Soil Properties.

Soil texture describes the coarseness or fineness of the soil particles and is determined by the proportions of sand, silt, and clay in the soil. These components affect the soil's ability to hold water, nutrients, and air, which in turn impacts plant growth. The soil texture can be determined through feel or more precise laboratory methods like sieving and sedimentation.

Soil particle size

1. Clay: Less than 0.002 mm
2. Silt: 0.002 - 0.05 mm
3. Sand: 0.05 - 2.00 mm

Soil texture analysis:

1. Feel method: You can feel soil to determine if it is sandy, silty, or clayey:
   • Sand feels gritty and lacks cohesion.
   • Silt feels smooth and soap-like.
   • Clay feels sticky and plastic when wet.
2. Sieving method: This method involves passing the soil through sieves with varying mesh sizes. Coarse particles remain on top, while finer particles are filtered based on their size.
3. Sedimentation method: Soil is placed in water, shaken, and allowed to settle. Larger particles (sand) settle quickly, while smaller particles (silt and clay) take longer to settle.

Soil texture classification (USDA vs WRB):

1. Sandy soils: These soils have coarse particles that allow rapid drainage but retain little moisture. They are well aerated, suitable for crops like groundnuts and peas, but prone to erosion and leaching.
2. Silt soils: Fine-grained soils, smooth and cohesive, but prone to sheet erosion during heavy rains. Silt particles are smaller than sand but larger than clay.
3. Clay soils: Fine particles that are sticky and cohesive when wet. They retain a lot of moisture, are rich in plant nutrients, and are suitable for paddy fields, but difficult to till when wet.
4. Loam soils: A mix of sand, silt, and clay, loam soils are the most fertile, well-drained, and ideal for farming. They are loose and open, allowing for easy water movement and root growth.

1. Water retention and drainage: Sandy soils drain quickly, whereas clay soils retain more water.
2. Nutrient retention: Clay soils hold nutrients better than sandy soils.
3. Plant growth: Soil texture affects root penetration and seedling establishment. Loam soils, with balanced texture, are ideal for most plants.
4. Soil erosion: Sandy soils are more prone to erosion compared to clay soils.

Soil structure refers to the arrangement of soil particles into aggregates, also called peds. These aggregates can be formed by organic matter, microbial activity, and physical processes.

1. Single grained structure: Particles do not stick together, commonly found in sandy soils.
2. Massive structure: Particles are cemented together, often found in clay and other compacted soils.

Types of soil structure:

1. Crumb: Round aggregates found in loam soils. Well-aerated, good for plant growth.
2. Granular: Small particles in non-porous, compacted soils.
3. Platy: Plate-like structure found in compacted clay and silt soils. It restricts water flow and root penetration.
4. Blocky: Irregularly shaped blocks, typically found in clay-loam soils, well-drained and productive.
5. Prismatic: Vertical column-like structures found in subsoil, good for water movement and root development.
6. Columnar: Vertical structures, dense and difficult for root penetration, found in saline or alkaline soils.

Soil color can indicate fertility, mineral content, and water availability. For instance:

1. Red: Indicates iron oxide and good drainage.
2. Dark: Associated with high organic matter content.
3. Yellow: Suggests the presence of hydrated iron oxides.
4. Black or Mottling: Indicates high levels of manganese oxide or poor drainage conditions.

Soil color can also be used to assess the suitability of soil for specific crops and agricultural activities.

Soil color parameters:

1. Hue: The dominant color (e.g., red, yellow).
2. Value: The lightness or darkness of the color.
3. Chroma: The purity or strength of the color.

Indication of organic matter content

1. Dark-coloured soils are often fertile because they contain a high proportion of organic matter. Organic matter is essential for soil productivity and plant growth.
2. Light-coloured soils indicate low fertility, usually resulting from minimal organic content or extensive leaching.

Indicator of soil development

1. Young soils: Light-coloured soils may reflect the nature of their parent rock material.
2. Mature soils: Soil colour can indicate the climatic conditions under which the soil formed. For example:
   • Warm climates often result in red soils due to oxidation of iron minerals.
   • Light colours may result from excessive leaching, removing iron and other minerals.

Mottling (spotty colours)

1. Indicates intermittent wetting and drying cycles in soils.
2. Mottling is caused by oxidation and reduction reactions, suggesting poor drainage or temporary waterlogging.

Definition

• Soil temperature is the measure of warmth or coldness in soil, influenced by solar radiation and other environmental factors.

Factors affecting soil temperature

1. Climate: Tropics have warm soils due to high solar radiation, while polar regions have cold soils.
2. Slope of land (topography): North-facing slopes are cooler compared to south-facing slopes due to differences in sunlight exposure.
3. Vegetation cover: Vegetation insulates soil, making it cooler in summer and warmer in winter compared to bare soil.

Impact of soil texture and moisture

1. Sandy soils warm up faster because they hold less water.
2. Clay soils warm slower due to higher water content and heat retention.
3. Moist soils heat up slower than dry soils because water has a higher specific heat capacity than soil.

Importance of soil temperature

1. Influences seed germination, microbial activity, chemical weathering, and decomposition of organic matter.
2. The ideal temperature range for plant growth is 18°C to 24°C (65°F-75°F).

Definition

• Soil porosity refers to the percentage of pore spaces in soil, affecting water and air movement.
• Formula:

$\text{Porosity (P)} = \frac{\text{Volume of voids (Vv)}}{\text{Total volume of soil (Vt)}} \times 100$

Factors influencing porosity

• Particle shape and size: Loosely packed particles (e.g., sandy soils) have higher porosity than densely packed ones (e.g., clay soils).
• Soil structure: Well-aggregated soils (granular or crumb) have more interconnected pores.

Impact of porosity

1. High porosity allows better water infiltration and drainage but decreases water retention.
2. Low porosity retains water, which can lead to waterlogging.

Types

1. Particle density: Mass of soil solids per unit volume (typically 2.65-2.75 g/cm³ for mineral soils).
   • Lower in organic soils due to lighter organic matter.
2. Bulk density: Mass of dry soil (including pore spaces) per unit volume (ranges from 1.0-1.8 g/cm³ for mineral soils).

Importance

• Bulk density indicates soil compaction and porosity. Lower bulk density means better aeration and root penetration.

Soil colloids

• Tiny charged particles in soil that retain and exchange ions.

Types:

1. Organic colloids: Humus particles.
2. Inorganic colloids: Clay minerals and oxides of iron and aluminum.

Their small size and large surface area make them essential for nutrient retention and soil fertility.

Ion exchange

1. Cation exchange: Exchange of positively charged ions (e.g., Mg²⁺, K⁺, Ca²⁺) between soil and solution.
2. Anion exchange: Exchange of negatively charged ions (e.g., Cl⁻, NO₃⁻) between soil and solution.

Cation exchange capacity (CEC)

1. A measure of the soil's ability to retain and supply cations to plants.
2. High CEC soils (e.g., clays, organic matter) are more fertile than low CEC soils (e.g., sands).

Anion exchange capacity (AEC)

• Similar to CEC but involves negatively charged ions. Affects soil pH and nutrient availability.

1. Soil pH is measured using a soil-water suspension with a pH meter to determine how acidic or alkaline the soil is.
2. Soil pH directly influences the cation exchange capacity (CEC), which refers to the soil's ability to hold and exchange positively charged ions (cations). The higher the CEC, the better the soil can retain nutrients that plants need.
3. Soil base saturation (the percentage of exchangeable bases like calcium, magnesium, potassium, and sodium in relation to the total cation exchange capacity) also plays a role:
   • Alkaline soil: Base saturation above 50%.
   • Acidic soil: Base saturation below 50%.

1. Soil pH is crucial for determining which crops can thrive in a particular soil, as different plants have varying pH tolerance ranges.
2. Ideal pH for most plants: Generally, plants grow best in soil with a pH ranging from 6 to 7.5, though they can tolerate soils with a pH from 5.5 to 8.5.

Several methods can be used to adjust soil pH for specific crops:

1. Organic matter (compost): Adding decomposed organic matter gradually increases soil acidity, making it more suitable for certain crops.
2. Aluminum sulphate and sulphur: These are used to lower soil pH quickly, making the soil more acidic.
3. Lime: This is used to raise soil pH, making the soil less acidic and more alkaline.

Nutrient availability:

1. Most nutrients (nitrogen, calcium, phosphorus, potassium, magnesium) are most available to plants when the soil pH is between 6.5 and 7.5 (slightly acidic to slightly alkaline).
2. Acidic conditions increase the availability of nutrients like iron, copper, and zinc but reduce the availability of molybdenum (which is most available at a pH of 0-6).

Microbial activity:

• Soil pH affects the activity of soil microbes involved in organic matter decomposition. Extreme acidic or alkaline pH slows down microbial processes, which in turn impacts nutrient cycling and soil health.

Disease prevention:

• Some plants, like cotton, benefit from more acidic conditions because they can help prevent diseases such as cotton mosaic.