Soil Science: Horizons, Properties, Problems, and Management

Humic Horizon (A-horizon / Topsoil):
- Derived from humus, which is a stable form of carbon.
- Formed over long periods from decaying plant parts.
- Primarily found in forest soils.
- Topsoil: Mixture of soil-forming minerals (from parent materials) and organic matter.
- Where agriculture is possible.
- A good topsoil layer is generally considered to be at least 10 \text{ cm} deep.
E-Horizon (Eluviated Zone):
- Located below the A-horizon.
- Characterized by leaching, where minerals and nutrients are washed down from this layer.
B-Horizon and C-Horizon: These are the main layers that significantly contribute to soil formation, leading to the development of the top layer (soil) over thousands of years.
Bedrock: The underlying solid rock material.

Soil orders represent the diverse types of soils, often categorized by different colors.
Soil order classification mainly depends on:
- The types of soil horizons present.
- The depth of these horizons.
- The materials within the horizons.
Examples of soil orders include types found in Manitoba, such as Brunisols.
Understanding soil type allows for determining what amendments could be beneficial.

Soil Texture:
- Defined as the percentage composition of sand, silt, and clay particles.
- These particles originate from parent materials (rocks) through the process of weathering.
- They are the primary components or building blocks of soil; organic matter and other elements are secondary additions.
- Particle sizes:
  - Sand: Porous particles.
  - Silt: Finer than sand, coarser than clay.
  - Clay: The finest particles, very powdery.
- The relative proportions of sand, silt, and clay determine the soil's texture (e.g., in a soil texture pyramid/triangle).
- For example, soils that are too clayey are generally less desirable for certain agricultural purposes.
- Soil texture influences crop choices and required management practices.
Soil Structure:
- Refers to how soil particles are bound together.
- It describes the arrangement and aggregation of sand, silt, and clay particles.
- This property is related to soil management.
- Light soil is a term sometimes used for coarse-particle, sandy soils.

Acidic Soil:
- Occurs when soil pH is too low (acidic).
- Primarily caused by excessive washing away of positively charged ions like Ca^{2+} (calcium) and Mg^{2+} (magnesium) by running water, which are essential nutrients.
- Consequences:
  - Disrupts the availability of micronutrients and other essential nutrients.
  - Leads to aluminum toxicity due to increased soil acidity.
- Highly problematic in regions such as Southeast Asia, South America, and Sub-Saharan Africa.
Salinity (High Salt Concentration) and Alkalinity (High pH/Sodium):
- Caused by irrigation water with a high mineral content, leading to the accumulation of salts.
- Consequences:
  - Causes toxicity to plants.
  - Leads to higher pH (alkalinity) and high sodium levels, which are also problematic for plant growth.

Preventative Practices:
- Using careful management practices.
- Making the right choice of crops.
- Implementing crop rotations.
- Providing protection for the soil.
Terracing:
- An agricultural practice where sloped land is converted into stepped flat areas (terraces).
- Example: Loess Plateau in China, where terraces help produce food and protect the soil from erosion.
Cover Cropping:
- Involves planting crops like grasses or leguminous plants (e.g., red clover, which can fix nitrogen) immediately after harvesting the main crop.
- Purpose: To protect the soil and prevent nutrient loss (e.g., nutrients that arrive with water can also be washed away by water if the soil is bare).

Pests:
- Organisms that live as parasites, depending on cultivated crops for part or all of their life cycle.
Disease Management:
- Protection: Taking measures to prevent diseases from occurring.
- Treatment: Once a disease is observed, treatments such as fungicides are used (bactericides are not commonly used for crop bacterial treatments).