LECTUER 9 SOIL MORPHOLOGY Soil Morphology Is the Aspect of Soil Science That Deals with Field Observable Attributes of a Soil Wi

LECTUER 9

SOIL MORPHOLOGY Soil morphology is the aspect of Soil Science that deals with field observable attributes of a soil within the various soil horizons or layers and the description of the arrangement of such horizons. They are properties of the soil that can be observed either by sight of feel. The observable attributes described in the field include: Soil Colour, composition, form, soil structure, organisation of the soil, features such as mottling, distribution of roots and pores, evidence of translocated materials (e.g. carbonates, iron, manganese, carbon and clay) and the soil consistence. These observations are typically performed on a soil profile. A profile is a vertical two dimensional cut in the soil that binds one side of a pedon.

A Typical Soil Profile

Soil Horizons can be designated as Master or Transitional Horizons.

MASTER HORIZONS Horizontal layers of soil called horizons can be described by their different morphological characteristics. Capital letters designate master horizons. They are as follows: O Horizon: O horizon is an organic horizon. It is a surface layer characterized by accumulation of organic matter which may be dominated by partially decomposed or undecomposed organic material. A Horizon: The A horizon is the uppermost mineral layer. It may lie below the O horizon. An A horizon has a high concentration of humus and it is usually regarded as an elluvial surface horizon (an horizon characterized by the movement away of materials e.g carbonates, clay etc) E Horizon: The E horizon is also an elluvial surface horizon. It has experience the loss of clay, organic matter, iron and aluminum oxides with the resultant accumulation of quartz and other resistant minerals. It is also characterized by bleached appearance because of the loss of the above materials. B Horizon: The B horizon is a subsurface mineral horizon showing evidence of illuvial accumulation of silicate clay, iron, aluminum, gypsum, silica, carbonate, sesquioxides, coating of sesquioxides etc moved down from A and E horizons above. C Horizon: The C horizon is a layer of minimal alteration. It consists majorly the weathered parent materials. Material may be similar to or may be unlike that of the other horizons formed above. R Layer: An R layer refers to hard consolidated bedrock. The R layer is presumed to be the material from which the overlying horizons are developed. If it is a different material from that of the overlying mantle, it is represented by IIR, indicating what is called Lithological Discountinuity.

TRANSITIONAL HORIZONS Transitional horizons occurs when the soil layers are dominated by properties of one master horizon but have the subordinate properties of another. These are designated by two capital letters, for example, AB, EB, BE, or BC. The first letter represents the dominant horizon characteristics while the second represents the subordinate one.

SOIL MORPHOLOGICAL CHARACTERISTICS 1. COLOR The most obvious soil characteristic is color. Soils come in a wide range of colors – shades of brown, red, orange, yellow, gray, and even blue or green. Color alone does not affect a soil,

but it is often a reliable indicator of other soil properties. Although color is not used as a quantitative measure, it does give a good indication of the organic matter contents, drainage conditions and the mineralogical compositions of a soil at a specific point in time. For example, a black soil may indicate the presence of organic matter while red soil colouration may indicate the presence of oxidized iron while gray color indicates water saturation. Soil color is described by three attributes: Hue, Value, and Chroma. Hue: is the dominant spectral color. It is related to the wavelength of light reflected by soil particles. Value: Value is the lightness or darkness of the color. It is a measure of the amount of light it can reflect. Chroma: Chroma is the strength or purity of the color. Munsell Soil Color Charts is the book or chart used in the determination of soil color and contain several series of distinctively colored chips. Each page represents a different hue. The chart normally has 15 pages, each with a number (e.g. 10, 7.5, 5, or 2.5) followed by a letter or letters indicating red (R), yellow (Y), green (G), blue (B), or combinations of these. Value units range between 0 and 10 with the numbers ascending vertically on the page from the lowest to the highest numbers. This indicates dark to light values. Chroma units are arranged horizontally across the page from 0 to 10, increasing in numbers from left to right. Low numbers indicate an increase in grayness, while high numbers signify a pure color with little mixing with other hues.

Colour Mottles The mottles must be described by their abundance, size, and contrast to the background. Abundance: Abundance is the relative amount of mottling. Abundance can be classified as few; common or many. Size: Size is a measure of the estimated average diameter of individual mottles along their greatest dimension. It could be fine; medium; or coarse. Contrast: Contrast is an indication of the relative difference in color between the soil matrix and mottles. Mottle contrast may be described as faint, distinct or prominent

2. TEXTURE A soil’s texture depends on its content of the three main mineral components of the soil: sand, silt, and clay. Texture is the relative percentage of each particle size in a soil. Texture differences can affect many other physical and chemical properties and are therefore important in measures such as soil productivity. Soils with predominantly large particles tend to drain quickly and have lower fertility. Very fine –textured soils may be poorly drained, tend to become waterlogged, and are therefore not well-suited for agriculture. Soils with a medium texture and a relatively even proportion of all particle sizes are most versatile. A combination of 10 to 20 percent clay, along with sand and silt in roughly equal amounts, and a good quantity of organic materials, is considered an ideal mixture for productive soil.

3. AGGREGATION Individual soil particles tend to be bound together into larger units referred to as aggregates or soil peds. Aggregation occurs as a result of complex chemical forces acting on small soil components or when organisms and organic matter in soil act as glue binding particles together. Soil aggregates form soil structure, defined by the shape, size, and strength of the aggregates. There are three main soil shapes: platelike, in which the aggregates are flat and mostly horizontal; prismlike, meaning greater in vertical than in horizontal dimension; and blocklike, roughly equal in horizontal and vertical dimensions and either angular or rounded.

Soil peds range in size from very fine-less than 1mm to very coarse-greater than 10 mm. The measure of strength or grade refers to the stability of the structural unit and is ranked as weak, moderate, or strong. Very young or sandy soils may have no discernible structure.