Soil Basics
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The soil in your lawn or garden is much more than just a medium for plants to anchor in.
It's a complex structure of organic material, mineral particles, air, water, and living
micro-organisms. All of these elements together determine the availability of nutrients to
growing plants, and thus the health of your lawn and garden. An ideal soil contains
balanced amounts of important plant nutrients, and is neither too acidic nor too alkaline. It
must be porous enough to allow air to reach roots, and excess water to drain, while
retaining enough moisture to meet plants' water needs and prevent nutrients from
leaching away. It should support a healthy community of beneficial microbes that break
down organic matter, make nutrients available to plants, and compete against harmful
microbes.
It's a complex structure of organic material, mineral particles, air, water, and living
micro-organisms. All of these elements together determine the availability of nutrients to
growing plants, and thus the health of your lawn and garden. An ideal soil contains
balanced amounts of important plant nutrients, and is neither too acidic nor too alkaline. It
must be porous enough to allow air to reach roots, and excess water to drain, while
retaining enough moisture to meet plants' water needs and prevent nutrients from
leaching away. It should support a healthy community of beneficial microbes that break
down organic matter, make nutrients available to plants, and compete against harmful
microbes.
The main component of most soils is inorganic mineral particles, rocks weathered away to
tiny bits.
The size of these particles is the biggest factor in soil texture. Particles are classified
in three general categories. From largest to smallest, these are sand, silt, and clay. Most
soils will contain all three, but the amounts of each vary widely.
A sandy soil provides excellent drainage and air penetration, but water passes through it
so easily that it doesn't retain moisture well and allows nutrients to be leached away.
Clay soils are great at retaining nutrients and water, but they often drain poorly, and can
become packed into a nearly impenetrable hardpan layer. The tiny particles of clay soil
bind together so tightly that they allow little room for air penetration or root growth. Here in
the Clark County/Portland area, clay soils are the norm.
The ideal balance of these soil particles is loam. Even if your soil falls short of the loamy
ideal, though, there are other ways of improving it.
tiny bits.
The size of these particles is the biggest factor in soil texture. Particles are classified
in three general categories. From largest to smallest, these are sand, silt, and clay. Most
soils will contain all three, but the amounts of each vary widely.
A sandy soil provides excellent drainage and air penetration, but water passes through it
so easily that it doesn't retain moisture well and allows nutrients to be leached away.
Clay soils are great at retaining nutrients and water, but they often drain poorly, and can
become packed into a nearly impenetrable hardpan layer. The tiny particles of clay soil
bind together so tightly that they allow little room for air penetration or root growth. Here in
the Clark County/Portland area, clay soils are the norm.
The ideal balance of these soil particles is loam. Even if your soil falls short of the loamy
ideal, though, there are other ways of improving it.
The remainder of soil is made up of organic material in various stages of decay. The vast
bulk of this is plant matter, as everything from leaves and dead grass to entire trunks of
fallen trees is recycled back into the earth. Add a small amount of animal waste and the
carcasses of perished animals great and small, and you have the basic mix.
Organic matter is present in soil in all stages of decomposition, and different materials
decompose at different rates. Starches and carbohydrates break down much faster than
the cellulose and lignin that make up wood, for instance. All of this is done by living things
in the soil - microbes, fungi, earthworms, and others. As they consume and digest the
dead matter, nutrients are released to be taken up by plants again.
Eventually, through decay and a process called humification, organic matter is transformed
into a substance known as humus. Gardeners often use the term humus to describe any
well-rotted compost or soil amendment, while soil scientists use it only for the final, stable
end product of decomposition and humification. Humus is what gives fertile soil its dark
color.
Humus and decaying organic material retain moisture, increasing the soil's drought
resistance. They bind to clay particles, increasing the average particle size and allowing
more pore space between them. Humus and other organic compounds bond with ions of
nutrient elements to form chelates that are readily absorbed by plants and resist being
leached out of the soil.
bulk of this is plant matter, as everything from leaves and dead grass to entire trunks of
fallen trees is recycled back into the earth. Add a small amount of animal waste and the
carcasses of perished animals great and small, and you have the basic mix.
Organic matter is present in soil in all stages of decomposition, and different materials
decompose at different rates. Starches and carbohydrates break down much faster than
the cellulose and lignin that make up wood, for instance. All of this is done by living things
in the soil - microbes, fungi, earthworms, and others. As they consume and digest the
dead matter, nutrients are released to be taken up by plants again.
Eventually, through decay and a process called humification, organic matter is transformed
into a substance known as humus. Gardeners often use the term humus to describe any
well-rotted compost or soil amendment, while soil scientists use it only for the final, stable
end product of decomposition and humification. Humus is what gives fertile soil its dark
color.
Humus and decaying organic material retain moisture, increasing the soil's drought
resistance. They bind to clay particles, increasing the average particle size and allowing
more pore space between them. Humus and other organic compounds bond with ions of
nutrient elements to form chelates that are readily absorbed by plants and resist being
leached out of the soil.
| Organic Matter |
| Minerals |
| Soil Structure |
One of the first things you'll notice when working a soil, and one of the most important
factors in how well it will support crops, is its structure, sometimes referred to as texture or
tilth. Even a soil with abundant nutrients can fail to produce a good crop if it has poor
structure. Soil structure can be described in terms like light, loose, heavy, dense, crumbly,
sticky, and so on. The best soils are often described as friable - loose and crumbly.
The structure of a soil is determined mainly by the mixture of sizes and kinds of particles
that make it up.
factors in how well it will support crops, is its structure, sometimes referred to as texture or
tilth. Even a soil with abundant nutrients can fail to produce a good crop if it has poor
structure. Soil structure can be described in terms like light, loose, heavy, dense, crumbly,
sticky, and so on. The best soils are often described as friable - loose and crumbly.
The structure of a soil is determined mainly by the mixture of sizes and kinds of particles
that make it up.
| Improving Soil Texture |
If your garden plot isn't blessed with rich, crumbly loam (as most of ours aren't!), don't
despair. There are several ways to improve it. Gypsum is commonly applied to loosen
heavy clays; it works by causing clay particles to bond together into larger crumbs. Lime
is said to have a similar, but more limited, effect. There are also soil conditioners
consisting of ceramic granules.
There are two ways we recommend for more long-term benefit. The first is adding
organic matter such as compost to the soil. The other is planting cover crops (or green
manures, as they're sometimes called) which not only add organic matter but help to
loosen compacted soil. Although most soils in our area tend toward heavy clay, these
methods will help correct the deficiencies of any soil, at either end of the sand-clay
spectrum and in between.
despair. There are several ways to improve it. Gypsum is commonly applied to loosen
heavy clays; it works by causing clay particles to bond together into larger crumbs. Lime
is said to have a similar, but more limited, effect. There are also soil conditioners
consisting of ceramic granules.
There are two ways we recommend for more long-term benefit. The first is adding
organic matter such as compost to the soil. The other is planting cover crops (or green
manures, as they're sometimes called) which not only add organic matter but help to
loosen compacted soil. Although most soils in our area tend toward heavy clay, these
methods will help correct the deficiencies of any soil, at either end of the sand-clay
spectrum and in between.