Topic 4.3 Notes – Soil Composition and Properties

What Soil Is and Why Composition Controls Its Properties

Soil is not just dirt. It’s a mixture of:

• mineral particles from weathered rock
• organic matter such as dead plant material and humus (stable, decomposed organic material)
• water
• air
• living organisms like bacteria, fungi, insects, and earthworms

A productive surface soil is often shown as about:

• 45% mineral matter
• 25% water
• 25% air
• 5% organic matter

Idealized soil composition

This breakdown also helps you see that water and air share the same pore space, so they shift constantly. After rain, pores fill with more water. As soil dries, air moves back in. Compaction squeezes out pore space, which hurts both drainage and aeration.

A quick horizon reminder helps here. Different soil horizons can vary in particle size, organic matter, mineral content, porosity, permeability, and fertility. That’s why the topsoil and subsoil can behave very differently.

The big idea for this whole topic is simple. Soil composition controls water movement, nutrient availability, and productivity.

Soil Texture and the Main Soil Types

Texture means the relative percentages of sand, silt, and clay in the mineral part of soil. It does not mean structure. Structure is how particles clump into aggregates.

Sand, silt, and clay together

• Sand is $0.05$ to $2.0$ mm. It has the largest particles and feels gritty.
• Silt is $0.002$ to $0.05$ mm. It has medium particles and feels smooth, almost flour-like.
• Clay is less than $0.002$ mm. It has the smallest particles and feels sticky when wet.

Particle size matters because it affects surface area, pore size, and how strongly soil holds water and nutrients.

Property patterns by texture

• Sandy soils have large pores, high permeability, low water-holding capacity, low nutrient retention, and high leaching risk.
• Silty soils are in the middle. They feel smooth and usually have moderate water retention.
• Clay-rich soils have many tiny pores, slow water movement, high total water-holding capacity, and often higher nutrient retention. Some of that water is held too tightly for plants to use.
• Loam has a balanced mix of sand, silt, and clay. It usually gives the best mix of drainage, water retention, and nutrient retention.

The soil texture triangle

The soil texture triangle classifies soil using percent sand, silt, and clay.

$$\%\text{sand} + \%\text{silt} + \%\text{clay} = 100\%$$

Only two percentages are needed, because the third must make the total 100%. The triangle helps identify classes like sand, sandy loam, loam, clay loam, clay, and silt loam.

Soil texture triangle

Loam is the class you should care about most for productivity. In the triangle, it sits near the center because it represents a relatively balanced mix of sand, silt, and clay.

How Soil Properties Affect Water and Fertility

Porosity is how much pore space a soil has. Permeability is how easily water moves through connected pores. Those are related, but not the same. Clay can have high porosity and still low permeability because its pores are tiny.

Other key ideas:

• Infiltration means water entering the soil surface. It depends on texture, vegetation, slope, crusting, and compaction.
• Water-holding capacity means total water the soil can retain.
• Sandy soil drains fast and holds less water.
• Clay-rich soil holds more water, but some is unavailable to plants.
• Loam usually gives the best balance.

Fertility means the soil can provide nutrients and good growing conditions. It depends on:

• nutrient levels
• organic matter
• pH
• water availability
• aeration
• biological activity
• texture and structure

CEC, or cation exchange capacity, is the soil’s ability to hold positively charged nutrients such as K$^+$, Ca$^{2+}$, Mg$^{2+}$, and NH$_4^+$. Clay and humus usually raise CEC. Sandy soils usually have lower CEC and lose nutrients more easily by leaching.

pH also matters because it changes nutrient availability and microbial activity. Very acidic or very basic soils are less suitable for many crops.

Compaction is a classic APES cause-and-effect point. It lowers porosity, permeability, infiltration, and root growth, and it increases runoff.

Soil Testing and What the Results Are Used For

Soil quality is measured through physical, chemical, and biological tests together.

Physical tests

• Texture can be tested by feel or with a settling-jar test.
• Permeability can be compared by drainage through equal soil columns.
• Water-holding capacity can be tested by saturating, draining, and reweighing soils.
• Bulk density helps measure compaction.

Chemical tests

• pH is often tested with a soil-water slurry.
• Nutrient tests often look at nitrogen, phosphorus, and potassium.
• Other tests can measure salinity, organic matter, CEC, and contaminants.

Biological tests

• Organism abundance
• Microbial respiration through CO$_2$ production
• Decomposition rate
• Root condition

How results guide decisions

1. Irrigation depends on texture. Sandy soils often need smaller, more frequent watering. Clay soils need slower, less frequent watering.
2. Fertilizer depends on deficiencies, but also on texture and CEC. Sandy soils have greater leaching risk, so timing matters.
3. Amendments like compost improve water retention, aggregation, and biological activity.

The APES takeaway here is that no single test tells you overall soil quality. Texture, pH, permeability, CEC, organic matter, and biological activity work together.