Soil Texture and Classification

• Soil is a dynamic natural resource vital for plant life and ecosystem functions.
• Soil properties are classified into physical, biological, and chemical characteristics.

Soil Texture

• Soil texture is the most fundamental soil property, greatly influencing other soil traits.
• It is defined as the relative proportion of sand, silt, and clay in a soil mass.
• Soil texture affects water-holding capacity and aeration.
• Sand, silt, and clay are the three principal size groups of inorganic soil particles, also called separates.

Soil Textural Classes

• Twelve textural classes are recognized based on the proportion of soil separates in a soil mass.
• These classes are divided into:
  • Fine-textured soils: clay, silty clay, clay loam, silty clay loam, and sandy clay loam
  • Medium-textured soils: silt, silt loam, and loam
  • Coarse-textured soils: sandy loam, loamy sand, and sand

Particle Size Analysis

• Determining the percentage of sand, silt, and clay in a soil sample is necessary for textural classification.
• Particle-size or mechanical analysis involves separating soil particles in an aqueous suspension.
• Vigorous stirring of a soil sample in distilled water is used to achieve this separation.
• A dispersing agent prevents flocculation of particles in the suspension.
• The percentage of soil separates is determined using the hydrometer or pipette method once the soil suspension is dispersed.

Importance of Soil Texture

• Soil texture influences:
  • The amount of water the soil can hold
  • The rate of water movement through the soil
  • The workability and fertility of the soil
• Sandy soil is well-aerated but holds little water and has low nutrients.
• Clay soils hold more water and supply nutrients better.
• Texture often changes with depth, affecting root penetration.

Soil Profile Types

• Uniform: Same texture throughout the soil profile
• Texture-contrast: Abrupt texture change between topsoil and subsoil
• Gradational: Texture gradually increases down the soil profile

Soil Separates: Particle Size

• Soil separates are specific ranges of particle sizes.
• Clay particles are less than 0.002 mm in diameter and are plate-shaped, increasing specific surface area.
• Silt particles range from 0.002 mm to 0.05 mm in diameter (USDA classification).
• Sand particles are larger than 0.05 mm in diameter.
  • Large sand particles are coarse
  • Intermediate sand particles are medium
  • Smaller sand particles are fine
• Different countries have varying particle size classifications.

Soil Textural Class Characteristics

• Sand or Loamy Sand:
  • Dry: Loose, single-grained, gritty, with no or very weak clods
  • Moist: Gritty, forms easily crumbled ball, does not ribbon
  • Wet: Lacks stickiness but may show faint clay staining
• Sandy Loam:
  • Individual grains are easily seen and felt
  • Dry: Clods break easily
  • Moist: Moderately gritty, forms a ball that stands careful handling, ribbons very poorly
  • Wet: Stains fingers, may have faint smoothness or stickiness, but grittiness dominates
• Loam:
  • Characteristics of sand, silt, and clay are present but none predominates
  • Dry: Clods are slightly difficult to break and somewhat gritty
  • Moist: Forms firm ball, ribbons poorly
  • Wet: Gritty, smooth, and sticky all at the same time,stains fingers
• Silt or Silt Loam:
  • Grittiness of sand is well masked
  • Dry: Clods are moderately difficult to break, rupture to a floury powder that clings to fingers
  • Moist: Smooth, slick, velvety feel, forms firm ball, may ribbon slightly, shows good fingerprint
  • Wet: Smooth with some stickiness from clay, stains fingers
• Sandy Clay Loam:
  • Dry: Clods break with some difficulty
  • Moist: Forms firm ball, forms short ribbons, may show poor to good fingerprint
  • Wet: Grittiness of sand and stickiness of clay are balanced, masks smoothness of silt, stains fingers
• Clay Loam:
  • Dry: Clods break with difficulty
  • Moist: Forms firm ball, ribbons fairly well, shows fair to good fingerprint
  • Wet: Moderately sticky with stickiness dominating grittiness and smoothness, stains fingers
• Silty Clay Loam:
  • Resembles Silt Loam but with more stickiness of clay
  • Dry: Clods break with difficulty
  • Moist: Shows a good fingerprint, forms a firm ball, ribbons fairly thin
  • Wet: Stains fingers, has sticky-smooth feel with little grittiness of sand
• Sandy Clay:
  • Dry: Often cloddy, clods are broken only with extreme pressure
  • Moist: Forms a very firm ball, shows fingerprint, squeezes to a thin, somewhat gritty ribbon
  • Wet: Stains fingers, clouds water, quite sticky and plastic, has some grittiness
• Silty Clay:
  • Dry: Same as Sandy Clay
  • Moist: Forms a very firm ball, shows fingerprint, squeezes out to a thin, smooth ribbon
  • Wet: Stains fingers, clouds water, stickiness dominates smoothness, grittiness is virtually absent
• Clay:
  • Dry: Cloddy, clods often cannot be broken even with extreme pressure
  • Moist: Forms firm, easily molded ball, squeeze out to a very thin, long ribbon
  • Wet: Stains fingers, clouds water, very sticky, stickiness masks smoothness and grittiness, wets slowly

Determining Soil Texture

• Take 2 tablespoons of soil and add water until it reaches a sticky consistency.
• Squeeze the wetted soil between thumb and forefinger to form a flat ribbon.
• Determine the texture based on the length of the ribbon.

Sandy Soil Properties

• Ribbon length: 75 mm
• Root growth is moderately to severely restricted
• High susceptibility to mechanical compaction
• Water drains very slowly except in self-mulching soils