CLAY particles are flat and microscopic in size. There is very little air space between and around these flattened structures. They take up air and water very slowly, they drain slowly and they contain very little air. Clay soils are also known as "heavy soils" because they are so dense.
They can be amended with organic matter to drain faster and to take up air, water, and nutrients more readily. Clay soils are often very rich in nutrients.
LOAM is the happy medium. It contains approximately 40% by volume sand, 40% by volume silt, and 20% by volume clay. Loam has good drainage, a good ability to take up and hold moisture and nutrients, a good overall structure and is relatively easy to work. Loam with a generous quantity of organic matter is the ideal garden soil.
SOIL REACTION: The symbol, pH, represents the "negative logarithm of hydrogen ion concentration" which means the relative "sweetness" (alkalinity) or "sourness" (acidity) of the soil. The scale goes from 1 to 10. The pH of chemically pure water is 7.0 which is considered as neutral. The lower the pH away from 7.0, the greater the acidity. The higher the pH away from 7.0, the greater the alkalinity.
The term, "reaction" refers to the relative acidity-alkalinity of the soil. Most plants do best when the soil is just a bit on the acid side with a pH around 6.2 - 6.4.
The pH of the soil is also important in terms of plant nutrition. Some of the minerals become less available as nutrients in alkaline soils. This will be covered in the next session on plant nutrition.
SOIL ORGANISMS: Soil organisms are
totally basic to life as we know it. They recycle just about
everything on the face of the earth. They are usually categorized as
soil microorganisms or soil macro-organisms depending on their size.
The microorganisms are microscopic. The macro-organisms can be seen
with the naked eye. There are a few, minor exceptions.
ACTINOMYCETES (Ak-ti-no-my-CETS):
These organisms are present in the soil in huge numbers, second in
population only to bacteria. They seem to be an intermediate form
between a bacterium and a fungi. Their specialty is breaking down
particularly resistant materials such as cellulose and chitin. They
are drought resistant.
ALGAE: These organisms are usually present
in the soil in relatively small numbers. They live close to the soil
surface and do best in moist and mildly acidic conditions. They feed
on simple organic materials. Their function is not clear.
MOLDS: Molds are multi-cellular fungi with
thread like hyphae or feeding structures. They feed on organic matter
in moist conditions. They can develop vigorously and they can tolerate
wide variations in soil pH. They are particularly adept at breaking
down organic residues in highly acidic forest soils.
MYCORRHIZAL FUNGI: Certain fungi
form symbiotic relations with the roots of certain seed plants. The
mycorrhizal fungi have the potential to be highly beneficial. The
mycorrhizal fungi and the roots feed on both the nutrients in the soil
and on each other. The roots get supplemental moisture and nutrients
from the mycorrhizal fungi. The mycorrhizal fungi get plant sugars
from the roots. The mycorrhizal fungi take very little from the root
in comparison to what the root gets from the mycorrhizal fungi making
them beneficial. There are ectomycorrhizal fungi that live on the
surface of the root and there are endomycorrhizal fungi that actually
penetrate into the root's interior.
These microorganisms will be discussed later in the session on plant
nutrition.
SOIL BACTERIA: Bacteria are
single-celled organisms, one of the simplest and smallest forms of
life known. There are more numerous in the soil than any of the other
microorganisms. They exist as mats, filaments, or clumps called,
"colonies". They rival or exceed all of the other soil microorganisms
in their ability to breakdown organic compounds, to form humus, to
stabilize soil structure, and to re-cycle nutrients.
SOIL FUNGI: Fungi are present in the
soil in huge numbers. Fungi include yeasts, molds, and mushrooms. The
yeasts are single-celled. The molds and mushrooms are multi-celled and
have thread like hyphae. Their principal function is to breakdown
organic residues and to aid in the production of humus. They breakdown
cellulose, starch, sugars, gums, lignin and some protein.
YEASTS: Yeasts are single celled fungi
that live primarily in waterlogged, anaerobic soils. There specialty
is breaking down simple organic materials when conditions are
anaerobic.
Microfauna - "That part of the ANIMAL population which consists of
individuals too small to be clearly distinguished without the use of a
microscope". . . Brady's "The Nature and Properties of Soils".
NEMATODES: Nematodes or "ellworms" are
found in almost all soils. They are tiny, unsegmented round-worms that
feed mostly on decaying organic matter, soil microorganisms, and
insect larvae. Most nematodes are beneficial to the garden but some do
feed on plant roots and are considered as "pests".
PROTOZOA: Protozoa are mobile,
single-celled creatures that capture and engulf their food. They feed
mostly on bacteria and decaying organic material. They do best in
well-drained soils but there usually aren't enough of them to make a
significant difference in the decay of organic matter or nutrient
release.
BREAKDOWN A WHOLE VARIETY OF COMPOUNDS OR
MATERIALS: The soil microorganisms, as a group, can breakdown
or bio-degrade just about any organic, mineral, or synthetic material
and make it into a form that can be picked up by the plants.
"FIX" A VARIETY OF CHEMICAL ELEMENTS: Certain
soil bacteria are able to "fix" or convert a variety of chemical
elements and compounds from a form that plants can not use as
nutrients to forms that they can. Nitrogen fixing bacteria, for
example, cause the reaction of gaseous nitrogen from the air with
organic mater in the soil to form organic nitrites, takes those
organic nitrites and then makes them into organic nitrates. The
organic nitrates are in a form that is usable by the plants as
nutrients. The gaseous nitrogen is not.
AID IN THE PRODUCTION OF HUMUS: Humus is the
residue after the decomposition of added plant and animal is complete.
It is mostly a mixture of fulvic and humic acid. It concentrates in
the rhizosphere, the area directly around the roots and root hairs.
Humus is often called "Black Gold" because of its dark color and the
many benefits it gives to the garden. Some of the benefits include:
helping in the transfer of nutrients from the soil to the roots and
the transfer of waste products from the root to the soil, helping to
make minerals contained in insoluble rock particles available to the
plants as nutrients, providing a rich diet for the soil organisms,
serving as a buffer to resist changes in soil pH, and helping soil
aggregates to form and stabilize. There are a number of other less
important benefits to numerous to mention as well.
PRODUCE CARBON DIOXIDE: Soil organisms
produce carbon dioxide which can combine with soil moisture to produce
mild carbonic acid which helps to breakdown insoluble rock particles
to make the contained minerals available as plant nutrients.
A FOOD FOR THE PLANTS AND OTHER SOIL
ORGANISMS: The life span of the various soil microorganisms is
relatively short. Their bodies and/or remains are a major source of
nutrients for both the plants and the living soil organisms. The
quantity of soil organisms is measured in terms of their bio-mass and
in the case of soil microorganisms, it is substantial.
SOIL PATHOGENS: Most of the soil
microorganisms are beneficial and promote the health of the garden.
There are, however, a number that cause problems and are looked at as
pathogens causing plant disease. Ways to prevent and control these
pathogens will be discussed later. At this time, it is simply
important to recognize that plant disease is caused by living
organisms and that many of these organisms live in the soil. Plant
disease is not caused by wet or cold or poor air circulation. It is
caused by the microorganisms. The wet or cold or poor air circulation
are conditions that allow the pathogens to thrive.
SOIL MACRO-ORGANISMS: Many, if
not most, of the creatures that live in the garden go through a stage
in their life cycle when they are living in or on the soil surface.
Not all of these creatures are considered as soil macro-organisms.
Generally speaking, when a gardener views that creature as beneficial
to the life and health of the soil, the creature becomes known as a
soil macro-organism. If the gardener views that creature as
destructive, the creature becomes known as a soil pest. If the
creatures are not of any particular benefit or threat, they are simply
considered as creatures.
Keep in mind that "pests" are in the eyes of the beholder. A soil grub
feeding on the roots of a favorite plant is a "pest" in the eyes of
the gardener. A bird would more likely see the grub as a fine meal.
The more important soil macro-organisms include:
EARTHWORMS: Earthworms are probably
the most important and most beneficial of all the soil
macro-organisms. They literally eat their way through the soil
consuming huge quantities of organic matter as they go. They do not
eat living plant material or their roots. They open up the structure
of the soil aiding nutrient uptake and passage, aeration, and
drainage. The casts or waste products they produce are rich in
nutrients. They are even able to eat through and destroy hard pan.
The two most important earthworm species are Lumbricus terrestris,
commonly called "Nightcrawlers" and Eisensia foetida, commonly called
"Red Wigglers". The "nightcrawlers" are deep boring. The "red
wigglers" live closer to the surface. "Red wigglers" are the
earthworms of choice for vermicomposting.
GROUND BEETLES AND OTHER SOIL SCAVENGERS:
‘The Principles of Gardening" by Hugh Johnson says, "Everything that
grows is food for something else. Innumerable hosts of predators, both
animal and vegetable, are constantly at work, not only on our precious
plants but on each other. The best any gardener can do is to referee
the performance with a bias in favor of our side."
Many of the organisms that live in or on the surface of the soil are
scavengers feeding on dead organic material and converting it into
nutrients for members of the food chain below. Some times, however,
they cross over and feed on living garden plants. The line between
"pests" and "beneficials" is not always self-evident.
SOIL AMENDMENTS: Soils are amended
or treated with a variety of materials to achieve a variety of goals.
This section is limited to the basic issues of building organic
content, improving structure and drainage, and the very common problem
of amending rock hard, dry, clay soils.
BUILDING ORGANIC CONTENT:
The assigned chapter in Brady on "Soil Organic Matter" may take a
couple of readings. It's a complicated subject but at bottom line,
most gardens need an inch or two layer of compost or well rotted
manure in Spring and in Fall as a mulch, as a soil amendment, and as a
fertilizer.
It's function as a mulch is to serve as a blanket to minimize problems
with weeds, to provide a layer of thermal insulation and to minimize
water's evaporation from the soil. It's function as an amendment is to
serve as a source of organic material for the soil, organisms, and
plants. It's function as a fertilizer is to be as a rich source of
nutrients for both the plants and the soil organisms.
IMPROVING STRUCTURE AND
DRAINAGE: Heavy soils, that is soils that contain excessive
quantities of the small clay particles, have a dense structure that is
short on porosity and slow draining. Light soils, that is soils that
contain excessive quantities of the big sand particles, have an open
structure with too much porosity and they are sharp draining. Adding
organic content helps both kinds of soils to build better porosity and
drainage. Specifically, added organic material helps clay soils build
soil aggregates which opens up the structure. Adding organic material
helps sandy soils by reducing the open spaces.
In a choice between digging the organic material into the soil or
simply putting it down as a layer, a light "scratching in" seems the
better answer. Digging the organic matter into the soil would defeat
it's function as a mulch. Digging around the plants would also
endanger the plant roots and the aeration would trigger too much
activity on the part of the soil organisms.
The light "sctratching in" helps to incorporate just enough.
AMENDING HARD, CLAY SOILS:
Un-amended, rock hard, bone dry, water impervious, clay soils are a
challenge. The best way is to first soak it down with water for
several consecutive days and then to work over the surface with a pick
making as many nicks as back and time will allow. Add a 2" (5 cm )
layer of compost or well rotted manure over the entire soil surface,
pick and/or dig in the organic material as best you can. Add another
layer of compost or well composted manure and again pick and/or dig it
in as best you can and perhaps repeat the process several times until
the top 12" (30 cm) or so is mostly amendment with pits and pieces of
clay mixed in.
Be conscientious in terms of adding a 2" (5 cm) layer of compost or
well rotted manure every Spring and every Fall thereafter. The soil
organisms use up a lot of this organic material in the bio-degrading
process.
ASSIGNED READING:
KINDS OF SOIL MICROORGANISMS: