Introduction to Agriculture Soil Science Unit 2 Physical

Introduction to Agriculture Soil Science Unit 2 Physical Properties of Soil

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

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 » p. H

» 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 Texture

» The p. H 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 (-) » p. H 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 p. H

» 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 p. H

» 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 Soil Structure