Soil Structures Part 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

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

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.

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

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

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

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.

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.

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

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.

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

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

Soil capping

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

Soil compaction pan

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