Soils Study Guide

Leaching

the process where water-soluble substances, such as minerals, nutrients, or pollutants, are dissolved and carried downward through the soil by percolating water 

Humus

the dark, complex organic matter in soil resulting from the partial decomposition of plant and animal remains

Material which nourishes and supports growing plants 

what is soil?

Water-25%,Air-25%,Mineral Particles-45%,Organic Matter-5% 

The “ideal” soil

O (organic) Horizon

mainly organic matter like litter and humus—organic:  

• It consists of fresh and decaying plant residue 

• The O horizon is dark because decomposition produces humus

A (surface) horizon

zone of organic matter accumulation (topsoil)—mineral: 

• Mainly mineral material and is darker in color than the lower layers because of the varying amounts of humified.  

• Usually, the most productive layer of soil 

• Where most root activity occurs 

• may be referred to as a surface layer 

E (eluvation) Horizon

zone of eluviation (loss of clay, Fe, Al)—mineral 

• Generally, is bleached or whitish in appearance 

• As water moves down through this horizon, soluble minerals and nutrients dissolve and some dissolved materials are washed (leached) out 

• Main feature is loss of silicate clay, iron, aluminum, humus, or some combination of these, leaving a concentration of sand and silt particles 

B (subsoil) Horizon

zone of accumulation (clay, Fe, Al, CaCO₃, salts…) -- subsoil—mineral 

•  Is usually lighter colored, denser, and lower in organic matter than the A horizon 

• Commonly is the zone where leached materials accumulate 

• Further defined by the materials that make up the accumulation which identifies that clay has accumulated 

• Other illuvial concentrations or accumulations include iron, aluminum, humus, carbonates, gypsum, or silica 

C (substratum) Horizon

little or no pedogenic alteration, unconsolidated parent material, soft bedrock—mineral 

• may consist of less clay, or other less weathered sediments 

R (bedrock) Horizon

hard, continuous bedrock—rock 

• Bedrock can be within a few inches of the surface or many feet below the surface

water and wind

Two main ways soil is moved?

Cl—Climate

A major factor in determining the kind of plant and animal life on and in the soil. It determines the amount of water available for weathering minerals and transporting the minerals and elements released. Climate through its influence on soil temperature determines the rate of chemical weathering.

O—Organisms

• Plants affect soil development by supplying upper layers with organic matter, recycling nutrients from lower to upper layers and helping to prevent erosion. 

 

• Deep rooted plants contribute more to soil development than shallow rooted because the passages they create allow greater water movement, which in turn aids in leaching. 

 

• Leaching is the process where water-soluble substances are washed away and carried downward through soil layers by percolating water. 

• Organisms eat and break down organic matter, releasing plant nutrients. Some change certain elements, such as sulfur and nitrogen, into usable forms for plants. 

R—Relief

• The slope of the surrounding landscape affects erosion and drainage 

 

• Steeper, longer slopes will cause water to run downhill quicker and for longer periods of time, causing more erosion 

 

• Erosion causes the soil near the bottom of a slope to be higher quality than the soil higher up on the slope 

 

• Also, certain parts of slopes tend to receive different amounts of sunlight, leading to some soil along slopes being more dried out. 

 

Parent Material

• serves as the starting point for soil formation 

 

• Rock is broken down by weathering, forming mineral materials that serve as the basis for soil 

 

• Soil generally inherits properties such as color from this

T—Time

• Soil can take many years to form 

 

• Over time, it begins to lose the characteristics of its parent material 

 

• After long periods, it begins to pile up, forming soil layers that extend deep into the ground 

 

• It begins to look less and less like the parent material 

Additions

Materials deposited onto the soil from above or from below, including organic matter from decomposing leaves and roots. The most obvious addition is organic matter. As soon as plant life begins to grow in fresh parent material, organic matter begins to accumulate. Most organic matter additions to the surface increase the cation exchange capacity and nutrients, which also increases plant nutrient availability

Examples: organic matter input, soil from wind erosion 

Losses

Processes where soil components are removed from the soil profile, leading to a decline in soil quality and productivity. Most losses occur by leaching. Water moving through the soil dissolves certain minerals and transports them into deeper layers. Some materials, especially sodium salts, gypsum, and calcium carbonate, are relatively soluble 

Examples: leaching (nitrogen and phosphorous), erosion 

Translocations

The movement of soil materials within the soil profile from one location or horizon to another, driven by water, gravity, and soil organisms. Translocation means movement from one place to another. In low rainfall areas, leaching often is incomplete. Water starts moving down through the soil, dissolving soluble minerals as it goes. There isn't enough water, however, to move all the way through the soil. When the water stops moving, then evaporates, salts are left behind. Soil layers with calcium carbonate or other salt accumulations form this way. If this cycle occurs enough times, a calcareous hardpan can form. Translocation upward and lateral movement is also possible. Even in dry areas, low-lying soils can have a high water table. Evaporation at the surface causes water to move upward. Salts are dissolved on the way and are deposited on the surface as the water evaporates.  

Examples: movement of inorganic matter to organic matter 

Transformations

Processes that change the physical, chemical, and biological composition of soil constituents, breaking down and altering them into new forms. changes that take place in the soil. Microorganisms that live in the soil feed on fresh organic matter and change it into humus. Chemical weathering changes parent material. Some minerals are destroyed completely. Others are changed into new minerals. Many of the clay-sized particles in soil are actually new minerals that form during soil development. 

• Can change the form of certain materials. Iron oxides (ferric form) usually give soils a yellowish or reddish color. In waterlogged soils, however, iron oxides lose some of their oxygen and are referred to as being reduced. The reduced form of iron (ferrous) is quite easily removed from the soil by leaching. After the iron is gone, generally the leached area has a grayish or whitish color. 

Examples: weathering of primary particles in place 

Available water capacity

an estimate of how much water a soil can hold and release for use by plants measured in inches of water per inch of soil. Influenced by soil texture, content of rock fragments, depth to a root-restrictive layer, organic matter, and compaction. 

Cation Exchange Capacity (CEC

a measure of the ability of soil to hold and exchange cations. One of the most important chemical properties in soil and is usually closely related to soil fertility.

Drainage class

refers to the frequency and duration of periods of saturation or partial saturation during soil formation. Seven classes of natural drainage are used in soil surveys.

Erosion Factor (K) and (T) (RUSLE – Revised Universal Soil Loss Equation

A=R⋅K⋅L⋅S⋅C⋅P 

 

• K: The soil erodibility factor is a relative index of the susceptibility of bare, cultivated soil to particle detachment and removal and transport by rainfall. K values range from 0.02 to 0.64 or more. Higher values indicate greater susceptibility. Soil that has more silt and very fine sand are generally more erosive because of weaker bonding. 

• The T factor is the soil loss tolerance used in the RUSLE. It is defined as an estimated maximum rate of annual soil erosion that will permit crop productivity to be sustained economically and indefinitely. The five classes of T factors range from 1 ton per acre per year for very shallow soil to 5 tons per acre per year for very deep soil that can more easily sustain productivity. 

High water table

the highest average depth of free water during the wettest season. The ground water level, or water table, may be high year round or just during heavy rains. How high the water table rises and how long it stays there affect what can be done on that soil. 

Organic matter

estimated for each layer. One percent organic matter is equivalent to 0.6 organic carbon. It encourages granulation and good tilth, increases porosity, lowers bulk density, promotes water infiltration, reduces plasticity and cohesion and increases available water capacity. It has a high cation adsorption capacity and its decomposition releases nitrogen, phosphorus, and sulfur. 

Permeability (Saturated Hydraulic Conductivity –water movement through  

     saturated soils)

is influenced by texture, structure, bulk density, and large pores. Soil structure influences the rate of water movement through saturated soil, in part, by the size and shape of pores. 

• Permeability is used in drainage design, irrigation scheduling, and many conservation practices. 

Reaction (especially pH)

an expression of the degree of acidity or alkalinity of a soil. It influences plant nutrient availability. A very acid soil (pH <5.0) typically has lower levels of nitrogen, phosphorus, calcium, and magnesium available for plants, and higher levels of availability for aluminum, iron, and boron than a neutral soil at pH 7.0. At the other extreme, if the pH is too high, availability of iron, manganese, copper, zinc, and especially phosphorus and boron may be low. 

Salinity

Salts, mainly sodium, magnesium, calcium, and chloride or sulfate, may interfere with the absorption of water by plants. They also create a nutrient imbalance in some plants. Soils that have more than 2 mmhos/cm of electrical conductivity in soil solution are considered saline. 

Slope

the gradient of the elevation change. A 10 foot rise in 100 feet is a slope of 10 percent. Ranges of slope assigned to map units represent practical breaks on the landscape that are important for the use and management of the area. Terraces, irrigation, and tillage practices are all considered.

Soil productivity

he output or yield per acre of a specified crop or pasture under a defined set of management practices

Irrigation

for most crops, the most favorable soils for irrigation are deep, nearly level, and well drained. They have good surface soil permeability and a high available water capacity

Drainage

is the removal of excess water from soil. Soils that have intermediate saturated hydraulic conductivity (permeability) respond well to subsurface drainage open ditches or a combination of these

Erosion control practices

need for ___ depends on the potential for erosion and the cultivars grown. Some crops, such as hay and pasture, protect against soil erosion. For others, such as row crops, specific management practices are needed

Hydric soils

Wet soils defined as a group for the purpose of implementation of legislation for preserving wetlands and for assessing the potential habitat for wildlife. The soils considered to be hydric were selected on the basis of flooding, water table, and drainage class criteria. Hydric soils developed under wet conditions (anaerobic within 12 inches) and can support the growth and regeneration of hydrophytic vegetation

Ball Test

Wet the soil, try to form a ball, if it forms and holds together it is either silt or clay, if not, pure sand. Next, throw it in the air, if it stays together, definitely not sand material

Ribbon Test

Push out the soil from middle of hand. If it ribbons even more than half of an inch, it is loam material (mixture of silt, clay and sands), If it is super sticky, that is indicative of clay. Clay will have at least a 2-inch ribbon.

Sand

• Size: 0.05mm-2mm, visible without microscope 

•  Feel/texture: gritty, noncohesive (does not stick together unless very wet) 

•  Color: white (if from quartz), red, yellow, or brown (from other minerals) 

• Water and nutrient holding capacity: Low ability  

•  Water movement: High permeability  

•  Soil mechanics: Large particles maintain volume and density, wind and water erosion, erodes on slopes 

•  Agricultural use: Commercial farm crops 

•  Non-agricultural use: Glass and ceramic production 

• Mostly quartz 

• Low chemical activity 

• Large pore space 

• Low water holding capacity, the finer the sands: higher water holding capacity 

• High conductivity 

Silt

• Size: 0.002mm-0.05mm, only visible with microscope 

•  Feel/texture: smooth, like silly putty 

•  Color: light brown, can be gray or red 

• Water and nutrient holding capacity: Holds water; fertile, high nutrient content 

•  Water movement: Well drained 

•  Soil mechanics: Compresses easily, can form surface seals when flooded, washes away easily 

•  Agricultural use: Good for crops, drought tolerant 

•  Non-agricultural use: Used in construction 

• Feels like flour 

• Medium pore spaces 

• Better water holding than sand 

Clay

• Size: <0.002mm, only visible with microscope 

•  Feel/texture: sticky and plastic 

•  Color: light gray to deep red 

• Water and nutrient holding capacity: High, stays wet and holds nutrients; often phosphate deficient and has high alkaline pH 

•  Water movement: Slow, drought tolerant 

•  Soil mechanics: Swells (when wet) and shrinks and hardens (when dry), resists erosion 

•  Agricultural use: Good for crops that like moisture  

•  Non-agricultural use: Human waste treatment 

• Chemically active (negatively charged) and bonds with positive cations in water) 

• Small pore space 

• Low conductivity