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Soil Layers and Soil Health
Soil horizons, composition, and signs of healthy soil
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Soil is a living system made of minerals, organic matter, water, air, and organisms. Its layers, called horizons, record how climate, organisms, topography, parent material, and time shape the ground beneath us. Healthy soil matters because it supports plant growth, stores water, filters pollutants, cycles nutrients, and helps regulate carbon in the environment. Understanding soil profiles helps farmers, builders, and conservationists make better land use decisions.
A typical soil profile begins with organic-rich material near the surface and changes downward into weathered rock and bedrock. Water, roots, worms, fungi, bacteria, and chemical reactions move materials through the horizons, creating differences in color, texture, nutrients, and structure. Soil health is often judged by properties such as organic matter, pH, porosity, infiltration rate, nutrient content, and biodiversity. Practices like adding compost, reducing erosion, rotating crops, and avoiding compaction help keep soil productive and resilient.
Key Facts
- Common soil horizons are O, A, E, B, C, and R from surface to bedrock.
- Soil texture depends on particle size: sand is largest, silt is medium, and clay is smallest.
- Porosity = pore volume / total soil volume.
- Bulk density = dry soil mass / soil volume.
- Infiltration rate = water depth absorbed / time.
- Soil pH affects nutrient availability, with many crops growing best near pH 6 to 7.
Vocabulary
- Soil horizon
- A soil horizon is a distinct layer in a soil profile with its own color, texture, composition, and biological activity.
- Topsoil
- Topsoil is the upper mineral-rich layer, often called the A horizon, where many roots, organisms, and nutrients are found.
- Humus
- Humus is dark, stable organic material formed from decomposed plants and animals that improves soil fertility and water holding capacity.
- Infiltration
- Infiltration is the process by which water enters and moves downward through soil pores.
- Soil compaction
- Soil compaction is the squeezing together of soil particles, which reduces pore space and makes it harder for air, water, and roots to move.
Common Mistakes to Avoid
- Calling all soil dirt is misleading because soil is an organized living system with horizons, organisms, water, air, and chemical processes.
- Assuming darker soil is always healthier is wrong because color can suggest organic matter, but pH, texture, drainage, nutrients, and contamination also matter.
- Ignoring soil texture leads to poor predictions because sandy soil drains quickly while clay-rich soil holds more water and can drain slowly.
- Measuring soil health from only one sample can be inaccurate because soil properties vary across a field, slope, garden bed, or depth.
Practice Questions
- 1 A soil core is 10 cm long and has a cross-sectional area of 20 cm2. Its dry mass is 260 g. Calculate the bulk density in g/cm3.
- 2 During an infiltration test, 3.0 cm of water enters the soil in 24 minutes. Calculate the infiltration rate in cm/min and cm/hour.
- 3 A garden has compacted clay soil with standing water after rain and shallow plant roots. Explain two soil properties that are likely limiting plant growth and describe one practice that could improve the soil.