1. Soil Structures
Part 2

2. Learning outcomes
Distinguish between the arrangements of particles in the soil.
Explain the factors influencing soil structural stability
Discuss soil structure improvement
Explain what causes soil structural problems

3. Student activity
In your group discuss and list what contributes to soil structure.

4. This is the arrangement of particles in the soil
This is the arrangement of particles in the soil. They are arranged into distinct groups or aggregates e.g. Crumb structure. It refers to the size and shape of these groups. It also means that the structure in concerned with the pore spaces in between the soil aggregates.

6. Soil structural stability
Important because it allows:
The soil to be worked without structural damage
A tilth can be prepared easily on the top 50mm of soil which will allow seedlings to germinate successfully and develop into healthy seedling

7. Factors influencing soil structural stability
Soil texture Fine sand/silt soils easily damaged. Clay soils – less easily damaged as more strongly developed structures. Course sand soils much more difficult to damage. Soil organic matter content A soil with a high organic matter content/level is more structurally stable than one with a low organic matter level. Reasons: Decaying organic matter leaves channels in the soil to aid aeration/drainage

8. Cont.
‘Gums’ from decaying organic matter help bind soil particles together. Soil moisture content If there was too much moisture in the soil (i.e. saturated ) the water acts almost like a lubricating oil so that when the soil is worked structures break up as a result. A dry soil is more stable structurally in the short term

9. Soil structure improvement
Root growth Exploring roots probe the soil, opening up channels. Sideways pressure forces the soil into structures. On dying the channels are stabilised by the roots decomposing remains. Earthworm activity Continuous worm activity leaves channels which aid s drainage/aeration . They help to stabilise structures by incorporating organic matter into the soil.

10. Cont...
Weathering Frost shatters clods in the winter although only on the surface and mainly in clays. Wetting and drying in clays helps to maintain structural improvement due to swelling and shrinkage. Flocculation Humus and calcium appear to combine with clay so that when it dries cracking occurs. In clay particles remain in groups, they are said to be flocculated, if they separate and form a sticky mass – dispersed or deflocculated.

12. Influence of calcium
Calcium is a positively charged ion called a cation.  Cations are absorbed by the plant roots and also held on exchange sites in soils. The positive charge of calcium is attracted to negative electrical charges found on the exchange sites on clay particles.  The more clay, the stronger the attraction.  This attraction of the positive Ca with the negative charges in the clay particles -- binds the clay soil particles together so they become flocculated and resist dispersing and soil structure breakdown

13. Cont....
Cultivations Usually carried out to provide a suitable rooting environment for crops. However the use of cultivators /ploughs especially when wet can lead to a platy layer or pan (no vertical cracks =poor drainage=poor root penetration) Organic matter incorporation Regular organic matter addition improves and maintains structure in most soils especially silts/fine sands and clays.

14. Soil Structural Problems
Capping
Cause:
low organic matter in soil
Over cultivation of soil
High % silt in soil
Heavy rain usually causes crust on the surface
Symptoms:
Crust on surface of silt soils
Seedlings cannot emerge easily
Poor gaseous exchange

15. Cont... Preventions: Avoid over cultivation even if dry
Regular addition of organic matter
Mulching, if possible

16. Soil capping

17. Cultivation, compaction and smear pans
Cause:
Rotating at the same depth every time/ploughing.
Trampling/cattle/heavy machinery on wet land
Type of soil
Symptoms:
Water lying on the surface
Poor root growth
Preventions:
Applying organic matter
Only work the soil when dry
Change the depth of cultivation/ploughing

18. Soil compaction pan

19. Salt pan Cause: UK Flooding by salt water
Tropics: irrigation by slightly salty water.
Rapid evaporation leaves salt behind
Applying too much fertiliser (build up of Sodium Chloride)
Symptoms:
Clay disperse – deflocculated
Structure destroyed
No plants grow apart from salt tolerant
Prevention:
Flood with fresh water
Leaching from rainfall