Table of Contents
The experiment covers the analysis of different soil particle sizes, permeability and soil water retention capacity which are affected by the size of soil particles. To separate the different soil particles, the soil samples are passed through sieves. The particles obtained are gravel, fine gravel, coarse and fine sand, and silt and clay. The apparatus used in the experiment comprise of four plastic sieves, mesh of different sizes, a bottom pan, a lid to prevent excessive dust and labels for identification purposes. Other additional materials include paper towels, funnels, beakers, a stopwatch and a graduated cylinder. To obtain the proportions of different soil particles in percentage, the previously obtained total mass is divided into the mass of each particle size. To determine the soil water properties, the length of time the water takes to seep through different soil samples, is measured using a stopwatch. The amount of water retained in different soil samples is obtained by subtracting the water collected in the graduated cylinder from the initial capacity of 50 millimeters. Acceptability of the test results is determined by considering Precision and bias during the experiment.
Soil is the naturally occurring thin layer of loose sediments forming the earth crust. It is a natural medium in which plants grow. Soil acts as a reservoir of water and nutrients for plants and provides mechanical anchorage.
Soil is formed through the process of rock weathering or from the decomposition of organic materials. When rocks have broken down, soil may be classified as stones, sand, silt or clay depending on the particle size. Soil is also composed of organic materials obtained from the decay of once-living organisms. These materials form humus which is a chemically active component that bonds itself to other soil constituents. Soil texture depends on the relative proportions of clay, silt and sand present in a given soil. For example, clay, bonds sand and silt together into tiny soil particles. Large portions of clay in a given soil cause too much binding and the soil becomes a hard and lumpy mass. Soil also contains other components like air, water, and plant nutrients. The proportions of soil components vary depending on both environmental and human factors resulting in different soil characteristics and properties.
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Soil characteristics, even in a small field can vary significantly from one point to another. Soils vary in color, depth and texture. To get the most accurate results in the investigation of soil properties samples should be taken everywhere in the selected area. Chemical composition analysis of soil should be done soon after extraction. This is because the soil chemistry starts changing once the soil is removed from its natural ecosystem. Preservation techniques such as freezing or air drying are employed to slow down the chemical changes in the soil after extraction. Soil samples are analyzed to establish their composition, nutrient and contaminant content. Other soil characteristics analyzed are acidity and PH level.
Separation of soil particles
The soil samples obtained from two different locations were used in the experiment. One sample was collected from a level grass-covered surface after all the vegetation had been removed. The other sample was obtained from a sandy area. The soil samples were spread on paper towels to dry. After drying, the soil aggregates, were crushed in the hand, and the weight of both samples obtained. The Screen Sieve Kit was used to separate the soil samples into various particle sizes. The screen sieves were arranged with the largest screen size on top, decreasing proportionally in size to the closed-bottom container. One of the soil samples was placed in the uppermost sieve and covered. It was lightly shaken using a back and forth motion. The particles were then removed carefully from each sieve, and their separate weights obtained. The mass of each particle size was converted into a percentage by dividing the previously obtained total mass into the mass of each particle size. The sieves were then cleaned and the procedure repeated for the other soil sample.
Examining permeability and water retention in soil
The sieves were arranged in the same order as in the previous procedure. The particles were then taken from the sieves containing fine gravel and coarse gravel and the bottom pan with silt and clay. Three funnels were placed side by side with a loose wading of cotton in the neck of each. Equal amounts of fine gravel were placed in the first funnel, the coarse sand in the second funnel with the silt and clay particles placed in the third funnel. Three beakers were filled with 50 millimeters of water separately. To determine the rate at with which water seeped through the different constituents, the length of time the water took to drain through each funnel was determined using a stopwatch. To obtain the amount of water in the bottom pan the amount collected in the bottom pan for each sample was subtracted from the original 50 milliliters in the beaker. The results were used to determine the soil’s ability to hold moisture.
RESULTS AND OBSERVATIONS
Individual gravel and sand particles were easily distinguishable with the naked eye though the gravel particles were larger than the sand particles. Clay and silt particles were difficult to distinguish with the naked eye.
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Water was observed to drain through gravel within the shortest time compared with the other soil components. The second highest infiltration rate was observed in fine gravel. Water took the longest time to drain through silt and clay. Also, it was observed that sand retains only 5 millimeters of the initial 50 millimeters of water which is the lowest water retention. The mixture of clay and silt retains 25 millimeters of water.
It was observed that of all the soil samples sand contained the greatest amount of coarse particles. This is because the top soil had had not enough time to develop completely. Silt and clay were the predominant particles in the soil sample extracted from the level grass covered surface. Another observation made when the soil samples were set out to dry was that, the size of the soil particles is directly related to the soils ability to hold moisture. This phenomenon further presents itself when investigating the soil water permeability. Clay and silt particles were found to retain most water. These two soils have small pores in between the soil particles. Clay soil having the finest particles takes the longest to dry. While investigating soil permeability, the funnel containing fine gravel drained within the shortest time thus indicating that gravel has the highest permeability. This is due to the large pores spaces in between the large soil particles which provide a direct path for water to pass through easily.
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Soil infiltration rate depends on two main factors. These are soil texture and soil water content. The soil infiltration rate might have been affected by the soil samples not drying thoroughly. Water infiltrates faster in dry soil than soil containing moisture. The quantity of water contained in the soil does not remain constant but varies with time. The mixture of silt and clay can be separated by placing it in a beaker containing water and stirring. After the mixture has settled for about 30 minutes, clay particles, which remain suspended in the water, can be obtained through evaporation. The upward movement of water in clay soil is slower than in sand soil but covers long distances.
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