Soil Chapter 2 Soil Physical Properties Pages 26 – 60

Density... A measurement of mass divided by volume (in grams (g)) per cubic centimeter (cm 3 ) Density = mass/volume = g/mL or g/cm 3

Bulk density... Term typically used in soils implies a measure including air spaces a ‘non-uniform’ material with air spaces

Bulk density... In general – bulk densities range from 1 g/cm3 to 1.5 g/cm3 Clays – 390 g/300 ml (1.3 g/mL) Sands – 572 g/300 ml (1.91 g/mL)

Sand... Defined as the largest particle of broken rock that qualifies as soil ≤ 2mm to 0.5mm

Clays... Formed by the precipitation of soluble crystalline materials in solution settling out in bodies of water Ex. lakes, seas, oceans

Porosity... Measure of the total volume of voids in a sample of soil to the total volume of soil... measured as a percentage

Porosity... Pore space volume / total soil volume (100%) = porosity % Porosity = 100% total volume — % solids Good garden soil porosity is about 50%

Soil pores... Divided into two main categories: a) macropores 2) micropores

Sands... Have a predominance of macropores or large pore spaces provides minimum forces required to hold water water drains readily leaving large gas- filled voids

Clays... Have a predominance of micropores or small pore spaces attracts and holds water very well reduces the amount of air spaces amongst the clay particles

Cake and eat it too... Good gardening soils should have a good distribution of both micropores and macropores Sandy-loam soils contain a good combination of pore sizes

Soils structure... Arrangement of sand, silt and clay particles into aggregates or ‘peds’

Soils structure... Can be affected by: textural elements – sand, silt and clay chemicals – iron, calcium, aluminum, etc. mechanical alterations – tilling, compaction, etc. organic matter – humus, compost, mulch, etc.

Organic matter Nature’s soil elixir

Organic matter... Added to clay soils... clay particles collect together and bind into larger particles becomes crumbly with the formation of macropores increases air spaces and drainage

Organic matter... Added to sandy soils... fills macropores spaces between sand particles increases nutrient-holding capacities reduces macropore spaces increasing water holding capacities

Humus This is NOT chickpeas, garlic and extra virgin olive oil! End product of organic decomposition Binds individual clay particles into larger aggregate particles

Particle density... Particle density = mass of solids/volume of solids Based on an average of sand, silt and clay As a general rule particle density is 2.65 g/cm 3

Remember... Bulk density = total mass / total volume = mass of solids/total volume Where total mass = total solids

Some relationships... % Solids = bulk density / particle density = (mass of solids/total volume) / (mass of solids/volume of solids) = (volume of solids/mass of solids) x (mass of solids/total volume) = volume of solids/total volume

Physical properties of soil (review) Texture – percentage of sand, silt and clay Sand 2mm – 0.05mm Silt – 0.05mm – 0.002mm Clay < 0.002mm Texture is determined by a settling test and soil texture triangle

Physical properties of soil (review) Texture helps define the characteristics of a soil, including: water infiltration rates irrigation rates irrigation frequencies structural abilities – compaction, etc.

Physical properties of soil (review) Structural abilities – compaction, etc. Fertility – lower in sandy soils, higher in loam and clay soils Plant selection Texture not easily changed

Physical properties of soil (review) Structure Arrangement of texture particles Aggregates or peds Structure can be changed mechanically or chemically

Physical properties of soil (review) Addition of organic matter is the most effective and economical way to change structure by increasing... water holding capacity... drainage and... nutrient holding capacity

Physical properties of soil (review) Structure can be damaged by... compaction... tillage of wet soils... use of sodium-softened water and... excess irrigation

Bulk density mass of soil / volume of soil Bulk density can be modified by compaction or tillage

Particle density Mass of solids / volume of solids

Porosity %Porosity = 100% total volume – % total solids Measured as a percent

Consistency Soil’s response to mechanical forces – resistance to rupture Soft or hard when dry Friable (crumbly), firm or rigid when wet Plasticity – ability to tolerate deformation without breaking Stickiness – affects workability

Expansiveness Changes in soil volume from wet to dry

Color Affects soil temperatures Light soils tend to be cooler Dark soils tend to be warmer

Color Color determinations made using the Munsell Color Charts Colors may indicate drainage or lack of (gleying – blotchy gray soils due to poor drainage)

Temperature Affected by... topography... seasons... soil color... cover... moisture...