Soil Profiling: Horizons and Properties

• Soil profiling in soil science involves examining soil layers (horizons) to understand their composition, structure, and properties.
• Soil profiling provides insights into soil formation, fertility, and suitability for agricultural and environmental use.

Objectives

• Examine and identify different soil horizons and their characteristics (texture, color, structure).
• Analyze soil properties at varying depths to assess fertility and suitability for plant growth.
• Understand soil formation processes and how they influence soil profile development.
• Develop soil sampling techniques and accurately record observations.
• Interpret soil profile data for environmental and agricultural applications.

Materials

• Auger or soil corer
• Shovel/Spade
• Tape measure
• Bamboo sticks
• Labeling materials
• Soil pH meter
• Soil moisture meter
• Munsell soil color chart
• Tissue paper
• GeoCam application

Site Selection & Preparation

• Use GeoCam to capture images of the sampling area.
• Look for terrain variations or crop growth differences indicating soil composition differences and signs of nutrient deficiencies.
• Record GPS coordinates and relevant features (waterlogged areas, slope, etc).
• Take photos with GeoCam to document the site, ensuring GPS functionality is active.

Conducting the Soil Profile

• Dig a 1mx1mx1m soil pit or use a soil corer.
• Record changes in soil color, texture, and composition during digging.

Soil Horizon Identification

• O Horizon (Organic Layer): The topmost, dark brown or black layer of decomposed plant material.
• A Horizon (Topsoil): Below the O horizon, typically dark brown or brown, containing mineral particles and organic matter.
• B Horizon (Subsoil): Lighter in color (yellowish, reddish, or brown), denser layer often containing clay and minerals leached from upper horizons.
• C Horizon (Parent Material): Weathered rock or unconsolidated material that may contain larger particles like gravel or stones.
• R Horizon (Bedrock): Solid bedrock, often inaccessible in the field.

Observations

• Record the depth of each horizon, e.g. O horizon is 0-10 cm, A horizon is 10-30 cm.
• Describe the soil color using terms like dark brown, reddish, light gray.
• Observe the texture and structure of each horizon and note observations about soil composition.

Soil pH and Moisture

• Soil pH testing involves taking soil samples from O, A, and B horizons of soil, mixing with water (1:1 ratio), and measuring pH with a meter or kit.
• Soil moisture can be measured using a soil moisture meter.
• The manual squeeze test involves squeezing soil in hand to determine if it feels moist, dry, or wet.

Interpretation of Data

• The O horizon (32 cm) deep of the experiment contained decomposed organic matter and had a sandy texture.
• The grittiness of sandy soils allows water to pass through quickly, washing away essential nutrients.
• The A horizon (56 cm) deep of the experiment was loamy and more fertile, as it retained moisture and nutrients well.
• The B horizon (12 cm) appeared lighter, indicating leaching and reduced fertility.
• Soil pH ranged from slightly acidic to neutral, favorable for crops, but moisture levels were low.
• Earthworms and ants improve aeration and nutrient cycling, while tree roots compete for resources.
• The A horizon is most suitable for plant growth with loamy texture and high humus, while O and B horizons have limitations.

Study Questions and Answers

• Soil texture varied across the horizons. The O horizon was sandy and crumbly, A horizon loam (balanced), and B horizon loam but more compact.
• Soil color varied depending on the organic matter content and mineral content. O and A horizons were darker.
• Soil erosion was observed on the surface, with bald vegetation. Compaction was noted due to dry moisture.
• Moisture content was generally dry across horizons. The pH was 7.0, 6.8, and 6.4.
• The upper soil layers have access to nutrients, but the ability to retain them depends on the O horizon's composition.