Soil Surveys: How-To

How do you know what’s in your trail site soil horizon? By taking samples and testing them using one or more of several methods outlined below.

  1. Difference between various Standards, i.e. USDA, USCS, AASHTO…
  2. ^image
  3. Soil Survey Maps USDA Interactive GIS DATA
  4. Soil Texture Calculator USDA
  5. Examination and Description of Soil Profiles USDA
  6. Determination of Grain Size Distribution USDA
  7. Field Book for Describing and Sampling Soils: 300pp
  8. Soil Taxonomy: Making and Interpreting Soil Surveys 800pp
  9. Soil Climate and Moisture regimes
  10. Soil Classification, mostly British System, but still some relevant info

METHOD 1: the Ribbon Test

Part of METHOD 2: Roll a handful of moist soil into a tube shape with both hands. Squeeze it between your thumb and forefinger to form the longest and thinnest ribbon possible.

Texture Feel Ribbon
Sand Grainy Can’t form a ribbon
Loam Soft with some graininess Thick and very short
Silt Floury Makes flakes rather than a ribbon
Sandy Clay Substantial graininess Thin, fairly long—50 to 76 mm (2 to 3 inches)—holds its own weight
Clay Smooth Very thin and very long—76 mm (3 inches)

METHOD 2: USDA guide to texture by feel (shown below the horizons)


First let me say that trail tread should be on the subsoil/mineral soil B and/or parent rock C, or bedrock R, a little A is OK. O should be removed during construction if it exists (in desert climates it may be very thin or non-existent). Enough use can start to displace O depending on its depth and user numbers. This approach however, can create a trough, and a berm that won’t allow water to escape the trail. It’s best to remove O and the outside berm as well to establish outslope that will allow water to sheet flow off the trail, not down it.

soil-particle-size3METHOD 3: Mason Jar Method (link to protocol)



1) Divide the layer type depth or thickness by the total depth or thickness 2) multiply by 100 to find the percentages 3) locate the soil type in the chart to the left using the percentages

Soil color never lies: Color is a physical property of soils that allows us to know some of its most important characteristics, such as mineral composition, age and soil processes…

soil color is used as a criteria for classifying diagnostic horizons (albic, anthric, fulvic, hortic, hydragric, melanic, mollic, plaggic, sombric, spodic, terric, umbric and voronic horizons must meet certain color parameters), diagnostic properties (albeluvic tonguing, aridic properties, gleyic and stagnic color patterns), and diagnostic materials (limnic and sulphidic materials), and is used for supporting field indentification in many cases.

  • Dark or black. Dark colors are usually due to the presence of organic matter, so that the darker the surface horizon more organic matter content is assumed (this is not an exact and one-to-one correspondence). It is characteristic of the surface horizon, but can be found in other exceptional cases…where climatic conditions favor the accumulation of humus. At other times, the dark color is due to the presence of compounds of reduced iron and manganese…
  • Clear or white. Usually due to the presence of calcium and magnesium carbonates, gypsum or other more soluble salts. Carbonates may show continuous or discontinuous patterns: nodules, fine powder or films on soil aggregates (pseudomycelia)…. In other cases, light color is due to a relatively high proportion of sand (quartz crystals), either in the whole soil or in profile horizons that have suffered extensive washing under very humid climate.
  • Red. The red color is usually a result of alteration of clay minerals, so it usually occurs in the argillic horizons. Weathered clay minerals release aluminium and iron oxides such as hematite (Fe2O3). This process is favored in hot climates with a long and intense dry season, as the Mediterranean climate. Reddish colors indicate good drainage and aeration of the soil, allowing the existence of oxidizing conditions to form oxides.
  • Yellowish brown/orange. Yellowish or orange colors may be due to the presence of goethite, FeO(OH), and bound to the clay and organic matter. Therefore, although composition is similar to some red soils, we know that these soils were formed under conditions of increased moisture availability red soil.
  • Gleyic color pattern. The Russian word “gley” is used for mucky soil mass or clay. This color pattern is due to the presence of ferrous and ferric compounds. These colors are characteristic of Gleysols, developed under alternating reducing and oxidizing conditions. The mottled or marbled is presented as groups of spots of red, yellow and gray colors. This property appears in soils or horizons that are waterlogged for at least one part of the year. Sometimes it may be due to the activity of plant roots living in ponding. Usually, soils presenting this color pattern show reddish/brownish/yellowish colors in the surface of aggregates, pore surfaces or in the upper soil horizons (due to oxidized Fe3+ and Mn4+), and greyish/blueish colors inside the aggregates or in the deeper soil horizons (due to insoluble Fe2+ and Mn2+ compounds).
  • Green. The green color in the soil may be due to the presence of minerals such as glauconite, an iron potassium phyllosilicate mineral (mica group) or melanterite, a hydrated iron sulphate mineral.

“Iron minerals provide the most and the greatest variety of pigments in earth and soil.”

Properties of Minerals, and Colors

Mineral Formula Size Munsell Color
goethite FeOOH (1-2 m m) 10YR 8/6 yellow
goethite FeOOH (~0.2 m m) 7.5YR 5/6 strong brown
hematite Fe2O3 (~0.4 m m) 5R 3/6 red
hematite Fe2O3 (~0.1 m m) 10R 4/8 red
lepidocrocite FeOOH (~0.5 m m) 5YR 6/8 reddish-yellow
lepidocrocite FeOOH (~0.1 m m) 2.5YR 4/6 red
ferrihydrite Fe (OH)3 2.5YR 3/6 dark red
glauconite K(SixAl4-x)(Al,Fe,Mg)O10(OH)2 5Y 5/1 dark gray
iron sulfide FeS 10YR 2/1 black
pyrite FeS2 10YR 2/1 black (metallic)
jarosite K Fe3 (OH)6 (SO4)2 5Y 6/4 pale yellow
todorokite MnO4 10YR 2/1 black
humus 10YR 2/1 black
calcite CaCO3 10YR 8/2 white
dolomite CaMg (CO3)2 10YR 8/2 white
gypsum CaSO2H2O 10YR 8/3 very pale brown
quartz SiO2 10YR 6/1 light gray






Additional info: