RAINDROP SPLASH
Falling raindrops contain a high level of energy. In thunderstorms raindrops are larger, fall faster and have higher energy. Plants help to protect soils from erosion and bare soils are easily eroded.
BARE SOILS
Beating raindrops hit bare soil surfaces forming small craters.
Raindrop splash occurs when droplets carrying small soil particles are flung into the air.
Raindrops break down clods and destroy soil structure forming a surface seal.
Beating raindrops slow down infiltration into soils and runoff increases.
Erosion is more severe when surface clods are broken down into small particles and become easily carried by water. Higher runoff will increase erosion.
On a sloping surface more raindrop splash occurs downhill than uphill and this results in downhill erosion occurring directly from beating raindrops.
Raindrop splash increases sheet erosion.
Runoff is able to carry more soil particles when raindrops stir up surface water.
PLANT PROTECTION
Growing plants intercept raindrops, breaking them up into smaller droplets and reducing their velocity.
The soil surface is protected by plants from the beating action of raindrops reducing structure breakdown. Leaf litter also protects soil.
Infiltration of rainfall into soils is greater when plants protect the soil surface.
Erosion is less when plants reduce runoff and improve surface structure. Plant roots, soil organic matter and microbes especially fungi bind soil particles into stable aggregates below the soil surface.
Soil Structure
Structure is the arrangement of small soil primary particles into aggregates or peds.
Sand silt and clay are the most common inorganic particles in soils. Organic matter, living organisms and many different types of minerals occur in differant soil types. In healthy soils these small partices are held together and form aggregates or peds.
Aggegates are bound together by
1 electrostatic forces; clays have negative charged surface.
2 cementing agents; colloidal organic matter, aluminium and iron hydroxides.
3 fungi hyphae and roots bind particles together.
Soil structure is more stable when organic matter is plentiful in soils. Organic matter produces cementing agents helping to bind sand, silt and clay together. Bacteria bind particles together when they produce a sticky gum. Strands of fungi hyphae and actinomyces bind small particles into aggregates. Plant roots help to prevent soil aggregates from breaking up.
A stable soil structure improves soil health.
Increases water penetration
Improves aeration
Reduces erosion
Enables easier root penetration
Increases seed germination and emergence
Prevents formation of surface crust
Increases growth of microbes and small animals
Plant growth increases
Soil structure often breaks down when organic matter is reduced. When land is regularly ploughed for crop production organic matter is reduced and structure breakdown increases. Heavy grazing decreases the plant material added to soils reducing organic matter and increasing structure breakdown.
Reduction of soil organic matter, followed by structure breakdown is often the first symptom of land degregation and serious erosion.
Aggregation of soil particles
Clay particles have a plate like shape. Domains are a number of clay particles stacked up together. The surfaces of clay particles are negatively charged and the electrostatic forces can form either attraction or repulsion forces between clay particles. Calcium ions increase attraction forces and the flocculation of clay particles. Sodium ions increase repulsion forces and the dispersion of clay particles.
Organic colloids can cement soil particles together. Iron and aluminum hydroxides also are cementing agents.
Fungi and actinomycetes hypha bind soil particles together. Plant roots help to form a stable structure.
Bacteria are surrounded by a sticky gel binding soil particles together.
