Physical Properties

of Soil

Introduction to AgricultureSoil Science

Unit 2

Physical Properties

»Physical properties or characteristics of the soil determine to a large degree how useable and productive soil will be

Productivity of soil is affected by the physical properties of soil

Factors of productivity• How quickly water penetrates• How well water stays in the soil• How well soil holds up under machinery• The ease of root penetration• The aeration of the soil

• Important because influences:• Plant and root growth• Microorganism population and activity• Growth of toxins• Water and nutrient absorption• Disease development

Physical Characteristics Include

»Texture»Structure»Consistency»pH

Soil Texture

» Texture refers to the size of the individual soil particles called SOIL SEPARATES˃ The larger the size of the separates, the coarser the

soil feels˃ Largest of the soil particles is sand˃ Silt particles are smaller than sand˃ Smallest particles are referred to as clay˃ Can compare the separates to a basketball, golf ball,

and BB for relative size difference

Soil Texture

Soil Texture

» Soils rarely are composed of pure sand, pure silt, or pure clay… rather a combination of all three

» Ideal soil˃ Loam Soil: nearly ideal for growing most

crops˃ Soil that is <52% sand, 28-50% silt, and 7-

27% clay

Soil Texture

» Sandy soils˃ Coarse˃ Does not hold water well…particles are so large, water from rain or

irrigation pass through the soil and little is retained for plants˃ Coarser soil results in fewer soil nutrients that will be in the soil and

available for plants+ Coarse or sandy soils are less productive because they do not hold

water or nutrients as well as finer soils+ When particles are large, water moves out of the pores taking

water-soluble plant nutrients with it+ LEACHING

Soil Texture

» Clay soil˃ May not let enough water through˃ Cause the water to run off or hold the water too long

Soil Texture

»Mixture of finer textured silts and clay˃ Typically more productive˃ Slow water down and hold a portion of water for

plants to use˃ Make nutrients available longer for plants˃ Have more nutrients adhere to the greater surface

area of clay

Soil pH

» The pH of the soil has to do with how acid the soil is˃ Acid – a substance containing hydrogen that forms hydrogen ions when

dissolved in water˃ Hydrogen ion – a hydrogen atom with a single proton (+)˃ Hydroxide ion – a hydrogen atom with two electrons (-)

» pH scale – a measure of the relative strength of the hydrogen ion or hydroxide ion activity in a substance˃ Neutral substance – has as many hydrogen as hydroxide ions; ranks 7.0 on

the scale˃ Hydrogen ions > hydroxide ions = acid; score lower than 7 on the scale˃ Hydrogen ions < hydroxide ions = alkaline; score higher than 7 on the scale

Soil pH

» Soils that are either too acidic (higher _________ ion concentration), or too alkaline (higher ___________ ion concentration) may not be ideal for production of certain crops

» Examples:˃ Potatoes, tomatoes, peppers, beans, onions ˃ Cabbage, Brussels sprouts, mustard,

Soil Structure

» Water flow through soil is greatly influenced by the structure of the soil

» Factors that cause soil particles to clump together are:˃ Affected by freezing/thawing˃ Root movement˃ Earthworms/other life forms˃ Variation of moisture content

Soil Structure

» Group of clinging soil particles are called peds, or soil aggregates˃ Peds are held together by the

clay and humus in the soil˃ Peds categorized by

+ Type (shape)– Spherical – Plate-like (flat and

thin)– Block-like (cubic)– Prism-like (long with

several sides)

Soil Structure

+ Class (size)– Very fine– Fine– Medium– Coarse– Very coarse

+ Grade– Strength, how stable, not changing or fluctuating – permanent– Structureless soils – no real structure… either single grains (sand in a

dune) or massive (clays stuck together with no distinguished peds)– Weak structure – peds are hard to distinguish and only a few can be

separated in moist soil– Moderate structure – peds are visible and can be handled without

breaking up– Strong structure – most of the soil is formed into peds and can be

handled without breaking up