R2102 1.4 Define soil texture and describe the root environment Flashcards
Define soil structure
The arrangement of soil particles in the soil and the spaces between them.
Individual soil particles are arranged into groups called soil aggregates or peds, held together by the adhesive properties of clay and humus.
Structured soil types (4)
- Prismatic
- Blocky
- Platey
- Crumb
What is prismatic soil?
Where aggregates form clods in columns with flat tops and seperated by deep cracks, allowing water to penetrate quite well
What is platey soil?
Has flat horizontal aggregates like an alluvial floodplain soil. Drainage and permeability are poor in these soils
What is blocky soil?
The aggregates are blocky and soil is moderately permeable
General, what is crumb soil?
The ideal type of soil structure for horticulture. It has small rounded aggregates of soil particles loosely adjoining other aggregates, and so it is porous and permeable, yet retains moisture.
Formation of good crumb structure. Function of small and large pores (3)
- Soil crumbs formed by aggregation of large and small soil particles
- Small pores inside soil crumbs which contain water
- Large pores between crumbs allow movement of air and water
Influence of crumb structure on plant growth (4)
- Encourage root penetration so that the plant can develop and thrive
- Allow easy passage of water and nutrients that the plant requires
- Hold air that the plant needs to respiration
- Allows space for the growth of micro-organisms
What is crumb structure?
Where a mixture of particle sizes (sand, silt, clay) clump together around lime or humus.
Good soil structure is present when the soil forms stable aggregates which don’t break down too easily
What is tilth?
The crumb structure of the seedbed
What is friable soil?
The consistency of the soil when it is easily cultivated i.e. readily forms crumbs
How can crumb structure be destroyed? (4)
- Cultivating at the wrong time
- Over working the soil
- Compaction of the soil
- Vulnerable to collapse when wet - puddles of water, rainfall, irrigation.(Formation of soil caps)
What are soil pores?
The spaces between the soil particles
What does soils ability to withstand compactation depend on? (3)
- Structural stability - Sand and silts easily collapse when they soak up water (slaking). Soils with a lot of clay, humus and chalk hold their form more strongly
- Water content - high water level in the soil destroys soil structure.
- Do not cultivate soil when it is too wet or take machinery over wet soil
What is the cultivation window?
The number of days that soil is in a suitable condition to be cultivated.
Typical mineral soil composition
Air 25%
Water 25%
Minerals 45%
Organic matter (5%)
Compostion of the organic matter component in a typical mineral soil
Organisms 10%
Roots 10%
Humus 80%
Relationship between pore space, air and water
The size of soil pores affects the balance of air and water held in them.
What is a saturated soil?
- One which has all the soil pores filled with water.
- There is no air
- Most plants will not grow in saturated soil
What is the saturation point?
The saturation point of a soil is when water has filled all the soil pores (i.e. no air in pores)
What is field capacity (FC)(3)
- This is the water that remains after gravitational water has drained away.
- Represents the optimum growing situtation
- Clay soils will have a higher FC than sandy soils
Why is soil air necessary? (3)
- Repiration of roots
- Respiration of living organisms in the soil
- Air competes with water for the pore spaces between the soil particles
What is Air Filled porosity (AFP)
Amount of air in soil or growing media
Why is AFP important for seeds and cuttings?
- As seeds germinate their respiration rate increases- so does their need for oxygen
- Cutting need an open textured compost to give oxygen for the high rate of respiration taking place, fuelling cell division where new roots are developing.
Why is AFP important for seeds and cuttings?
- As seeds germinate their respiration rate increases- so does their need for oxygen
- Cutting need an open textured compost to give oxygen for the high rate of respiration taking place, fuelling cell division where new roots are developing.
What is water holding capacity?
The amount of water held at Field Capacity of the soil
What can affect the porosity of the soil? What are 3 problems the soil can face?(3)
- Compacted soil
- Soil cultivation pans
- Soil capping from rain
The porosity of the soil must reach the surface to referesh the air
What can postively affect porosity of the soil?
- Any increase in air space (digging / rotivating etc) will reduce the carbon dioxide concentrations and increase oxygen
- A healthy population of soil organisms improves structure
Point 1 - Compaction has the opposite effect
Oxygen concentration & needs
- Generally if oxygen dips below 5% growth ceases
- Varies by plant - peas need 21%, rice only 0.5%
Soil pore sizes
- Macropores (large pores) > 0.1mm Will drain easily within hours after being saturated. Do not hold water long enough for plants to make use of. Allow for root penetration (transmission pores)
- Mesopores (medium sized) Contain water that is available to plants and the air moves in as the water is removed by plant roots. Also called storage pores.
- Micropores (Small pores) Contain only water which rarely dries out and is unavailable to plants because it clings so tightly to the smallest soil particles. Also called residual pores. The water in these pores has a great effect on the mechanical strength of the soil - its resistance to compaction/
What is the permenant wilting point (PWP)
The point at which plants wilt due to lack of available water in the soil. PWP is reached sooner in sandy soils than on clays. Plant does not recover overnight
What is Available Water Content (AWC)
The water held in soil between Field Capacity and Permenant Wilting Point
What is Soil Moisture Deficit (SMD)
The amount of water needed to restore the soil to FC from PWP
