Lecture 2 - Physical properties of soil

Question 1

what is soil texture

Answer 1

proportions of sand, silt and clay

Question 2

What is sand

Answer 2

particulate matter from rock disintegration (non-plastic) (2-0.02mm)

Question 3

What is silt

Answer 3

fine mineral material, crumbles when rolled (0.02-0.002mm)

Question 4

What is clay

Answer 4

fine grained soil, display plastic properties (<0.002)

Question 5

Characteristics of clay soils

Answer 5

• retain moisture and nutrients
• most stored water is not usually available for plants
• prone to waterlogging
• easily compacts

Question 6

characteristics of sandy soils

Answer 6

• struggle to retain water and nutrients
• allow plants to extract water and nutrients easily
• prone to nut. leaching
• prone to water repellence (need to add clay to help)

Question 7

what impacts soil colours

Answer 7

• organic matter content = darker
• mineral matter = retains colour of rock
• form and abundance of iron = avg conds. = yellow oxide, well drained/dry = red oxide, waterlogged = grey/green/blue. More CO2 turns iron more brown
• moisture content = darker

Question 8

define soil structure

Answer 8

how soil components are arranged into aggregates (combination of sand, loam, clay, OM, fungal hyphae into clusters)

Question 9

Importance of soil strucure

Answer 9

• permeability to water and air, root penetration and seedling emergence, resistance to erosion (stability)
• better structure (more aggregates) = more pores/voids = movement of air and water

Question 10

3 ways soil water exists in soil

Answer 10

• gravitational water = water is held in large pores and drains out of soil due to gravity (smaller pores = less draining)
• capillary Water = held in pores small enough to hold against gravity (can mean that plants can’t remove this water as too difficult)
• chemical water = water tightly held by electrostatic forces to clay surfaces (unavailable to plants)

Question 11

What is field capacity

Answer 11

after saturation, all gravitational water has drained out

Question 12

what is the wilting point

Answer 12

plants have drawn all accessible water out of soils

Question 13

What is the issue with water resistance

Answer 13

• uneven pasture germination
• seeds buried in soil may not get any water if only light rains
• established and young plants in same pasture = difficulty using chemicals (different ages)
• increased risk of erosion

Question 14

What destroys soil structure

Answer 14

• cultivation (turning over soil)
• loss of OM
• mixing dispersive subsoils (wetting with freshwater disperses clay particles)