Soil Part 2

What are the four soil forming processes?

1) Additions - the addition of organic material (i.e., addition of dead root mass in grassland ecosystems). 2) Transfers - the downward movement of soluble material in water (i.e., redistribution of calcium carbonate in a soil profile). Note that while the distribution of material with depth has changed, the material has not been completely lost from the soil. 3) Transformations - the weathering of primary minerals to secondary minerals, or the decomposition of organic matter. 4) Removals - the complete loss of material from the soil profile (i.e., the loss of calcium carbonate in a soil forming in a humid environment).

What accounts for variations in soil organic matter decomposition both within a site and between sites?

1) Chemical structure of SOM: some structures are more easily decomposed than others. 2) Stabilization by minerals: Clay colloids attract and stabilize organic matter. Minerals may also react with SOM. 3) Aggregation: SOM held in aggregates decomposes slowly. 4) Climate: Precipitation and temperature can affect the rate of decomposition. 5) Disturbance: Cultivation reduces inputs (harvest) and increases outputs (erosion).

Name five different types of soil contamination.

1) Salinization 2) Acidification 3) Radionuclides 4) Accumulation of heavy metals 5) Accumulation of toxic organic chemicals

Name three types of organic molecules that chelate with metals.

1) substances that are synthesized by roots, 2) various humic substances that have multiple carboxyl groups, and 3) synthetic substances.

What is a chelate?

A chelate is an organometal complex in which an organic molecule combines with a metal atom by way of two or more chemical bonds.

How does the DDL influence whether colloidal particles are dispersed or flocculated?

A colloidal particle with a thick DDL tends to stay dispersed, because it strongly repels other such particles. On the other hand, colloidal particle with a thin (collapsed) DDL is susceptible to flocculation.

What are some human activities that may lead to accelerated erosion?

Activities that can lead to accelerated erosion include, but are not limited to: Cutting forests, overgrazing, plowing hillsides, recreational activities (including ATV use), indiscriminate or arbitrary burning, construction of roads and buildings.

How are aggregates formed?

Aggregation = Flocculation + Cementation. Flocculation occurs when primary particles remain close together due to interactive forces. Cementation is the stabilization of these floccules by a cementing agent.

In the space between peds:

Air is exchanged between the soil and the atmosphere and root growth occurs.

If all four processes are active in all soils, why aren't all soils alike?

All four processes can and do occur at the same time in any given soil. It is the balance of these processes that determines the type of soil that forms. The balance of the four processes is controlled by the five soil forming factors.

Which of the three types of water erosion results in greater soil losses?

Although gully erosion looks more catastrophic, far more soil is lost by sheet and rill erosion.

What is ammonium (NH4+) fixation?

Ammonium fixation is a way through which an available N form (i.e., NH4+) is lost from the soil solution. Interlayer spacing of some phyllosilicate clays (illite or fine-grained mica) are just large enough to hold ammonium ions. The ammonium ions tend to hold together the adjacent layers of illite minerals (due to charge sharing and the attraction of opposite charges). Since these layers are held together so strongly, ammonium ions cannot readily escape again. This trapping is called ammonium fixation (which has nothing to do with biological N-fixation). The fixed ammonium is not exchangeable, though it can be gradually released by weathering.

Is there a Bm, Btj, Bf, or Bfj at least 5 cm thick?

Brunisolic order

What is the significance of CEC?

CEC 1) contributes to soil buffering, 2) contributes to nutrient retention in available forms, and 3) contributes to retention of various contaminants.

Why is carbon considered to be the building block of life?

Carbon is essential to life on earth. It is found in all living organisms, in the atmosphere, in sediments on the ocean floor and in fossil fuels. It is required for photosynthesis.

Is chernozemic A (Ah or Ap) present?

Chernozemic order

What is the difference between eluviation and illuviation?

Eluviation is the process by which clay is removed and illuviation is the process by which clay is deposited. Distinct eluvial and illuvial horizons may develop over time as water moves clay particles through the profile. The upper Ae (eluvial) horizon is depleted of clay and the lower Bt (illuvial) horizon is enriched with clay. Therefore, we can say that the eluviation of clay from the surface horizon created an Ae horizon, a horizon which is depleted of clay.

What function does organic matter decomposition serve in the soil?

Decomposition provides energy for microbial growth as well as carbon for the formation of new cell material.

Describe the importance of chelates.

Dispersed and dissolved chelates readily release metal ions and contribute to nutrient metal availability. Fulvic acids form chelates that provide nutrients to plants. Undispersed (flocculated) and undissolved or excessively stable chelates contribute to nutrient deficiency. Humic acids and humins form very stable chelates.

What are the four major forms of soil degradation?

Erosion, physical degradation, chemical degradation and biological degradation are the four major forms of soil degradation.

Why are A horizons more common in grassland soils than forest soils?

In grasslands, most of the organic matter is added directly to the mineral soil through dead roots and a significant amount of biomass is found underground. In forested ecosystems, above-ground biomass is much larger than below ground, while organic material is added to the surface of the soil where it tends to decompose before it is incorporated into the soil.

Give an example of why seasonal distribution of rainfall is an important factor in erosion.

In many temperate parts of the world, little erosion is caused by rainfall in the early spring because the soil is still frozen, while the same amount of rain a few months later may cause substantial soil loss.

Is there a Bg (or Cg within 50 cm of surface) at least 10 cm thick?

Gleysolic order

The Ae horizons are more commonly present in soils that are under a vegetative cover of trees, rather than grasses. Can you think of why this might be?

Grasses tend to grow more quickly than trees in the spring and soak up more of the snow melt and early rainfall. Hence, more leaching takes place in soils under a vegetative cover of trees than grasses leading to formation of an Ae horizon.

Why are soils with high CEC more fertile than soils with low CEC?

High CEC soils can adsorb more exchangeable cations, and therefore can store more plant nutrients than soils with a low CEC.

Is there a Bt horizon at least 5 cm thick?

Luvisolic order

What is one reason that mineral matter is a better conductor than organic matter?

Mineral matter is a better conductor than organic matter because organic matter is more porous and can therefore hold more air. Air is a poor conductor of heat.

List the major organic and mineral horizons.

Mineral: A, B, C Organic: L, F, H, O

Distinguish between mineralization and immobilization.

Mineralization is the process of conversion of an organic form of an element to an inorganic state, as a result of microbial decomposition. Inorganic forms of elements are forms that plants take up from the soil solution. Immobilization is the process of conversion of an element from the inorganic to organic form in microbial tissues, thus rendering the element unavailable to higher (vascular) plants. Immobilization is also mediated by soil microorganisms. Mineralization leads to reduction of the soil organic matter and increase of the available forms of nutrients, while immobilization increases soil organic matter content and reduces available forms of nutrients.

Which of the following chemical equations represents the cation exchange process in soils?

NH4+ + H-soil ----> NH4-soil + H+

Nitrates produced through the process of nitrification can be a problem for water supplies. Why?

Nitrate (NO3-), a negatively charged ion, does not get adsorbed by soil colloids, which are common in temperate regions. These soil colloids (silicate clay minerals, organic colloids, and some Fe/Al oxides and hydroxides) predominantly carry negative charges. Hence, NO3- ions stay in soil solution and are easily leached from the soil to groundwater.

What is nitrogen fixation?

Nitrogen fixation is a biochemical reduction of atmospheric nitrogen (N2) to ammonia. The site of N2 reduction is the enzyme nitrogenase, which is a two-protein complex containing Mo and Fe. Nitrogen fixation can be carried out by either free-living soil organisms (e.g., cyanobacteria and bacteria such as Azotobacter sp. and Clostridium sp.) or symbiotic organisms (e.g., bacteria Rhizobium sp., actynomicetes Frankia sp., bacteria Azospirillum sp.) that live in specific associations with vascular plants.

Why is soil quality of greater societal concern today than it was 100 years ago?

Over 100 years ago much of the world's potentially arable land was covered with natural forests or grasslands, nutrients were being recycled, and the soil was protected from erosion. The quality of most soil was not being degraded. The dramatic expansion in cultivated land needed to feed the growing human population has replaced natural vegetation with crop plants, which remove nutrients, leave the soil bare much of the year, and decidedly reduce biological diversity. This has resulted in widespread erosion, nutrient depletion and soil quality deterioration. If the quality of our soils declines further, the ability of the world to feed itself will also decline, as will the welfare of humans and all other organisms that depend on the soil.

Why do plants require phosphorus?

Phosphorus plays a number of important functions in plant development and growth. It helps during photosynthesis and respiration, provides energy transfer and storage, and helps with efficient use of water.

Is there a podzolic B horizon (Bf, Bhf, or Bh at least 10 cm thick)?

Podzolic order

Why do plants require potassium?

Potassium is used in photosynthesis, helps plants metabolize, controls absorption of water into plant pores, and helps control chemicals and water inside the plant that help it function well.

They do not have a B horizon (this is their diagnostic feature) and may even lack an A horizon. The reasons for this can be the following: youthfulness of the material, recent alluvium, nature of parent material (pure quartz sand), dry and cold climate, colluvium on steep slopes, etc.

Regosolic order

As soil pore space increases, particle density:

Remains unchanged. Particle density does not include pore volume and therefore is not subject to change when pore space increases or decreases.

List some human activities (other than agriculture and forestry) that can have a direct impact on soil formation.

Road building, excavation, construction of various buildings, and mining.

How deep would the layer of topsoil on a hilltop be as compared to a valley below?

Soil at the top of a hill would be much thinner than in a valley because sediments move down slope.

Why is soil biodiversity important?

Soil organisms are a key component in a number of processes that affect the functioning of entire ecosystems. They play a role in nutrient cycling, development of soil structure, nutrient retention, disease suppression, carbon sequestration and pollutant degradation. Each different type of organism has a different role to play and as such the diversity of organisms in the soil is as important as the total number.

How is soil quality measured?

Soil quality is measured through a series of indicators (physical, chemical, and biological soil properties) of a soil's ability to perform certain desirable functions (e.g., the accommodation of water entry, resistance to aggregate degradation, and the sustenance of plant growth).

Of what importance is soil quality to all organisms that live in or on the soil?

Soil quality is of vital importance to humans and all other organisms that depend upon the soil in any way. It is especially important in efforts to maintain biological diversity and sustainability of all soil functions.

What is soil quality?

Soil quality is the capacity of a soil to function in such ways as to sustain biological productivity and diversity, maintain environmental quality, and promote plant, animal and human health.

Is solonetzic B (Bn or Bnt) horizon present?

Solonetzic order

How do individual raindrops lead to increased surface runoff?

Splashing raindrops destroy soil structure. When a raindrop hits the soil surface, it detaches particles from soil aggregates and then places the loosened particles in suspension. When these particles are redeposited, they plug soil pores and consequently reduce water intake. Once the soil dries, a crust develops at the surface and runoff increases.

How does sulfur enter the atmosphere?

Sulfur enters the atmosphere through the combustion of fossil fuels, volcanic eruptions, gas exchange at ocean surfaces and decomposition of organic compounds.

Which would exhibit a higher water content at saturation - a sand, silt loam, or clay loam?

The clay loam would have the highest water content at saturation because it is a fine-textured soil. In a non-compacted clay loam the particles are organized into highly porous granules that have greater total porosity than either a silt loam or a sand. The greater the porosity, the greater the water content at saturation.

Respiration

The conversion of glucose and oxygen to water and carbon dioxide is called

What are factors of soil formation?

The factors of soil formation are climate, biota, topography, parent material, and time.

Which could store the most water in plant available form - a sand, silt loam, or clay loam?

The silt loam would contain the most plant available water. Even though the clay loam has a greater total porosity, the silt loam has a pore size distribution most favorable for storing water in plant available form.

Liming is necessary to neutralize:

Total acidity. When lime is added, hydrogen ions in the soil solution are neutralized. The hydrogen ions on the exchange complex are in equilibrium with those in soil solution. Thus, liming reduces active and reserve acidity.

Farmers can protect their soil from erosion by:

building terraces and contour plowing

What is a siderophore?

Under Fe-deficient conditions certain microbes and roots produce chelating ligands called siderophores. These ligands bond with Fe3+ and maintain relatively high concentrations of soluble iron.

What is the main purpose of a windbreak?

Windbreaks and shelterbelts are used to control wind erosion. A windbreak may be a single row of trees along the windward side of a field, or plantings of a number of species in several rows. They vary widely in composition and layout as a result of wide differences in soils and climate. Windbreaks also act as wildlife habitat, a shelter for natural predators (i.e., woodpeckers and flickers that consume cutworms, grasshoppers and beetles), provide fuelwood and act as recreational areas.

What practices can increase both thermal admittance and diffusivity?

a) Adding sand to organic soil, b) cultivation (mixing organic layers and mineral subsoil), c) adding water to dry soil, and d) removing the organic surface layer (scalping, burning).

What is the major factor limiting land use possibilities for each of the following soil orders? a) Solonetz b) Vertisol c) Gleysol d) Cryosol e) Podzol f) Organic soils

a) Solonetz: Low soil moisture levels; high pH and salts b) Vertisols: High shrink swell potential, poor for construction of homes and roads; difficult to till c) Gleysol: Wet for most parts of the growing season, poor aeration d) Cryosol: Low soil temperatures; permafrost e) Podzol: Low pH and base saturation, P sorption f) Organic soils: Oftentimes they are wet and make poor foundations

Erosion can be caused by which of the following:

a) rain falling on steeply sloped land b) a very hard rainfall c) rain falling on bare soil

Which of the factors below influences the erodibility of soil?

a) slope of land surface b) loss of vegetative cover c) soil texture

Top soil is the most productive soil layer because:

it is where most organic matter is found

What processes can lead to acidification of the soil?

n forested or parkland areas, the decomposition of vegetation releases carbon dioxide. This combines with soil moisture to form carbonic acid. Acid rain is formed by water combining with the emissions of certain gases such as sulfur dioxide. Soils can be affected, depending on the source, by prevailing wind directions and velocities. Some chemical fertilizers applied to the soil produce acidic compounds when they enter into solution.

Erosion is:

the movement of soil