Soil is a dirty word; dirty in the sense that it is misused and misunderstood. All of that brown granular stuff on the ground is not soil, some of it is sediment. The term sediment is not synonomous with soil. Technically, soils form whereas sediment is deposited.
Soils are made-up of two components--inorganic and organic. The inorganic component is rock which is "weathered" into particles. The organic component is that part resulting from microorganisms, earthworms, feces, decaying animals, plant litter, and even roots. In a very real sense, soil is a transition zone between rocks and plants.
Soil formation is the result of five factors--parent material, slope, climate, vegetation, and time. The underlying geology affects the nature of the inorganic component. For example, soils formed from limestones have different properties than those formed from granites or on redeposited sediments. Similarly, soils on flat surfaces tend to be deeper or thicker than those on slopes. Precipitation and temperature conditions affect weathering though chemical and physical means (e.g., moisture desolves rocks, ice cracks them apart). They can also affect flora. Desert soils are low in organic matter much in contrast to grasslands. Finally, the temporal element is all important. The longer the processes are operative the more fully developed soils become.
The combination of soil formation elements results in variations in several soil properties, which can be dichotomized as physical and chemical. Physical properties include the "texture" of particles (which is classified in percentages of sand, silt, and clay), the "structure" of peds (platy, prismatic, blocky, or spheroidal), and "rockiness" all three of which affect pore space and moisture retention capabilities, and "color." Chemical properties include such things as reaction (pH), nitrogen, phosporous, potassium, soluble salts, calcium, and magnesium.
Although soils form over time they also have morphological characteristics which are manifest in horizons, which are not unlike the layers of a cake [example]. In effect, soil scientists or pedologists infer soil formation processes by examining horizon chararcteristics after first describing them. Horizons can be seen in one of three ways--natural exposures (such as stream banks), pits or trenches excavated for the purpose of examining soil profiles, or cores (samples extracted with a specially designed tool). Perhaps not surprisingly, profiles vary considerably in depth.
The importance of soil depth varies considerably with its intended use. If a building is to be constructed on a particular site, the builder should be concerned about everything from the surface to bedrock, and perhaps deeper. Farmers, in contrast, need only concern themselves with the upper few decimeters. The nature of field investigations will, accordingly, vary with the purpose to which soils will be used.
Much of soil analysis takes place in the laboratory. Indeed, detailed analysis of both physical and chemical properties (e.g., the hydrometer method of particle size analysis, and nutrient assessments) can only be done in the lab. Other analyses can be done in the field, and certainly the collection of all soil data is done in the field.
Analyses done in the field are, as one might expect, rather crude and good only for general approximations. There exist various kits which contain bottled agents used for assessing chemical properties. Perhaps the most commonly used is the La Motte soil test kit. In addition, there are portable pH meters, soil moisture probes, and penetrometers. The pH meters are not anywhere near as accurate as larger desk-top models used in labs. Also, qualities such as soil moisture and compaction can change with a little rain, thereby rendering their reliability questionable.
Soil texture is perhaps the most important physical property and is best evaluated analyzing samples in a fully-equiped laboratory. Such analyses results in a detailed description by means of a soil texture triangle. However, field workers often fine it convenient to simply use a field determination of texture based on feel.
Sandy loam: Many sand grains visible. Moist cast bears careful handling. Squeezed when dry it forms a weak cast that crumbles at light touch.
Loam: Some sand grains visible. Dry cast bears careful handling. Wet cast can be deformed slightly without crumbling. Forms a rough broken ribben when rolled between thumb and forefinger. Has a relatively even mixture of sand, silt, and clay.
Silt loam: Dry cast may be handled freely. Moist cast bears handling. When rolled between thumb and forefinger it forms a rough broken ribben that will not bear its own weight. Dry material feels like flour.
Silty clay loam: Moist cast bears handling. Wet cast can be kneaded into different shapes but with a tendency to crack as moisture is worked out. When rolled between thumb and forefinger it forms a thin smooth ribbon that just bears its own weight.
Clay loam: Few sand grains visible. Wet cast can be molded into different shapes but tends to break as moisture is worked out. When rolled between thumb and forefinger it forms a thin smooth slick ribbon that will bear its own weight.
Silty clay, or Clay: Wet cast can be molded into different shapes without breaking. When rolled between thumb and forefinger it forms a thin ribbon that easily bears its own weight.
Soil color can be determined quickly and easily in either a lab or the field with a Munsell Soil Color Chart.
For the most part, field tests involve top soil and are agriculturally related. They are, quite literally, quick and dirty, and involve point samples collected with a hand trowel. For more precise assessments samples can be sent to the agricultural extension service at the state's land grant university [ Texas Example ]. Samples of approximately 250 ml can be bagged either in zip-lock plastic bags or plastic-lined paper bags provided by the state lab or your county's agricultural extension agent.[sample] The cost of analyzing each sample varies between $50 and $75, depending on which analyses are performed.[sample form][sample results] The typical procedure for collection such samples is not to scoop up a bag full from one spot, but rather to move around the parcel of land under investigation and scoop up small "pinches" from five or six spots and mix them together in one bag.
Analyses that involve depths greater than the surface--soil horizons--require different field and laboratory strategies. Samples have to be collected from each horizon [example]. One approach is to excavate a trench and describe the profile and collect samples to be analyzed later in the lab. Without doubt describing profiles in trenches is the best approach, but it also requires great knowledge and skill that few possess. Soil scientists always progress in their descriptions from the top down. Given that soils actually form from the bottom up, it really makes sense to progress in this direction. Descriptions are typically recorded on standard forms developed by the U.S. Soil Survey Staff. Recording depths and thicknessess of the horizons is most important. Trowels are important tools using in describing profiles. They are used to scrap clean, fresh faces, and to extract from the horizons samples to be analyzed in the lab.
An alternative approach to trenching is to take cores using a soil corer. This involves first selecting a sample point. Here, the corer is pushed into the ground to the first marked increment. The corer is then pulled up and the sample is then extracted and placed in a zip-lock back and marked appropriately--including site location and horizon. The upper and lower ends of the sample should also be indicated. The corer is then reinserted into the same hole, and pushed until the second increment reaches the surface. The corer is then extracted, the sample removed and placed in another appropriately labeled bag. The process is repeated until bedrock or the end of the corer is reached. All the samples are then taken to the lab for analyses.
One "quick-and-dirty" technique for obtaining information about soils from natural exposures is to take a digital image of the profile with something of known size for scale in the photograph. Samples from the more conspicuous horizons can be collected and brought back to the lab for analyses. Back in the lab the digital image can be used as the basis for drawing a profile, and the analyzed samples used for finalizing the description.
Created by William E. Doolittle. Last revised 26 June 2013, wed