Soil Erosion

SOIL EROSION

WIND (Entrainment) Moves particles by SUSPENSION, SALTATION & TRACTION. Works best on unvegetated surfaces. Wind speed greater than 30km hour begins to be effective. Saltation moves particles of up to 0.4mm in diameter by a series of jumps) - this is the dominant process. Fine sand is moved by a process known as a 'blow' (can have 'blowouts' on dunes, which can be triggered by the action of rabbits or humans) Topsoil texture becomes coarser as a result - a little like the 'armouring' of a stream channel by the removal of fine material. Most common in Eastern England from March to May. Due to bare fields and strong winds. Root crops, such as sugar beet need a fine seedbed, and are therefore vulnerable.

WATER EROSION RAINSPLASH, SHEETWASH & RILL / GULLY EROSION are the main processes. Raindrop impact breaks up the soil aggregates. Erosion is therefore influenced by rainfall intensity and raindrop size (greater in the tropics) A 'threshold' process: nothing will happen until a critical stage or intensity is reached and then the soil will start to be moved. Helped by weak soil structure - often caused by over reliance on fertilisers in intensive agriculture, and bare soil when fields are left fallow, or by setaside. Water accumulates in small channels (rills) which are then enlarged to form gullies - ultimate extent is the formation of 'badland' erosion.

There are several farming practices which can lead to erosion:

ploughing on slopes which were formerly under grass in order to increase area of arable cultivation

use of larger agricultural machinery increases soil compaction

removal of hedgerows and associated increase in field size. Larger fields cause an increase in slope length.

declining levels of organic matter from intensive cultivation & reliance on artificial fertilisers which leads to reduced aggregate stability

cultivation by machinery produces rills along wheel ruts and drill lines

use of powered harrows and rolling of fields after drilling breaks up clods

use of autumn sown cereals exposes bare fields to winter rainfall

PROBLEMS CAUSED BY EROSION Loss of humus: this is source of nutrients due to decomposition, helps to maintain a uniform pH, helps prevent drying and shrinking of the soil & improves aeration (in combination with clay) Also leads to sedimentation in reservoirs. Shortens their lives and reduces capacity. Reduces soil productivity.

PREVENTING SOIL EROSION WIND EROSION Maintenance of a good soil structure is important, so that particles do not become dispersed with cultivation. Keep a high level of cations e.g Calcium in the soil - this can be achieved by liming e.g on the fields near Knight's Hill, King's Lynn this has been done. Provide sufficient levels of organic matter - this is difficult when arable rotations are not used as only small organic residues are left, but ploughing in the remains of a clover or ley can help Keep the soil moist so that surface tension films (adhesion) keep the particles together - ploughing just before high winds can help as this brings wetter soil to the surface but can't be used very often as it depletes soil moisture, and can't be done if fields have been drilled. In some parts of the world, the soil is never turned over as the water that would be lost by evaporation as a result is necessary for plants to grow in these areas.

Try to maintain a loam texture - this can be done by marling: adding clay - this process has been used extensively in the Fens. A mixed farming system with manure production is a good compromise.

WINDBREAKS Large hedges or shelter belts of poplar trees are very useful. These are effective for about 5 times the height of the tree height to windward and up to twenty five times to leeward. They are ecologically preferable to fences, but can be expensive to plant and slow to grow. They take up valuable land, they shade out growing crops, and compete for soil moisture with anything growing in the field. With the large fields used in modem mechanised farming, they can only protect a fraction of the field. Intercropping: the planting of rows of crops of differing heights e.g a short crop in between a taller one e.g barley interspersed with carrots, root crops etc. Crop is sometimes removed by rotovator and is not grown for commercial gain. Movable hessian screens have also been used, around 1m high -labour intensive and only used on tender high-value crops. Tend only to be used in Class 1 land areas.

Can try to use methods which don't involve the traditional methods of producing a seedbed: minimum cultivation methods. Straw (called trash in US) can be left spread over soil, or loosen stubble with special harrows (stubble mulching) Need to protect roots which would otherwise be left exposed by removal of fine particles: remember that silt is the most susceptible to erosion: sand grains are larger, but clay has greater cohesion.

WATER EROSION

Basic principles are to keep the ground as covered as possible, in order to try to slow down overland flow.

Protect the soil from raindrop impact - can be achieved with very small levels of organic mulch.

Fields of potatoes in Norfolk are often subject to erosion losses after "earthing up' creates artificial rills.

Contour ploughing

Break up soil pans

Maintain ditches - can silt up quickly.

METHODS OF SOLVING PROBLEMS Wind Erosion: Maintain soil structure, Marling, Windbreaks, Mulching Water Erosion: Mulching, Contour ploughing, Soil and crop management, Terracing and 'Check dams' in the gullies

DUST BOWL 1930'S USA Try a search on 'Dust Bowl' USA too to see what effects this period had on American society, or read Steinbeck's 'The Grapes of Wrath'. There are some excellent sites on this area. This was, in part, caused by a series of hot, dry years which depleted the vegetation cover and dried out the soil. The effects of drought were exacerbated by years of overgrazing and unsatisfactory farming techniques. Prime cause was rapid expansion of wheat cultivation in the Great Plains, in states like Oklahoma. The number of cultivated hectares doubled during World War 1 as tractors rolled out into the fields. After the war, wheat cultivation continued, with development of combine harvester and government assistance. Farmers gained high yields and saw no reason to change, but the soil structure was breaking down, and strong winds led the infamous 'black blizzards'. One dust cloud covered an area of 1.35 million square miles, and stood 3 miles high, obliterating the sun. It reached from Canada to Texas. A 4 day storm in May 1934 transported 300 million tonnes of soil.

Run off volume Wind Strength Vegetation cover Steep Slope angle High Infiltration capacity Long slope length Soil management techniques used Terracing Planting of windbreaks to shorten fetch Rainfall erosivity level Poor quality soil in Capability study