LAB 11: ALLUVIAL SEDIMENT AND SOIL SAMPLES ANALYSIS
-To learn simple techniques for measuring soil properties.
-Compare samples of soil and alluvial sediment.
1. SOIL LAYER STRUCTURE
Soils begin to develop when either rocks or deposits of loose materials (e.g. glacial, river, wind etc derived-material) are colonized by plants. Once organic decomposition starts to take place among mineral particles or disintegrated rock, differences begin to develop from the surface down through the soil parent material. The vertical differentiation comes about originally from such simple factors as the gradual accumulation of organic material at the surface and removal of fine particles and dissolved matter from the top layers by downward percolating water, followed by deposition at lower levels. As this process continues through time the vertical differentiation becomes stronger and stronger and clear "layers" horizons develop.
In most soils in the mid-latitudes there are commonly several generally recognizable layers:
O horizon: At the surface in humid regions with sufficient vegetation and moderate rates of organic decomposition there is an O horizon. This is a layer of loose organic debris and raw humus.
A horizon: Comes immediately below - typically this horizon has a darker color toward the top due to a concentration of decomposed organic material. The A horizon is sometimes referred to as a zone of depletion since soil water removes particles from it in suspension (eluviation) and solution (leaching).
B horizon: Below the A horizon is the zone of accumulation - which is where most of the material removed from the A horizon is deposited.
C horizon: Does not reflect the movement of matter and organic activity in the upper zones. This layer is unchanged bedrock or unmodified material transported to the site by water, wind or glaciers. It is the mineral material from which the soil has developed (parent material). Under certain circumstances this may be more than one material e.g. if wind blown material is deposited on top of previously deposited glacial material.
The vertical cross-section of a soil from its surface down to the parent material from which it is formed is called the soil profile.
There is no fixed amount of time for a soil to become mature. This is because of the number and variability of the factors that influence soil formation. As a general rule it takes hundreds of thousands of years for soil to reach maturity.
2. CHARACTERISTICS OF SOILS
Soils have physical properties that are useful in differentiating horizons and in identifying the dominant processes occurring in their development. The most important include color, texture, structure, acidity or alkalinity, and capacity to hold and transmit moisture and air. We will consider color and texture here.
Color: This is the most visible attribute. Soils vary from black to brown to reds, yellows, grays etc and near whites. Each color offers a clue to the physical and chemical characteristic of a particular soil. For example, humus is black/brown, and soils high in humus content usually exhibit this color. As the humus content decreases, either as a consequence of low organic activity or leaching, the color gradually fades to light brown or gray. Red and yellow colors are usually due to the presence of iron compounds. In moist environments white indicates that iron has been removed and oxides of silicon and aluminum are present, while in dry climates the same color indicates a high proportion of salts. Rather than just giving a subjective assessment of the color of a soil an objective scheme has been developed where a color is described using charts - referred to as Munsell Color Charts. You will use these in the lab.
Texture: This varies according to the size of the particles that make up the soil. In clay soils particles have diameters less than 0.002 mm. The particles of silt are defined as between 0.002 and 0.065 mm, and sandy soil have diameters between 0.065 and 2.00 mm. Individual particles greater than 2.00 mm are regarded as inert gravel or rock fragments and technically not soil particles. Since no soil is made up of particles of uniform size, the proportions of particles in various size ranges determines the texture of the soil. A loam is a soil texture class in which no one of the three size grades (sand, silt, clay) dominates over the other two.
Soil texture is important in that it helps to determine the capacity of the soil to retain moisture and air, both of which are necessary for plant growth. Soil texture can be determined by sieving the soil through sieves of known diameter and determining the proportion retained in each sieve or by hand texturing (see handout attached to this lab). You will sieve samples and hand texture soil in this exercise.
II. GUIDE TO HAND-TEXTURING: Key to General Texture Groups
Rub the moist soil between thumb and forefinger. If you can feel individual sand grains distinctly as a major part of the mass, proceed to A1. If not proceed to B.
A1 Press the dry soil hard between thumb and forefinger. If it forms a cast that holds together slightly, proceed to A2. If the cast falls apart when pressure is released, it is probably coarse textured (sand or loamy sand). Check this by pressing when moist. If the cast formed can be transferred from one hand to another, proceed to A2. If it cannot, it is coarse (sand, loamy sand).
A2 If the cast formed of dry soil can be transferred from one hand to another, proceed to A3. If it cannot, it is probably moderately coarse textured (sandy loam; fine sandy loam). Check this moist. If the cast of moist soil can be handled readily without breaking proceed to A3. If it can be transferred from one hand to the other, but cannot readily be picked up intact, it is moderately coarse (sandy loam; fine sandy loam).
A3 Squeeze the fully moist (not wet) soil between thumb and forefinger with a shearing stress. If the soil forms a thin smooth "ribbon" that will bear its own weight, proceed to A4. If it forms a rough "ribbon" that breaks under its own weight it is medium textured (loam).
A4 Roll the fully moist soil into a worm-like roll about 1 cm in diameter between the palms of the hands. If the roll can bear its own weight when it is flexed (shake while holding one end), proceed to A5. If the roll breaks easily when flexed, it is moderately fine, (clay loam).
A5 If the roll does not break when flexed, it is fine textured (sandy clay).
B Sand grains cannot be felt distinctly as a major part of the mass (one may feel a few sand grains). Proceed to B1.
B1 Shear the fully moist (but not wet) soil between thumb and forefinger. If it forms a slick unbroken "ribbon" proceed to B2. If it forms a broken rough ribbon it is medium textured (silt loam or very fine sandy loam).
B2 Roll the fully moist soil between palms of the hands. If the roll bears its own weight readily when flexed, proceed to B3. If it breaks, it is moderately fine (silty clay loam).
B3 If the roll bears its own weight when flexed, it is fine textured (clay or silty clay).
In the last lab you surveyed a topographic cross-section from the edge of Williams Creek outward across point-bar deposits onto the adjacent flood plain and collected soil samples along the cross-section. In this lab you will relate the characteristics of the soil samples to their position relative to the stream and attempt to understand the relation between soil development and stream processes.
Lab 11 Exercises
Back to Table of Contents