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4.3 Soil Composition and Properties

3 min readdecember 30, 2022


AP Environmental Science ♻️

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Water-Holding Capacity and Retention 

The water-holding capacity of the soil is the amount of water that soil can absorb given the effects of gravity upon the soil. Particle size and amount of organic matter present plays a big role in water-holding capacity. When talking about water retention in regards to farming, soil is desirable when it's able to keep water in its pores rather than allowing it to penetrate further and further into the crust.
When talking about particle size and water retention, smaller particles correlate with higher levels of water retention. Larger particles will allow for the water to more easily flow to lower layers. In addition to this, organic matter in soil tends to increase water retention, because organic matter tends to absorb water. The type of soil that has the most water retention is loam which has an equal amount of small, medium, and large particles.

Particle Sizes and Properties

Various particle sizes can impact the characteristics of soil in a big way. Some of the characteristics that are impacted by soil size include porosity, permeability, and fertility. 
https://i.postimg.cc/FsrQNnGv/NZUUf-Iw-Imgur.jpg
Image Courtesy of Jesse Chippindale
Porosity describes how porous soil is. Larger particles have larger pore sizes, making soil more porous as the particle size increases.
Permeability is the ability of nutrients and water to move down the soil horizons. Larger particles increase the permeability of soil because there is more space between the particles. Soil is more permeable the more space it has for water to move through.
The fertility of the soil considers its nutrient levels and to what extent it is able to support vegetation. These nutrients can include elements like phosphorus or nitrogen and can be impacted by how much biomatter is in the soil. Some soils also have a capacity at which their nutrient limit is reached.
https://i.ibb.co/vJYYkt1/Screen-Shot-2020-03-09-at-4-14-00-PM.png

Chemical, Physical, and Biological Properties of Soil

CHEMICAL PROPERTIES

Some of the chemical properties of soil include pH and cation exchange. Soil pH is how acidic or basic soil is. This can shift based on current environment or pollutants and it can have an impact on the plants that are able to grow. Often, acidic soil is caused by pollution and acid rain. Cation exchange is another chemical property of soil that can be complicated. In its simplest form, cation exchange occurs when soil particles attract cations (atoms with a positive charge). Cation exchange helps to regulate pH of soil; when a soil's cation exchange capacity is high, it is able to maintain stability in its nutrient levels and pH.

PHYSICAL PROPERTIES

Some of the physical properties of soil are aeration, soil compaction, permeability, and particle size. A few of these properties were discussed earlier.
Aeration is the ability of soil to take in essentials like nutrients, water, and oxygen. Soil with good aeration is able to take in needed amounts of sunlight and water which is key to fostering plant growth.
Soil compaction is how compacted the soil particles are. This can affect porosity, permeability, and aeration given space between individual particles. When soil is heavily compacted, there are few large pores and space is limited. Thus, there are fewer pockets of water, air, or other essential nutrients.
Permeability is also negatively impacted by this because tightly packed soil won't allow nutrients to get through.

BIOLOGICAL PROPERTIES

The biological properties of soil are determined by the organisms and plants that live in it. This means soil composition and consistency will vary depending on biome or climate. Species like fungi and bacteria can help develop a soil's composition in ways unlike other species'.

Soil Texture Triangle

https://upload.wikimedia.org/wikipedia/commons/thumb/8/80/SoilTexture_USDA.png/800px-SoilTexture_USDA.png

Image Courtesy of Wikimedia

The soil texture triangle allows us to identify soil using the percentage of clay, silt, and sand. The angle of the numbers shows you the way the lines go for each type of particle. For example, clay lines go straight across, silt lines go down diagonally, and sand lines go up diagonally. To use a soil texture triangle, you follow the lines of each particle based on percent. The point where the lines intersect is the type of soil it is. For example, if we had a soil sample with 20% clay, 50% sand, and 30% silt, we would have loam.
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