Agriculture Reference
In-Depth Information
Soil Structure
While soil texture refers to the percentage of sand, silt and clay in your soil and is ulti-
mately summarized with words such as sandy loam or silty clay, soil structure looks at how
these individual soil granules clump together into various shapes and sizes and what that
means for factors such as water and air movement, compaction, biological activity and root
growth.
This soil aggregation is performed mostly by microbes, plants and small animals. Bacter-
ia excrete gluey carbohydrates that allow them to cling to soil, and help soil particles stick
to each other. Fungi send a latticework of mycelia throughout the soil, binding particles to-
gether. Certain fungi create a sticky protein called glomalin that excels at binding soil to-
gether. All of these microbes need the nutrients to be in balance in order to make a nice soil
structure, along with water and organic matter. Even worms aggregate soil by constantly
eating and excreting soil particles. As do other critters that live in the soil. Organic matter
is broken down into humus, which bonds electrically with clay to form aggregates, called
the clay-humus complex.
In a perfect world, your soil would be approximately 45% mineral soil, 5% organic mat-
ter, 25% air and 25% water. So that's 50% solid and 50% pore spaces. It rarely works out
exactly like this, but that's kind of a loose goal. The big pore spaces hold more air and the
small pore spaces hold more water. This provides just enough water and air for the plant
roots, microbes and insects — not too much and not too little. Some gardeners even strive
for higher organic matter content in relation to soil, but you don't want to add too much,
too quickly.
The reason small pore spaces hold water is because water molecules stick to each other
through cohesion, and to solid surfaces through adhesion. A small space has just enough
solid surfaces in close proximity for a water drop to stick. When water goes into a big
space, it just keeps going through, so air ultimately ends up occupying that space.
If your soil is dominated by sand, the resulting structure will likely allow for excellent
water infiltration and drainage. Infiltration is how water enters the soil from above, while
drainage is how it leaves the soil due to gravity. In a sandy soil, the water will more easily
get into the soil and even get down deep into the soil, but not that much of it will stay
around. This is because the relatively large size of the sand grains creates many big pore
spaces for air, but not as many small spaces for water.
While the water-holding capacity of a sandy soil isn't great, there is a lot of air and not a
lot of compaction in a sandier soil, like at the beach. That has its benefits, which is why
sand is used so much as the base for golf course greens. Golf greens get a lot of foot and
machine traffic, so the sand helps resist compaction. Roots and microbes can breathe and
extend their reach quite far in sandy soil.
