Soil Science Primer – Part 2

Soil Anatomy

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The Mineral Part

Most of the time soil forms on rocks, so it contains a lot of rock residuum.  Because rocks vary, the ways parent rocks cotribute to the soil are also different.  Granite breaks up into smaller chunks that have the same mineral composition as the parent rock and form a coarse gravelly admixture. Basalt breaks up into finer chunks with more surface area. Shale is what we have a lot of here in Pennsylvania. It is formed out of compressed sediments with lots of clay, so it breaks down into shard like layers and clay. Sandstone- sand.  Some soils are formed in sand to begin with.  In soils lingo, “clay” refers to particles that are super fine: <2um. Sand is >50um in diameter. A 1000um is a mm. That leaves a size between sand and clay that is very important called silt. It helps soil be very workable, but it is fine enough to hold nutrients and moisture. My MS thesis was about that particle size group.  Notoriously fertile Loam soils are predominantly silt. Some minerals don’t break up, though. They dissolve. Limestone is an example. Its lasting influence on the soil exclusively through water chemistry.

The Organic Part

As the minerals are exposed to the elements, living creatures interact with them. Some can eat the minerals. Lichens do this.  Mosses can get minerals from the rock once the minerals begin to dissolve. Microscopic fungi, bacteria, and protozoans are quick to inhabit young soils.  As soon as soils can support larger organisms, rooting plants, worms, beetles, and even burrowing vertebrates move on in. These larger animals mix organic matter more deeply into the soil. This process has a great word associated with it: “Bioturbation.”  Pigs are master bioturbators as they root around. Some earthworms are even better, taking fresh organic munchies from the surface several meters into the earth.

As decaying organic matter becomes incorporated more deeply in the soil, it releases acid causing more minerals from the rock to dissolve and become available to plants.

Air and Water

There are more ephemeral components to the soil: air and water. Air and water occupy the pores of a soil. The soil air is rich in CO2, a byproduct of the soil fauna. Some of the CO2 dissolves into the water, making it mildly acidic. This helps dissolve the rock particles and to make some of those nutrients available for plant.  Plant roots need oxygen that diffuses into the soil. Oxygen dissolves into the water where it can be absorbed by roots. Pores of air in the soil help get oxygen deeper in the soil,  thereby sustaining deeper rooting.

Continue to Part 3.

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