Agriculture Reference
In-Depth Information
Soil Formation
Soil gets started when rock is broken down by mechanical, chemical and biological
means. This process can take thousands of years.
The mechanical aspect of breakdown is brought about by water, wind and temperature
fluctuations. The chemical aspect consists of various reactions often caused by interactions
between oxygen, water, carbon dioxide and rock. The biological breakdown process de-
pends on the soil food web — microbes and plants working directly on the rock creating
their own organic acids. This process speeds up drastically as more organic matter is pro-
duced. They also make some of the chemicals mentioned above.
There's a very good chance that your soil did not come directly from the underlying
rock. Often, the parent materials or even the soil itself has been blown in by wind or
brought in by glaciers, volcanoes or historic waterways, but the soil formed somewhere as
virgin soil. Virgin soils don't have organic matter. Lichens — organisms made up of a sym-
biotic partnership between algae and fungi — are often first to arrive on the scene along
with pioneering nitrogen-fixing plants like alder, broom and black locust.
Nitrogen is needed for plants to grow. Alder, broom and black locust don't actually fix
nitrogen, but they partner with bacteria that do. These bacteria take nitrogen out of the air
and convert it into a biologically usable form. The plants can then grow bigger and this is
the start of making organic matter. Other plants will eventually come in when the soil is
ready and the nitrogen-fixers will gradually be phased out. This all takes time.
Our soils are often in poor shape, and while we should definitely bring in some compost,
I also like to take a cue from nature and plant some nitrogen-fixing plants. Actually, in a
new ornamental garden on poor soil, I often use a lot of nitrogen-fixing plants — at least
25% of my total. I use clover and vetch as a groundcover, beans and peas in my vegetable
garden, goumi, birch and many others in perennial plantings.
Another implication is that we can learn a lot about our soil by looking at the plants that
are naturally growing in it. If we're getting a lot of clover, we may have a lack of organic
matter and nitrogen in the soil. If we're getting a lot of dandelions, we have a calcium defi-
ciency or imbalance. We can actually learn to make some fertility decisions with a bit of
knowledge about weeds.
During the soil formation process, the quartz in the parent rock becomes sand and silt.
Most sand and silt are composed of this quartz, which is silicon dioxide. The clay, on the
other hand, is formed when oxides of silicon and aluminum come together into a molecule
and then many of these molecules join together. Sometimes it's silicon and iron. The other
minerals in the parent rock just float around as minerals. Some are leached and some attach
to the clay.
