Soil Physical Properties

Soil texture

• Qualitative (Feel)

• Quantitative (Proportion of sand, silt and clay)

Clay

ISSS- <2 um

FAO/USDA- <2 um

Silt

ISSS- 2-20 um

FAO/USDA- 2-50 um

Sand

ISSS- 20-2000 um

FAO/USDA 50-2000 um

Fine sand

ISSS- 20-200 um

Coarse sand

ISSS- 200-2000 um

Nature of Soil Separates

1. Sand

• larger smaller than 2 mm but larger than 0.05 mm (or 0.02 mm)

• feels gritty between fingers

• particles are generally visible to the naked eye (angular or rounded)

• grains contain a single mineral, usually quartz and other primary minerals

• the large particles of sand have low specific surface area

• possess little capacity to hold water or nutrients

• do not stick together into coherent mass

Nature of Soil Seperates

2. Silt

• particles smaller than 0.05 (or 0.02) mm but larger than 0.002 mm

• similar to sand in shape and mineralogy

• individual silt particles are so small as to be invisible to the unaided eye

• feels smooth or silky like flour

• even when wet, silt particles don't exhibit much stickiness or plasticity

Nature of Soil Seperates

3. Clay

• smaller than 0.002 mm

• possess very large specific surface area

• high water holding capacity and nutrient adsorption

• hard and massive when dry, sticky when wet

• clay particles tend to be shaped like tiny flakes of flat platelets.

• fine clay-sized particles behave as colloids (if suspended in water they do not readily settle out

Porosity

Number of pore spaces- Clay

Number of macropores- Sand

Peat

Organic material that is relatively undecomposed plant tissues

Muck

Highly decomposed organic material, and few plant tissue fibers are observe

Mucky peat

Intermediate decomposition

Feel and Roll method (Qualitative)

is a qualitative field technique used to estimate soil texture (the proportion of sand, silt, and clay) without laboratory analysis. It relies on touch and manipulation of moist soil in the hand.

sandy loam.

If a soil feels gritty but forms a short ribbon (<2.5 cm)

< 2.5 cm (1 inch)

Clay

If it feels smooth and sticky and makes a long ribbon (>5 cm)

5 cm (2 inches)

clay loam

2.5 – 5 cm (1–2 inches)

Spheroidal/Granular

• Small, rounded aggregates, like breadcrumbs.

• Common in topsoil (A horizon).

• Good for root growth and water infiltration.

• ½ inch in diameter

Crumb

• Smaller than Spheroidal structure. Typically found in surface layer of cultivated soil

Platy

• Thin, flat plates stacked horizontally.

• Restricts water movement and root penetration.

• Often found in compacted soils.

• Commonly founf in A or B horizon Virgin forest soil

Blocky

• Irregular, block-like peds, usually about the same length in all directions.

• Found in subsoil (B horizon).

Two kinds:

• Angular blocky → sharp edges.

• Sub-angular blocky → rounded edges.

Prism-like (Post-like)

• Prismatic

Vertical, column-like units with flat tops.

Common in subsoils with clay accumulation.

Restricts horizontal root growth.

• Columnar

Similar to prismatic, but with rounded tops.

Often found in soils with high sodium content (sodic soils).

Structureless

• Single-Grained

No real structure; soil particles are loose and don’t stick together.

Typical of sandy soils (e.g., beach sand).

• Massive

Large, hard clods with no visible structure.

Found in dense subsoils.

Poor for plant growth and water movement.

Soil Density

• refers to how much mass of soil is packed into a given volume.

• It affects water movement, aeration, root penetration, and soil strength.

Particle Density (ρs)

Mass of soil solids only per unit volume, excluding pores.

Bulk Density (ρb)

Mass of dry soil (solids + pores) per unit volume of soil.

Lower bulk density

more pore space, good for root growth.

Higher bulk density (> 1.8 g/cm³)

compacted soil, restricts water and roots.

Factors Affecting Bulk Density

Soil Structure

• Well-aggregated (crumb, granular) soils → lower BD (more pore spaces).

• Massive or compacted soils → higher BD.

Soil Depth

• Topsoil: usually lower BD (more organic matter, root activity, biota).

• Subsoil: higher BD (less OM, more compaction, less biological activity).

Soil Porosity

refers to the percentage of the total soil volume that is made up of pore spaces (the spaces between soil particles and aggregates). These pores can be filled with air, water, or a mixture of both.

Soil consistency

refers to the degree of cohesion and adhesion of soil particles and how resistant the soil is to deformation or rupture under applied forces such as wetting, drying, and pressure.

Wet Consistency

Soil is sticky and plastic.

Plasticity = ability of soil to be molded without breaking.

Clays show high plasticity, sands show little or none.

Moist Consistency

Soil can be broken apart or pressed between fingers.

Terms used: loose, friable, firm, very firm.

Indicates ease of root growth and tillage.

Dry Consistency

Soil is hard when dry.

Terms used: loose, soft, hard, very hard, extremely hard.

Sandy soils are loose when dry, while clay soils become very hard.

Loose

Soil flows through fingers.

Soil flows through fingers.

Soil breaks apart easily.

Firm

Soil requires moderate pressure to break.

Plastic

Can be molded into shapes without breaking.

Sticky

Adheres to fingers when wet.

Soil Moisture

• Most variable property of the soil

Classsification of water

• Water saturation

• Moisture at Field Capacity

• Moisture at Permanent Wilting point

• Hygroscopic or Adsorbed Water

• Chemically-combined Water

Movement of Water Within the Soil

• Capillary Movement/Rise

• Percolation

• Infiltration

Capillary Rise

• Upward movement

• There are more water movement in clay than sand

Percolation

• Determines drainage capacity of soil

• Influences groundwater recharge

Percolation Rate

Sandy soil → fast percolation (large pores, less water holding).

Clay soil → slow percolation (tiny pores, high water holding).

Loam soil → moderate percolation (best for crops).

Infiltration

Infiltration is the process by which water on the soil surface enters the soil

Soil Temperature

• Solar radiation

• Radiation from the sky

• Conduction of the heat from the atmosphere

• Condensation

• Evaporation

• Rainfall

• Insulation

• Vegetation

• Biology Activity

• Radiation

• Albedo

• Structure, Texture and Moisture

• Soluble Salts

Properties Affected by soil Temperature

• Soil Moisture

• Microbial Activity

• Soil Development

• Plant Growth

• Availability of Plan nutrients

Platy

Soil Structure Slowest percolation rate

Clay

Compacted than Sand

In Bulk Density it is lower than sand