Unit 1 Soil Physics (Labs, 3 and 4) Lab Soil Formation, Color, and Texture OBJECTIVES In this lab, you will be introduced to the concepts of soil formation and horizon development. You will observe several soil monoliths from New Jersey to accentuate how diverse soils in a region can be due to geologic events and how horizon development differs in different soils. Additionally, you will measure two very important soil parameters: texture and color. Both parameters are dependent upon geological events that dictate how soil will develop. Soil texture is one of the indicators of how water will move in a particular soil, how much lime will be needed to neutralize acidity, how well the site will support construction activities, etc. You will learn how to classify soil texture by the quick feel method, and by the more elaborate hydrometer method. You will learn how to utilize the Munsell Soil Color book to gain valuable insights on the properties of a soil. INTRODUCTION PARENT MATERIALS AND SOIL FORMATION While the underlying bedrock of some soils is the parent material, many soils did not form directly from the underlying bedrock. Outside forces such as wind, water or ice transported the parent material from one locality to another. For example, the northern part of New Jersey was covered with glaciers on at least two and perhaps as many as four occasions. Each glacier dragged material along with it and deposited it somewhere else. These glacial deposits constitute the parent material of the soil that developed after the glaciers receded. Different soils form from different parent materials. Properties of parent materials that are important in soil formation are rock type, mineral composition, and type of deposition. A mineral is a naturally occurring inorganic substance of fairly consistent chemical composition whose atoms are arranged in a regular pattern so as to create a crystalline structure. Minerals differ from each other in element composition, susceptibility to chemical and physical weathering and their appearance. As a group, minerals may be divided into primary and secondary. Primary minerals are those that formed from the cooling of molten rock. Secondary minerals are those that form from the precipitation or recrystallization of soluble substances. Secondary minerals form from products of primary minerals. The primary minerals in a soil are inherited from the parent material. Soil formation occurs through the action of weathering processes on parent materials over time. The properties of parent materials will affect the weathering rate, and the properties of the soil that is formed. A parent material that was physically broken up at the time of its deposition will weather faster than a residual parent material that is solid rock. If it has been deposited, the parent material must have undergone physical weathering (disintegration) before being transported; hence, soils formed on transported parent materials are generally younger than those formed on residual (sedentary) parent materials. Transported parent materials may be separated by particle size by the mode of transport. 1
SOIL COLOR Soil color is an important soil property that is reported in all soil profile descriptions because it constitutes a useful first approximation of soil conditions and properties. Color can be estimated with a spectrophotometer or other mechanical device; but it is frequently done by visual inspection. The practice of describing soil color first began in Russia, where attempts were made to form a cohesive system of soil color identification. In America, soil colors were occasionally mentioned in reports of the early 1900 s, but no formal system was agreed upon until the 1940 s, when the work of Dorothy Nickerson and Albert H. Munsell led to the use of the color chip system now employed. The system has led to a uniform and systematic description of soil color employed in all current scientific literature. Soil color is used for both soil classification and evaluation. From color, inferences regarding such things as reduction status (i.e., whether or not a soil remains waterlogged for long periods of time), organic matter content, and mineralogy are possible. For example, red, yellow, or reddish brown colors suggest the presence of oxidized iron and are indicative of good aeration and adequate drainage. Poor aeration and imperfect drainage are indicated by blue and gray soil colors, denoting reduced iron. Similarly, a dark brown soil color is usually attributed to organic matter. Minerals can be distinguished by inspection from the differing values of redness; acid sulfate soils are frequently in the gray-green-black spectrum; and types of clays present have also been characterized by color. 1 To classify soil color, a moist representative soil sample is compared to the color chips in a Munsell color book. The Munsell color system describes color in three parts: hue, value, and chroma. For example, a complete color description reads 10YR 4/3. Such a notation translates to: a hue of 10YR, a value of 4, and a chroma of 3. Hue is the spectral or rainbow color and is described by such notations as 10YR (yellowred), 7.5YR (more red, less yellow),.5y (yellow), etc. Each page in the Munsell color book is a different hue. Value is defined as the relative blackness or whiteness, the amount of reflected light, of the color. The value designation is found on the left side of the color book, and increases from the bottom (0 = pure black), to the top (10 = pure white). The chroma notation is the purity of the color or the amount of a particular hue added to gray. The chroma designation is located at the bottom of each page of the color book and increases from left (grayest) to right (least gray or brightest).
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SOIL TEXTURE The relative amounts of different sizes of mineral particles present in a soil have a profound influence on the behavior of soils. The particle size distribution of a soil is summarized by its texture. In order to determine the textural class of a soil (i.e. loam, sandy clay, etc.), the percentage of sand, silt, and clay size particles must be known. Sand is defined in the USDA system as being mineral particles of.0 to 0.05 mm diameter. Silt is defined as having diameters 0.05 to 0.00 mm, and clay is smaller than 0.00 mm ( microns) diameter. Particles larger than.0 mm diameter are not included in the determination of texture but can modify textural class names (e.g., gravelly loam or stony silt loam). Determining Soil Texture by Feel The determination of soil texture by feel is a qualitative technique. This technique is usually used in the field to approximate the texture of a soil when describing a soil profile or for estimating soil properties. To determine soil texture by this technique one simply takes a sample of soil in the hand, moistens the soil, and works the sample until it pliable and large aggregates have been broken down. The sample is then squeezed between the thumb and forefinger to attempt form a ribbon. The length of ribbon formed is determined by the amount of clay in the sample. A higher clay content should result in the formation of a longer ribbon. Specific instructions are given for this technique in the Thien: Texture-by-Feel Analysis diagram (Fig. 1). Determining Soil Texture by Hydrometer Method The hydrometer method is a more accurate and objective way of measuring soil texture. While the texture-by-feel method is utilized in the field, the hydrometer method is a laboratory technique. The hydrometer method is based on Stokes' Law, which describes the rate at which particles of different sizes settle down through a liquid. Stokes' Law states: V = D ( ρ ρ )g 18η (1) p Where: V = velocity of fall (cm/sec) g = acceleration of gravity (cm/s ), usually 980 cm/s D = "equivalent" diameter of particle (cm) (D = 4r ) ρ p = density of particle (g/cm 3 ), about.6 g/cm 3 ρ w = density of the solution (g/cm 3 ), about 1.0 g/cm 3 η = viscosity of the solution (g/cm-s), about 0.010 poise at 0 C, about 0.008 poise at 30 C (1 poise = 1 g/cms) The densities, gravity, and viscosity can be expressed by a constant (k), so: V = kd or V 8, 711D () Since soil separates have different diameters, different fractions will fall at different rates. For example, sand (with a large diameter) will fall faster than clay (with a small diameter). This allows us to calculate the relative proportions of sand, silt, and clay in a sample from the settling time. Precise measurement of particle size distribution by this method requires destruction of the organic matter with hydrogen peroxide; however we will be eliminating this step. It is also good to remember that, with the hydrometer method, you are actually measuring the buoyancy of the w 4
hydrometer in the soil suspension. Measuring the support of the hydrometer provided by the suspension at specified times allow us to estimate the grams of particles that have not yet settled to the bottom. This gives us values for the proportions of (silt + clay) and clay alone, and allows us to calculate proportions of sand and silt. You need to take the temperature of the solution to make corrections for the density of the water. 5
PROCEDURES Soil Texture: Hydrometer Method Each group will do two of the soils on their bench. For each soil: 1.) Weigh out 50 g of soil and transfer to a mixing cup..) Fill the cup approximately 3/4 full with distilled water. 3.) Add 10 ml of dispersing agent (10% sodium hexametaphosphate). The Na + in the (NaPO 3 ) 6 replaces Ca + from the soil clays causing them to disperse. This is a chemical dispersion. 4.) Place on mixer for 10 minutes. Do not start mixer until all cups are in place, and do not remove cups until mixer has completely stopped. This is a mechanical dispersion, which breaks up any soil aggregates. Soil aggregates have a larger diameter and a lower density than individual components and therefore would give a false reading. Take care with blades on mixer! They are sharp! 5.) Transfer the suspension to the glass cylinder, washing all soil from the mixing cup by squeezing a jet of distilled water from a squeeze bottle. 6.) Carefully place the hydrometer in the glass cylinder and add water to bring the volume to the line marked 1130 ml. Be sure the hydrometer is in place before filling to the line. 7.) Remove hydrometer, placing it carefully in its box until needed again. 8.) Holding cylinder steady with one hand, mix suspension vigorously with mixing tool for at least one minute, lifting all particles from the bottom. As soon as you stop mixing, start timing. 9.) Immediately place hydrometer in the cylinder. 10.) Take hydrometer reading, in grams per liter, 40 seconds after you removed mixing stick. Record reading. (The reading is the number visible at the surface of the water line.) 11.) Repeat steps 8-10. Record reading. Average the two 40 second readings. Leave the hydrometer in the suspension. We are assuming that all the sand has settled in this time. 1.) Carefully insert thermometer into suspension. Read and record temperature of the suspension. Temperature changes the viscosity of water, so if the temperature is > 68 F add 0. to hydrometer reading for each degree above, if it is < 68 F subtract 0. for each degree below. 13.) Wait one hour** and take second reading. DO NOT MIX AGAIN. Record reading. We are assuming that all the silt has settled in one hour. Two hours is more accurate. 14.) Read and record temperature. 15.) Calculate percentages of sand, silt, and clay using the equations given below. Determine soil textural class using the Textural Triangle. ** DO THE SOIL COLOR AND TEXTURE-BY-FEEL EXERCISES WHILE WAITING TO TAKE THE 1 HOUR READING. 6
Data Sheet for the Hydrometer Method Soil Name (letter) Sample Weight 50 g 50 g 40 s reading - (1) - () Average of readings Temp. degrees F Corrected 40 s reading Grams sand in sample (50 - corrected 40 s reading) % Sand in sample (calculated) 1 h reading Temp. degrees F Corrected 1 h reading Grams clay in sample (corrected 1 hr reading) % clay in sample % silt (from subtraction) CALCULATIONS Corrected 40 s reading = grams (silt + clay)/liter % Sand = {[(sample weight) - grams (silt + clay)]/ (sample weight)} x 100 Corrected 1 h reading = grams (clay)/liter % Clay = {grams (clay)/(sample weight)} x 100 % Silt = 100 - % Sand - % Clay Soil Texture by Feel 1.) Follow the flow chart titled Thien: Texture-By-Feel Analysis..) Record the texture in the Summary chart under Texture: Feel. Soil Color 1.) Place a pinch of soil in the white spot plate and determine the color using the Munsell color book..) Moisten the sample and determine the color of the moistened sample. 3.) Repeat for each soil and record in the Summary chart. 7
Thien, 1979. J. Agronomic Education (vol. 8). 8
Summary of Results Soil Name (letter) Textural Class: Feel Textural Class: Hydrometer % Sand % Silt % Clay Soil Name Soil Color Dry Soil Color Wet 1 3 REFERENCES 1.) Bigham,J.M. and E.J. Ciolkosz(eds,.) Soil Color. Soil Science Society of America Special Publication # 31. Soil Science Society of America, Inc. Madison, WI. 1993. 9