Welcome to the 2004 Massachusetts Envirothon Workshop Soils Overview Workshop Part III Tom Cochran USDA-NRCS Franklin Co., MA Some material courtesy of Jim Turenne USDA-NRCS, Rhode Island
So what? Physical differences The size of sand and clay give a horizon different physical & chemical properties. Sand particles are much larger than clay particles and, sand is blocky shaped while clay is platy. A collection of sand particlescreateair spaces that are larger and more connected than those created by a collection of clay particles. Chemical differences Sand particles have no charge on their surface. Clay particles have negative charge on their surface and adsorb elemental nutrients such as Ca, Mg, Fe, NO3, PO4.
Particle surface area A sand particle that is 1 mm in diameter has a volume of approximately 0.5 mm3 and a surface areaof6mm2 with no electrical charge. A clay particle (0.002mm dia.) has a volume of approximately 4 x 10-9 mm3, so 12.5 million clay particles will fit inside the 1-mm sand particle. Each clayparticle has a surface area of about 0.012 mm2. Therefore, 12.5 million clay particles will provide 150,000 mm2 of surface area with negative charge sites. The colloidal surface area of a 15-cm thick slice of a hectare of clay soil could be 700,000 km2 (270,000 mi2), which is greater than the area of France (Brady & Weil, 1996).
Soil Structure Definition: Soil structure is the natural organization of soil particles into units called peds. When structure is examined, its type, grade, & size is determined, and recorded in that order. Most structure types in New England are granular, subangular blocky, massive, or single grain because clay contents are usually less than 40%. Grades: Structureless – no discrete unit observable Weak – units are barely observable Moderate – unit well-formed & evident Strong – units are distinct and separate cleanly when disturbed
Granular crumb size units; often associated with A horizons that contain organic material Sub-angular blocky rounded edges and faces; often associated with B horizons Massive No structural units; material is a coherent mass Single grain No structural units; loose sand
Soil structural size Names for structure size are as follows, and size values vary by the structural type. For all structure except platy, Very fine Fine Medium Coarse Very coarse Extremely coarse For platy structure, Very thin Thin Medium Thick Very thick Extremely coarse
Soil Color The end product of organic matter decomposition is humus, which is a black color that stains surfaces. The humus is responsible for most of the black colors of an A horizon. Iron oxide coats soil particles and gives them the reddish-brown color of rust. Sand grains are mostly quartz material, which is naturally a gray color. Red sand is just quartz coated with iron rust. Mottling in a well-drained soil is usually due to the mineral coatings on the particles. Mottling in wetter soils can be caused by the reduction of the iron in the coatings.
Major Forms of Iron and Effect on Soil Color FormChemical FormulaColor Ferrous oxide FeO Gray Ferric oxide (Hematite) Fe2O3 Red Hydrated ferric oxide (Limonite) 2Fe2O33H2O Yellow
Soil Color Book The 10YR page of the Munsell color book. The Munsell color book is used to document color in a standard notation. Hue: Dominant spectral color. Value found in th top right-hand corner of each page. Value: The degree of light/dark of a color in relation to a neutral gray scale. Values along the left-hand side of each page. Chroma: Strength of hue. Values along the bottom of each page.
Reading Soil Colors Optimum conditions Natural light Clear, sunny day Midday Light at right angles Soil moist NO sunglasses!
Redoximorphic features e- = Eating OM Free electrons microbes In the presence of air, free oxygen molecules (O2) take the electrons (reduction), but when the soil is saturated there is no free O2. Under saturated conditions other oxidized molecules take the electrons in this order. NO3 Fe2O3 SO4 CO2 Nitrate rust sulfate carbon dioxide Scientists use the reduction of rust as an indicator of frequent saturated conditions because it is the first reduction reaction that can be seen by the naked eye.
Redoximorphic features continued Rust is the oxidated state of iron (Fe2O3). Under saturated conditions, the free electrons produced by microbial respiration remove the O3 from the iron. The O3 then forms water with the hydrogen ions that are plentiful in soil solution. 6e- + 6H+ + Fe2O3 ------ 2Fe2+ + 3H2O The iron ion (Fe2+) is soluble in the soil solution, and as the water drains from the soil the iron moves with it. This causes the iron to be removed from the particle coatings and be either completely removed from the soil profile or deposited in another place in the profile. Scientists interpret the absence of iron and the patterns of ironconcentrations when evaluating the drainage class of a soil.
Redoximorphic Features After the matrix color is determined, record the color patterns of the redox features if present. Can be very complex. Describe color, abundance, size, contrast, shape, and location.
Contrast of Redox Contrast refers to the degree of visual distinction between associated colors Faint-- evident only on close examination. Distinct -- readily seen. Prominent-- contrasts strongly.
Abundance and Size of Redox Abundance Few -- less than 2% Common -- 2 to 20% Many -- more than 20% Size Fine -- < 5 mm Medium-- 5 to 15 mm Coarse -- > 15 mm
Soil Drainage Classes Drainage class is determined in the field by observing landscape position and interpreting redoximorphic features. The table below is used to interpret the redoximorphic features.
Using a Soil survey Organization: Begins with general descriptions of survey area. Information about the soil forming factors is found here. Next, are detailed descriptions of each soil map unit. Typically, information includes profile depth drainage class topographic position Horizonation Permeability Available water capacity pH range agricultural suitability Woodland suitability urban development suitability Capability subclass suitability
Organization (continued) The Use and Management of the Soils section follows the detailed descriptions. This section explains the interpretations presented in the tables that follow, which include Yields per Acre Land capability classification Woodland Management & productivity Recreational uses Wildlife habitat Engineering uses Building site development Waste applications Construction material suitability Water management
Using the interpretation tables Let’s practice by using a few of the Tables. We will use Table 11 (Building Site Development)from the Worcester Co. South survey to determine the suitability of a Paxton soil for a dwelling with a basement. We find the map unit of interest in the left-hand column of Table 11 and the use of interest in the top row. At the intersection of this column and row is the interpretation. Page 61 of the Use and Management of Soils section explains the meaning of the interpretation listed in the table.
Next, let’s see if we can economically install an on-site sewage treatment system with our dwelling. We will need to use Table 12 (Sanitary Facilities) to see if there are any limitations to the operation of a septic absorption field. Again, page 61 of the Use and Management of Soils section explains the meaning of the interpretation listed in the table.
Farmland Interpretations Look for thick dark topsoil layer. Textures of upper 20 inches should not be too sandy. No large stones or boulders. Not too steep, slope < 8%. Site may be wooded.