Examine the processes that affect soil characteristics.

I am going to discuss this question under three different aspects - the processes of humification, leaching and weathering and how they affect brown earth soils.

The first process I am going to discuss is humification. Humification is a vital process in the formation of brown earth soils and affects the characteristics of humus content, colour, ph, structure and water content. Humification in brown earth soils is significantly influenced by the cool temperate oceanic found in northwestern Europe and the native vegetation. The cool temperate climate provides up to 1500mm of rainfall per year and mild temperatures ranging from 6-16 degrees celsius. This mild climate allows humification to happen throughout the year as soil temperatures are rarely below

0 degrees.

The native vegetation in northwestern Europe is deciduous woodland. The leaf-fall in the autumn and decaying trees provides plentiful organic matter, which is converted into humus by micro-organisms. This results in a high humus content of brown earths, which in turn makes them fertile soils for farming. Humus also has an influence on soil colour, giving the brown earth their brown colour.

Humification also affects soil structure as the high humus content of brown earths binds the soil particles together and this results in the crumb structure. The crumb structure of the brown earths allows a higher water content as the water (and air) can move in the pores between the soil peds (small lumps of soil made up of soil grains).

This means the brown earths can hold more water, which allows plants to soak up water and dissolved nutrients.

Humification also affects the ph of the soil as the high humus content of brown eaths prevents them being too acidic or alkaline. The neutral or near neutral ph of brown earths means that it is much more suitable for micro-organisms and farming.

The relief in some areas also affect humification and this in turn affects soil characteristics. Humification is slowed in high mountains, where temperature are colder and where there is an absence of enough organic matter.

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Leaching is another major process in brown earth soils. The cool temperate climate of northwestern Europe provides plentiful rainfall year round (between 800 - 1500mm per year) and this results in the leaching of the humus content of the brown earth soils.

The leaching adds to the fertility of the soil by washing the nutrients down through and around the soil, which makes it easier for plants to absorb the dissolved nutrients such as iron, phosphorous and potassium.

Leaching also affects the soil colour as brown earths tend to have a light brown colour due to the affect of leaching. The light brown colour is the same down through the different horizons of the soil.

Leaching also affects the acidity of brown earth soils as the rainfall picks up acidity from the humus and washes it through the soil. In some areas acidic brown earth soils form because of the heavy rainfall leading to leaching by acidic rainwater.

Leaching can also affect the acidity of the soils through the effect of chemical weathering on the parent rock, which may produce slightly acidic brown earths, e.g. sandstone in West Cork. These acidic brown earths often have a pale brown colour.

Leaching can also cause chemical weathering or rocks, e.g. hydrolysis of granite produces clay particles, which are a component of brown earths.

In certain areas, where severe leaching occurs, brown earths may change into gley soils as the soil becomes waterlogged and lacks oxygen.

Weathering is the third major process in brown earth soil formation. Weathering is the break down of rocks into smaller components and happens in mechanical, chemical and biological forms. Mechanical weathering is very important process for the formation of brown earths. It is caused by freeze thaw action on rocks in upland areas, which provides the soil grains that make up the soil as the scree falls to lowland areas. Soil grains based on sandstone parent rock may provide slightly acidic soils and those based on limestone provide slightly alkaline brown earths.Living organisms and plants prefer slightly acidic or neutral soils, and for this reason brown earths contain a wide variety of living things, whcih aids humification. These soils are the most productive for farming. The parent rock can also influence the colour of brown earths, varying the

2 colour from pale in the case of sandstone parent rock to darker brown in the case of limestone.

Weathering provides the mineral content of the soils.Chemical weathering of rocks releases important nutrients into the soils such as phosphorous, calcium and iron. Calcium is released from limestone, clay particles from granite, sand particles from sandstone and iron form iron oxide. As brown earths formed on various parent material, it contains a good mix of nutrients.

As brown earths formed in the last 10000 years after the end of the last ice age, brown earths are based on a variety of parent material and for this reason they have a mixture of sand, silt and clay. The clay comes from the chemical weathering of granite and deposition by melting glaciers, and the mechanical weathering of sandstone and other rocks releases sand particles. The silt comes from river flood plains. This mixture results in a loam texture, where the brown earth soil is crumbly to touch and has a crumb structure. This structure allow water to pass between the soil peds (small lumps of soil made up of soil grains) and thus prevents waterlogging. The water that passes through brown earth soils allows plant absorption of nutrients, enables the survival of the micro-organisms and and binds together the soil to prevent erosion.

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4 aspect marking scheme 3 aspect marking scheme aspect heading = 4 marks aspect heading = 4 marks

6 SRPs x 2 = 12 marks 8 SRPs x 2 = 16 marks overall coherence = 4 marks overall coherence = 7 or 6 marks

Aspect Total: 20 marks Aspect Total: 27 or 26

Essay total: 4 x 20 marks = 80 marks Essay total: 27 + 27 + 26 = 80 marks

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