Magnetic susceptibility as an indicator of layering in soils at Bonamanzi Game Ranch, KwaZulu-Natal, South Africa.

Abstract

All matter has a specific magnetic signal , due to their magnetic properties. These range from a high susceptibility to become magnetised (ferrimagnetic) to a low ability to be magnetised (diamagnetic). Magnetic susceptibility measures the degree to which a substance can be magnetised, and this can be used to identi fy minerals within material and used as an indicator for processes Le., erosion. Therefore magnetic susceptibility has been widely used to investigate soil related research as the values obtained correspond with the types of magnetic mineral s in the soil, mainly the iron oxides such as magnetite and maghaemite (higher MS), and haematite and goethite (lower MS). Thus MS acts as a signature for different types of soils allowing them to be categorised. The amount of magnetic minerals present in the soil is largely dependent on soil processes active in the profile and external factors, such as parent material. Soil processes influence the type or amount of magnetic mineral in the soil, or the strength of the magnetic signal. Lower MS values are associated with horizons that have undergone gleying, eluviation, leaching and reductive weathering. Higher MS values are found in horizons that have undergone illuviation, and hydrolytic and oxidative weathering. Diamagnetic materials, such as calcium carbonate, decrease the magnetic susceptibility by diluting the magnetic signal. The relationship between MS and iron in soil is influenced by both external factors such as parent material ; climate; topography; land use history of the area and time. Similarly these factors significantly contribute to soil genesis and are highly interactive. Parent materials that are igneous are found to form soil with higher magnetic susceptibility, and sedimentary and metamorphic rocks form less magnetic soils. However, it has also been found that materials such as slate are related to soil with high susceptibility, which is argued to result from more rapid weathering and the release of iron in the ionic form. Climate has a direct affect on the soil processes that drive magnetic susceptibility, thus in warm tropical climates magnetic susceptibility is expected to have higher values . Topography has been shown to alter magnetic susceptibility values , and generall y the top and foot of the slope have higher values than the slopes. Land-use history influences magnetic susceptibility as cultivation is likely to disturb the soil causing magnetic susceptibility values to be lower due to mixing of the upper and lower soil layers. Time relates to the age of the soil and older soil either has large magnetic susceptibility values due to more prolonged pedogenesis, or the magnetic particles have weathered out of the profile and magnetic susceptibility is lowered. Past research has found topsoil to have higher frequency dependent magnetic susceptibility than lower soil horizons. Frequency dependent magnetic susceptibility measures fine-grained ferrimagnetic particles with grain sizes between 0.013 and O.027f.lm. Two measurements at low and high frequency (0.46 and 46 kHz respectively) are used to calculate frequency dependent magnetic susceptibility. These are measured using a Bartington MS2B sensor and certain measures need to be taken in order to obtain an accurate measurements of magnetic susceptibility. Frequency dependent magnetic susceptibility values are influenced by factors including burning, organic matter and pedogenesis. However, the nature of contribution of these is still highly debated. Many South African soils are considered to be old soils and the amount of magnetic minerals in these is unknown. It has been found that magnetic minerals such as maghaemite have been depleted due to erosion cycles. Very little research has been undertaken regarding the applicability of magnetic susceptibility for South African soils as most of the research on magnetic susceptibility in soils has been carried out in countries of the northern hemisphere, where soil materials are much younger than in South Africa. The ability of frequency dependent magnetic susceptibility to differentiate between topsoil and subsoil allows it to be used as a method of assessing topsoil erosion. Topsoil erosion has a detrimental effect on the environment and it is vital a rapid field indicator is developed to assess erosion in order to curb the process.