Soil Water and Soil Conservation

1. Soil Water: Types, Importance, and Properties

Soil water is the water held in the soil pores. It is crucial for plant growth, nutrient transport, and soil microbial activity. Soil water can be classified into three primary types based on their availability to plants and physical properties:

• Capillary Water: This is the water held in the micro-pores of the soil by surface tension (cohesive and adhesive forces). It moves in any direction and is the only form of water available to plants.
• Gravitational Water: This is the free water that drains rapidly through the macro-pores under the influence of gravity after heavy rain or irrigation. It is generally unavailable to plants because it drains below the root zone. Excessive gravitational water causes waterlogging.
• Hygroscopic Water: This is the water held very tightly as a thin film around soil particles by adhesive forces. It is held at extremely high suction pressure (above 31 atmospheres) and cannot be absorbed by plant roots, making it unavailable.

Soil Water constants and Terminology

• Water-Holding Capacity (WHC): The ability of a soil to retain water against gravity. Clay soils have the highest water-holding capacity due to their large surface area and high proportion of micro-pores, while sandy soils have the lowest.
• Field Capacity: The amount of water remaining in the soil after excess gravitational water has drained away (usually 2 to 3 days after heavy rain or irrigation).
• Permanent Wilting Point (PWP): The soil moisture stage where the water content is so low that plants can no longer exert enough suction to extract it. Plants wilt and cannot recover even when placed in a humid environment.
• Available Water: The water held between Field Capacity and the Permanent Wilting Point. Formula: Available Water = Field Capacity - Permanent Wilting Point.

2. Soil Conservation: Causes, Effects, and Control

Soil conservation involves practices aimed at maintaining soil fertility and protecting soil from degradation. Major soil degradation processes include erosion, leaching, continuous cropping, and organic matter depletion.

A. Soil Erosion

Soil erosion is the washing or blowing away of the topsoil by water or wind.

Causes of Erosion:

• Heavy rainfall and floods
• Deforestation and bush burning
• Overgrazing and overcultivation
• Cultivating along slopes (instead of across/contour)

Types of Water Erosion:

1. Splash Erosion: Raindrops striking bare soil and detaching soil particles.
2. Sheet Erosion: Uniform removal of a thin layer of topsoil over a wide area. Highly insidious because it occurs gradually.
3. Rill Erosion: Small, finger-like channels formed by running water. Can easily be smoothed out by normal tillage.
4. Gully Erosion: Deep, wide channels formed when rills join and deepen. Cannot be removed by normal tillage operations.

Control of Erosion:

• Afforestation (planting trees) and Reforestation.
• Contour ploughing and terracing on steep slopes.
• Cover cropping (planting melon, sweet potato, etc., to cover bare soil).
• Strip cropping and windbreaks (planting rows of tall trees to reduce wind speed).

B. Leaching

Leaching is the downward movement of dissolved plant nutrients (like nitrates and calcium) through the soil profile beyond the reach of plant roots.

Causes of Leaching:

• Excessive rainfall or over-irrigation.
• Sandy soil structure (which has high permeability).
• Lack of vegetation cover.

Control of Leaching:

• Cover cropping and mulching to reduce infiltration speed.
• Application of organic manure to improve soil water-holding capacity.
• Split application of chemical fertilizers to prevent nutrient wastage.

C. Continuous Cropping, Burning, and Oxidation

• Continuous Cropping: Repeated cultivation of crops on the same piece of land season after season. It depletes soil nutrients, destroys soil structure, and increases pest and disease build-up. It is controlled through crop rotation and fallowing.
• Bush Burning: Destroys organic matter, kills beneficial soil microbes, exposes the soil to erosion, and volatilizes vital nutrients like nitrogen and sulfur.
• Oxidation of Organic Matter: Occurs rapidly in warm, moist, and frequently tilled soils. Tillage aerates the soil, speeding up microbial decomposition of organic matter, which reduces soil structural stability.

3. Irrigation and Drainage Methods

A. Irrigation

Irrigation is the artificial application of water to the soil for crop production, especially in areas with insufficient rainfall.

Classification of Irrigation Systems:

• Surface Irrigation: Water is applied and distributed over the soil surface by gravity. Examples include:
  • Flooding: Water is guided over a flat field. Low efficiency, high water loss.
  • Basin Irrigation: Land is divided into embanked basins where water is retained. Excellent for rice.
  • Furrow Irrigation: Water flows down small parallel channels (furrows) between crop ridges.
• Sprinkler (Overhead) Irrigation: Water is sprayed into the air through nozzles, mimicking rainfall. Ideal for uneven terrains but expensive to install and maintain.
• Drip (Trickle) Irrigation: Water is applied slowly and precisely directly to the root zone of plants through emitters. It is the most water-efficient method but has high initial costs and risks emitter clogging.

Importance of Irrigation:

• Ensures all-year-round crop production.
• Improves crop yield and quality.
• Reduces crop failure risk in arid regions.

Challenges of Irrigation:

• High cost of design and maintenance.
• Can lead to soil salinization (accumulation of salts in the topsoil) if water quality is poor.
• Encourages waterlogging if drainage is inadequate.

B. Drainage

Drainage is the artificial removal of excess water from the soil surface and subsurface to prevent waterlogging and improve soil aeration.

Classification of Drainage Systems:

• Surface Drainage: Uses open ditches, trenches, or land shaping to guide excess surface water away. Easy to construct but reduces cultivable land area and interferes with farm machinery.
• Subsurface (Underground) Drainage: Uses perforated pipes, tiles, or mole drains buried beneath the soil surface to collect and carry away excess water. Does not obstruct farm machinery but is highly expensive to install and maintain.

Importance of Drainage:

• Prevents waterlogging and improves soil aeration (root respiration).
• Promotes warm soil temperatures.
• Prevents the build-up of toxic substances and soil salinity.
• Improves soil structure and trafficability for farm machinery.