R2102 1.4 Define soil texture and describe the root environment

Question 1

Define soil structure

Answer 1

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.

Question 2

Structured soil types (4)

Answer 2

1. Prismatic
2. Blocky
3. Platey
4. Crumb

Question 3

What is prismatic soil?

Answer 3

Where aggregates form clods in columns with flat tops and seperated by deep cracks, allowing water to penetrate quite well

Question 4

What is platey soil?

Answer 4

Has flat horizontal aggregates like an alluvial floodplain soil. Drainage and permeability are poor in these soils

Question 5

What is blocky soil?

Answer 5

The aggregates are blocky and soil is moderately permeable

Question 6

General, what is crumb soil?

Answer 6

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.

Question 7

Formation of good crumb structure. Function of small and large pores (3)

Answer 7

1. Soil crumbs formed by aggregation of large and small soil particles
2. Small pores inside soil crumbs which contain water
3. Large pores between crumbs allow movement of air and water

Question 8

Influence of crumb structure on plant growth (4)

Answer 8

1. Encourage root penetration so that the plant can develop and thrive
2. Allow easy passage of water and nutrients that the plant requires
3. Hold air that the plant needs to respiration
4. Allows space for the growth of micro-organisms

Question 9

What is crumb structure?

Answer 9

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

Question 10

What is tilth?

Answer 10

The crumb structure of the seedbed

Question 11

What is friable soil?

Answer 11

The consistency of the soil when it is easily cultivated i.e. readily forms crumbs

Question 12

How can crumb structure be destroyed? (4)

Answer 12

1. Cultivating at the wrong time
2. Over working the soil
3. Compaction of the soil
4. Vulnerable to collapse when wet - puddles of water, rainfall, irrigation.(Formation of soil caps)

Question 13

What are soil pores?

Answer 13

The spaces between the soil particles

Question 14

What does soils ability to withstand compactation depend on? (3)

Answer 14

1. 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
2. Water content - high water level in the soil destroys soil structure.
3. Do not cultivate soil when it is too wet or take machinery over wet soil

Question 15

What is the cultivation window?

Answer 15

The number of days that soil is in a suitable condition to be cultivated.

Question 16

Typical mineral soil composition

Answer 16

Air 25%
Water 25%
Minerals 45%
Organic matter (5%)

Question 17

Compostion of the organic matter component in a typical mineral soil

Answer 17

Organisms 10%
Roots 10%
Humus 80%

Question 18

Relationship between pore space, air and water

Answer 18

The size of soil pores affects the balance of air and water held in them.

Question 19

What is a saturated soil?

Answer 19

1. One which has all the soil pores filled with water.
2. There is no air
3. Most plants will not grow in saturated soil

Question 20

What is the saturation point?

Answer 20

The saturation point of a soil is when water has filled all the soil pores (i.e. no air in pores)

Question 21

What is field capacity (FC)(3)

Answer 21

1. This is the water that remains after gravitational water has drained away.
2. Represents the optimum growing situtation
3. Clay soils will have a higher FC than sandy soils

Question 22

Why is soil air necessary? (3)

Answer 22

1. Repiration of roots
2. Respiration of living organisms in the soil
3. Air competes with water for the pore spaces between the soil particles

Question 23

What is Air Filled porosity (AFP)

Answer 23

Amount of air in soil or growing media

Question 24

Why is AFP important for seeds and cuttings?

Answer 24

1. As seeds germinate their respiration rate increases- so does their need for oxygen
2. 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.

Question 24

Why is AFP important for seeds and cuttings?

Answer 24

1. As seeds germinate their respiration rate increases- so does their need for oxygen
2. 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.

Question 25

What is water holding capacity?

Answer 25

The amount of water held at Field Capacity of the soil

Question 26

What can affect the porosity of the soil? What are 3 problems the soil can face?(3)

Answer 26

1. Compacted soil
2. Soil cultivation pans
3. Soil capping from rain

The porosity of the soil must reach the surface to referesh the air

Question 27

What can postively affect porosity of the soil?

Answer 27

1. Any increase in air space (digging / rotivating etc) will reduce the carbon dioxide concentrations and increase oxygen
2. A healthy population of soil organisms improves structure

Point 1 - Compaction has the opposite effect

Question 28

Oxygen concentration & needs

Answer 28

1. Generally if oxygen dips below 5% growth ceases
2. Varies by plant - peas need 21%, rice only 0.5%

Question 29

Soil pore sizes

Answer 29

1. 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)
2. 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.
3. 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/

Question 30

What is the permenant wilting point (PWP)

Answer 30

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

Question 31

What is Available Water Content (AWC)

Answer 31

The water held in soil between Field Capacity and Permenant Wilting Point

Question 32

What is Soil Moisture Deficit (SMD)

Answer 32

The amount of water needed to restore the soil to FC from PWP