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Flashcards in Soils Management Deck (81)
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What is an anion

A negatively charged atom/molecule

Ex found in soils: phosphate, sulfate, nitrate, and chloride


What is a cation

A positively charged atom/ molecule

Ex in soils: Calcium, magnesium, sodium, potassium, and ammonium


Define cation exchange capacity

The amount of positively charged anions that can be held by a given weight of soil


Define anion exchange capacity

The amount of negatively charged anions which can be held by a given weight of soil


What is the unit for CEC/AEC

Centimole charge per kg soil (cmol/kg soil) or meq/100g of soil


What factors determine the CEC of soils

Percent clay

Type of clay

Amount of OM

Soil pH


What is the order of CEC for OM and various types of clay

OM 200 meq/100 g

Vermiculite 150

Montmorillonite 100

Illite 30

Kaolinite 10


What affect does pH have on CEC and AEC?

As pH increases, CEC increases, AEC decreases


What ions can become fixed to clay surfaces? How do they become fixed?

“Holes” in the clay surface allow K and NH4 to enter the space, which they clay then collapses around. This makes nutrients unavailable for plant uptake


Differentiate saline, sodic/matrix, calcareous, acidic and alkaline soils

A saline soil contained sufficient soluble salt to impair plant growth (soils that have an electrical conductivity greater than or equal to 0.4 Siemens per meter in the saturation extract are considered saline)

A sodic/natric soil has from 13-15% or more of the CEC occupied by Na. (These soils have poor structure and accompanying poor plant growth)

A saline-sodic soil has ECs greater than or equal to 0.4 Siemens per meter and from 13-15% or more of the CEC occupied by Na. These soils have good physical properties until the salty is removed and they revert to sodic

Calcareous soils contain free calcium carbonate

Acidic soils have a pH less than 7

Alkaline soils have a pH more than 7


What properties change as sand and silt decrease and clay increases?

Bulk density, particle size, and pore size decrease

Pore volume and surface area increase


What does higher surface area create?

Higher CEC/AEC

Higher surface area = more clay and/or OM


Describe how soil texture affects water holding capacity, amount of available water, and wilting point of soils

The distribution of pore size in soil impacts drainage and plant available water

Plant available water is that water which can be extracted by plants. The max value of available water is the difference between the amount of water a soil can hold after most free drainage has occurred (field capacity) and the amount of water in the soil when plants will wilt and not recover even if water is added (wilting or permanent wilting point)


How is water in the soil measured?

By weight (dry soil basis) percentage

Volume percentage

Height of water (cm/in)

Energy of retention (units are bars, atmospheres or pascals)


Define soil structure

The arrangement of soil particles into larger units of varying degrees of coherence called peds


How do Soil microorganisms affect soil structure

Soil microorganisms decompose organic matter, crop residues, and other organic amendments added to the soil.

Short term: increase aggregation through production of decomp products that “glue” soil particles together

Long term: can decrease OM levels and aggregation in conditions that favor decomp (frequent tillage, optimum temp, moisture, oxygen, and limited return of crop residues)


What are macroorganisms and how do they affect soil structure?

Termites, ants, earthworms, moles

Mix the soil and create large channels that improve aeration and drainage


Define bulk density and what determines it

The mass of oven dry soil per unit volume


Soil texture and structure determine bulk density of a soil


How is bulk density impacted by organic matter and farm equipment traffic?

Bulk density increases as soil organic matter decreases (due to incorporation, burning, or removal of crop residues)

Continuous tillage can increase bulk density and cause the creation of tillage pan

Compaction from implement wheel traffic and animal traffic, especially on wet soils, increases bulk density


What are sources of organic matter?

Pant/crop residues (top/root)

Green manures/ cover crops

Animal manures

Composts from ag, industry, municipalities

Biosolids (municipal sewage sludge) and industrial wastes (paper mill sludge)

Soil animals (micro/macroorganisms)


What are the physical properties of biomass?

There are pools of organic matter in soils including organic material decomposing, biomass, and organic material that has undergone various degrees of decomp.

Humus is the most stable form of OM and decomposes very slowly

When organic materials decompose, about 80% is converted to CO2 and lost from the soil. About 15% becomes humus, and 5% is biomass


What are the chemical properties of organic matter

OM retains nutrients by adsorption. Cations are held by caution exchange sites on OM. Negative charges on OM are pH dependent, as pH increases CEC (pH depended negative charge on OM) increases

Anions are held by anion exchange sites on OM. The positive charges on OM is pH dependent, as pH decreases he AEC (pH dependent positive charge on OM) increases

At normal pHs, the CEC is much larger than AEC of OM

OM increases buffering against pH changes


What are the beneficial effects of OM?

Soil particles can be linked together into aggregates by OM. As OM content increases, aggregates tend to become more water stable (resistant to breakdown by water)

Source of nutrients (primarily N, also P and S) when decomposed. Retains cations that are macro/micronutrients for plants.

Increases water holding capacity, plant available water and infiltration

Decreases crusting of soil

Increases pore size, increases aeration


How does temperature influence soil microbial activity?

Microbial activity increases as temp rises above freezing until optimum temp, once above optimum temp activity declines back to zero

For many microbial conversions, there is a two-fold increase in activity for each 10EC increase in temp from about 15-35c. Above 35c, a decline commonly occurs


How does moisture influence soil microbial activity

Aerobic microbial activity is usually optimum at 40-60% of a soil’s water holding capacity (slightly drier than field capacity)

Decline in activity typically occurs as soils become more dry, rapidly declining at a critical soil water content. Critical level varies with microbial reaction being considered

Anaerobic activity (-Oxygen) occurs in saturated souls and ceases in the presence of oxygen.

Facultative anaerobes can function in the presence or absence of oxygen


How does soil pH influence soil microbial activity?

Fungi are more active in acidic soils

Bacteria are more active in neutral to basic soils

Rates of reaction carried out by specific microorganisms change as pH changes


How does OM influence soil microbial activity

Additions of organic C to soil stimulates microbial activity

The more decomposable the organic C, the more rapid the increase in microbial activity


How does salinity influence soil microbial activity?

As salinity increases, microbial activity can decline depending on the microorganism and amount of salinity.

Microbes expend more energy to absorb water as salinity increases

Various enzymatic processes within the microbial cell can be impaired by soluble salts


How do nitrogen applications influence soil microbial activity ?

May have a wide range of impacts on soil microbial activity

In general, better fertility improves microbial activity

Can increase activity if Low N is limiting decomp

Can affect specific groups of microbes


How does tillage influence soil microbial activity?

Tillage increase aerobic microbial acivity

May have adverse long term effects (decreased OM)