4.3 Soil Composition and Properties

The water-holding capacity of the soil is the amount of water that soil can hold via cohesion against gravity. Particle size and amount of organic matter plays a big role in water-holding capacity. When talking about water retention as it pertains to crops, we want soil that holds water in its pores rather than just letting it soak through to lower layers. 

When talking about particle size and water retention, the smaller the particles, the better the soil will retain water. However, larger particles will retain less water. Larger particles 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 soak up water. The type of soil that has the most water retention is loam which has an equal amount of small, medium, and large particles.

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. 

Porosity describes how porous soil is. The larger the particle size, the more porous the soil will be. Thus, soil that is sandy will have a greater porosity. The pore size increases as 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.

The fertility of the soil is talking about the number of nutrients that soil has to support vegetation. These nutrients include nitrogen, oxygen, carbon, and phosphorus. This is highly impacted by the amount of organic material, but may also be impacted by the number of nutrients a soil can hold.

Some of the chemical properties of soil include pH and cation exchange. Soil pH is how acidic or basic soil is. This can have an impact on the plants that are able to grow and the nutrients found in the soil. Often acidic soil is caused by pollution (i.e. mining) 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 act like magnets and attract cations (atoms with a positive charge). These cations include magnesium, potassium, sodium, and calcium.

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 nutrients, water, and oxygen. Soil with good aeration will be able to take in a lot of nutrients and water which is beneficial to plants growing in that soil. 

Soil compaction is how compacted the soil particles are. This can affect porosity, permeability, and aeration. When soil is heavily compacted, there are few large pores. Thus, there are fewer pockets of water, air, nutrients.

Permeability is also negatively impacted by this because when the soil particles are packed tightly together, nutrients and water are less likely to be able to permeate through the soil horizons.

Finally, aeration is negatively impacted, because when the soil is packed together, nutrients, air, and water have few places to go in the soil.

The biological properties of soil are determined by the organisms and plants that live in the soil. This can differ from soil to soil. For example, the soil in a desert is going to have a different set of organisms than forest soil. On a microscopic level, bacteria and fungi can live in the soil. On a macroscopic level, worms, ants, and rodents may live in the soil.

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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