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5/1/2006

SoilGeneral

Properties of forest soils

Soil is defined as the upper part of the Earth’s crust. It may consist of solid rock, loose deposits such as till, or by organic formations such as peat. Four main materials are basic soil components.

  • Mineral particles

  • Organic matter

  • Soil air

  • Soil water

Mineral particles and organic matter are solid components whereas air and water are found in pores among the solid particles. Volumes of soil air and soil water together make up the soil pore volume-the porosity.

The proportions of the four components vary considerably among soils, with depth, and with climate. The surface layer of a Swedish forest soil typically can consist of equal proportions of each of the four components. With soil depth the share of organic matter is almost reduced to zero, and the porosity, thus, decreases from about 50 to about 30%. Soils are continuously affected by changes in temperature, weathering, living organisms, Lateral soil water movement, substances deposited from the atmosphere, etc. Through these processes different soil types are developed. The process also results in varying amounts of organic matter in these soils, which is often reflected in the thickness of the humus layer and the humus form; examples are mor and mull. The soil type forming factors controlling soil-type-forming processes are:

  • parent rock material

  • climate

  • topography

  • soil organisms

  • time (age)

The parent rock-mother material-influences soil type formation and soil properties through its mineral composition, grain size distribution-the Soil parent material-and thickness of soil cover. In Scandinavia most mineral soils have been transported by ice, water or air. Therefore they may have another composition than that of the parent rock at the location where they were deposited. In Sweden the most common Soil parent material is till, which is a mixture of materials of different grain size classes. These are normally coarse-grained but, due to young soils and the low degree of weathering, they are relatively rich in easily weathered minerals.

Variations in temperature, precipitation and frost influence soil processes through the regulation of production and decomposition of organic matter, and also through soil chemical, biological and physical processes like weathering and leaching. In Sweden precipitation as compared to evapotranspiration is larger, which results in leaching. Freezing processes in soils during winter positively affect soil structure. The climate factor may also include atmospheric wet deposition of acid compounds or other pollutants. Deposition affects nutrient availability and soil acidity.

The level of the ground water table and the Lateral soil water movement is determined by topography, which also influence local climate and soil organisms. The availability of oxygen in the soil is strongly related to soil water content. Thus, a high ground water table may cause a lack of oxygen, which in turn influences soil formation through decreased decomposition of organic matter, resulting in peat formation and increased solubility of different metal salts.

Organisms, including plants, influence soil through production and decomposition of organic matter and by affecting the storage of soil particles - the soil structure. Organic matter is added to the soil as plant litter and through its decomposition humus is formed. Through the decomposition process nutrients are released from the decomposing material and returned to the circulation between plant and soil. Also Man, through farming and forestry, has an influence on the properties of soils, which is overwhelming. Harvest of biomass, for example, results in loss of nutrients, and biological acidification.

Time is an important soil type forming factor. Once formed, soil types develop and change with time. Soil types in Sweden are all formed after the last glaciation, ending 12000 to 8000 years ago. In comparison soils in tropical regions may be millions of years of age. Changes themselves may also influence the continued soil type development. An example is the formation of dense layers caused by the precipitation of iron compounds or compounds of other substances. Such dense layers influence water movements as well as root distribution and thus the continued soil type development.

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