Soil Characteristics

Soil a layer of natural materials on the earth’s surface containign both organic and inorganic materials and capable of supporting plant life.

Soil The material covers the earth’s surface in a thin layer. It may be covered by water, or it may be exposed to the atmosphere.

Soil Soil contains four main components: inorganic material, organic matter, water, and air.

Soil Ideal soil should contain about 50% solid material and 50% pore space. About half of the pore space should contain water and half of the space should contain air.

Soil Inorganic material consists of rock slowly broken down into small particles. The organic material is made up of dead plants and animals varying in stages of decay.

Soil The percentages of the four main soil components varies depending on the kind of vegetation, amount of mechanical compaction, and the amount of soil water present.

Soil Soil is formed very slowly. It results from natural forces acting on the mineral and rock portions of the earth’s surface. The rock is slowly broken down to small particles resulting in soil.

Parent Material Soil parent materials are those materials underlying the soil and from which the soil was formed. There are five major categories of parent material: minerals and rocks, glacial deposits, loess deposits, alluvial and marine deposits and organic deposits.

Parent Material Minerals are solid, inorganic, chemically uniform substance occurring naturally in the earth. Some common minerals for soil formation are feldspar, micas, silica, iron oxides, and calcium carbonates.

Parent Material Rocks are different from minerals because they are not uniform. There are three types of rocks, igneous, sedimentary, and metamorphic.

Parent Material Igneous rocks are those formed by the cooling of molten rock. Sedimentary rocks are those formed by the solidification of sediment.

Parent Material Metamorphic rocks are simply igneous or sedimentary rocks which have been reformed because of great heat or pressure.

Parent Material During the ice age, glaciers moved across areas of the northern hemisphere. They ground, pushed, piled, gouged, and eventually deposited great amounts of rocks, parent material, and already formed soil material.

Parent Material Loess deposits are generally thought of as windblown silt. Alluvial and marine deposits are water borne sediments. Alluvial deposits are left by moving fresh water. Marine deposits are formed on ancient ocean floors.

Parent Material Organic deposits are partially decayed plants that live plants are able to root and grow in. These are found in swamps and marshes.

Weathering When minerals are exposed to weather, they begin to break down into smaller pieces. This is mostly done by heating and cooling of the minerals and rock.

Weathering Some minerals are water soluble which means they dissolve when exposed to water. Some rocks may contain some minerals that are water soluble and only that part of the rock will dissolve. Ex: some caves.

Weathering When a tree or other types of plants begin growing in the cracks of rocks, this may speed up the break down of the rock because of the pressure the roots may exert.

Weathering Ice can also speed up the weathering process on rocks. If a rock has a crack that can fill up with water, when the water freezes, it can literally crumble the rock into small pieces.

Weathering Rocks can also be broken down by mechanical grinding such as wind blowing sand at high speeds or glaciers causing rocks to grind each other.

Weathering New soil is continually being made, but it takes a long time to create new soil and if it isn’t managed properly, soil can be eroded away quicker than it can be made.

Organic Matter In most soils, the proportion of organic matter is relatively small (2-5%). Its importance in formation and production is much higher than the small % would suggest.

Organic Matter Soil organic matter decaying plant and animals. As they die, they are attacked by microorganisms: fungi, bacteria, and others.

Organic Matter There are two types of organic matter. Original tissue is that portion of the organic matter that can still be recognized. Twigs and leaves covering a forest floor are good examples.

Organic Matter Humus is organic matter that is decomposed to the point where it is unrecognizable. The brown color you sometimes see in soil is a good example.

Organic Matter Purposes of organic matter: affects the soil structure by serving as a cementing agent, returns plant nutrients to soil (P, S, N), helps store soil moisture, makes soil more tillable for farming, provides food (energy) for soil microorganisms, which makes the soil capable of plant production

Characterizing Soils The Soil Profile Most soils have three distinct layers called horizons. The horizons are called A Horizon (topsoil), B horizon (subsoil), and C horizon (parent material).

Characterizing Soils The top soil is the most productive because that is where all the nutrients are.

Soil Physical Properties Slope is defined as the angle of the soil surface from horizontal. It is expressed as the % of rise over run.

Soil Physical Properties Slope effects the productive potential in numerous ways: Rain runoff, soil erosion, the use of farm machinery, and contour farming.

Soil Physical Properties Texture refers to the proportions of sand silt and clay in the soil. Course-textured soils are and sandy and do not hold water well, while fine-textured soils contain clay and tend to hold more surface moisture.

Soil Physical Properties Flood hazard refers to the likelihood that the soil will flood. This may occur in flood plains near rivers and greatly reduce plant production.

Soil Physical Properties Erosion as a soil property, refers to the degree that the soil has already been damaged. May range from none to severe.

Soil Physical Properties A field used for crop production that has little or no erosion can continue to be used for crops. But a severely eroded field may need to be turned into pasture where it is always covered.

Soil Physical Properties Topsoil and subsoil thickness refer to the depth of those layers that are available for plant production. Thin topsoil and/or thin subsoil can greatly limit crop production

Land Capability Classification Land capability class categorize the productive potential of the soil. The classes generally range from class 1, the best land for agricultural production, to class VIII, the least productive.

Land Capability In general, class 1 through class IV are for row crop production, and V through VIII are not suitable for row crop production for various reasons.

Land Capability Class I is the best land for row crop farming. It is level, well drained, deep, medium textured, not subject to erosion or flooding, and easily cultivated.

Land Capability Class II is just as good, but it may have some limitations such as sloping land or slight erosion.

Land Capability Class III can still be cultivated, but it has some severe limitations. The land may have moderate slope, erosion or a shallow root zone.

Land Capability Class IV has severe limitations, but can still be cultivated with good management practices.

Land Capability Class V is nearly level, but has some property which makes it unsuitable for farming. It may be very dry, very rocky, or most often very wet. This class is quite suitable for pasture, wildlife habitat, or forest production.

Land Capability Class VI is just a more serious version of V. It has severe limitations, but can be used for the same things.

Land Capability Class VII has some severe limiting properties. It may be very steep or be severely eroded and have deep gullies

Land Capability Class VII may be very course. This can be turned into pasture but grazing must be controlled. It can also be used as forest or recreation.

Land Capability Class VIII has one or more extreme limitations. It should be left in its natural state for recreation and wild life. It has little agriculture value.

Soil Classification The first unit of classification is the order. All soils fit into one of ten orders. Each order is broken down into a suborder, which is broken down into great groups, then subgroups, and then families.

Soil Survey Soil survey is the process of classifying soil. The results of the surveys in certain areas is published in what is known as the Soil Survey Report.

Soil Survey Then they develop a map from the survey. Scientists then use this as tool for figuring out the land capability.