Soil Texture
A close look at soil will clearly indicate that the makeup of the mineral portion is quite variable. The soil is composed of small particles. These small particles are the result of massive rocks of different mineralogy that have weathered to produce smaller rock fragments and finally soil particles. Soil particles vary in size, shape and chemical composition. Some are so small they can be seen only with a microscope.
Three categories for soil particles have been established — sand, silt and clay. These three groups are called soil separates. The three groups are divided by their particle size. Clay particles are the smallest, while sand particles are the largest. The size ranges for the soil separates and the relative size of the particles are shown in //Figure 2.2//. Sand particles are clearly visible, but a microscope must be used to see silt particles. An electron microscope is needed to see clay particles.
The laboratory procedure used to identify soil separates is known as mechanical analysis. This process records the time it takes a specific weight of soil particles to fall to the bottom of a tall cylinder filled with water. A textural triangle can be used to determine soil textural class from the results of a mechanical analysis (//Figure 2.3//). Often 100 units of soil are used in the analysis, so that the sum of the weights of the three soil separates total 100 and can be easily converted to percentages. The textural triangle represents all possible combinations of soil separates.
By using samples of known texture, and with a lot of practice, it is possible to determine soil texture by hand texturing. With this procedure, moistened soil is worked between the thumb and fingers to form a ribbon. Sand and clay percentages are estimated. A guide for estimating soil texture by hand is given in the table below.
Some small rock fragments may be present in soil as stones or gravel. While these rock fragments play a role in the physical properties and processes of soil, they are not considered in the determination of soil texture.

Soil Structure
Soil structure refers to the arrangement of soil separates into units called soil aggregates. An aggregate possesses solids and pore space. Aggregates are separated by planes of weakness and are dominated by clay particles. Silt and fine sand particles may also be part of an aggregate. The aggregate acts like a larger silt or sand particle depending upon its size.
The arrangement of soil aggregates into different forms gives a soil its structure. The natural processes that aid in forming aggregates are:
1) wetting and drying,
2) freezing and thawing,
3) microbial activity that aids in the decay of organic matter,
4) activity of roots and soil animals, and
5) adsorbed cations.

The wetting/drying and freezing/thawing action as well as root or animal activity push particles back and forth to form aggregates. Decaying plant residues and microbial byproducts coat soil particles and bind particles into aggregates. Adsorbed cations help form aggregates whenever a cation is bonded to two or more particles.
Aggregates are described by their shape, size and stability.


Bulk Density
Soil weight is most often expressed on a soil volume basis rather than on a particle basis. Bulk density is defined as the dry weight of soil per unit volume of soil
Bulk Density = Mass of soil / Volume of Soil
Soil Color
Organic matter (humus), manganese and iron are the primary coloring agents in soil. The dark color of many productive soils in Nebraska and throughout the Midwest is due to organic matter. The dark soil color from organic matter at the soil surface aids in the absorption of heat from sunlight to warm the soil.
Soil shades of red, yellow and gray are due to the amount and chemical form of iron and manganese present. Red soils contain oxidized iron. Oxidized iron is also observed on metal objects that have been exposed to the atmosphere. We call it rust. Yellow soils contain hydrated iron. Gray soils indicate chemical reduction of iron and/or manganese due to wetness and lack of oxygen. Yellow and gray coloration can be found in the subsoil of some Nebraska soils which remain wet for some portion of the year. These subsoil colors serve as an important indicator of natural drainage conditions.
Soil Horizons
A soil horizon is a specific layer in the soil which parallels the land surface and possesses physical characteristics which differ from the layers above and beneath.
O) Organic matter: Litter layer of plant residues in relatively undecomposed form.
A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life. This layer eluviates (is depleted of) iron, clay, aluminum, organic compounds, and other soluble constituents. When eluviation is pronounced, a lighter colored "E" subsurface soil horizon is apparent at the base of the "A" horizon.
B) Subsoil: Layer of alteration below an "E" or "A" horizon. This layer accumulates iron, clay, aluminum and organic compounds, a process referred to as illuviation.
C) Substratum: Layer of unconsolidated soil parent material. This layer may accumulate the more soluble compounds that bypass the "B" horizon.