International Certified Crop Advisor Exam Soil and Water Management

Anion definition

Negatively charged atom or molecule

Cation definition

Positively charged atom or molecule

Cation exchange capacity definition

Amount of positively charged cations which can be held by a given weight of soil. Cations are held by negative charges in clay and OM. Meq/100g soil

Anion exchange capacity definition

Amount of negatively charged anions which can be held by a given weight of soil. Anions are held by positive charges in clay and OM. Meq/g soil

Soil factors that influence CEC

Percent clay, type of clay, amount of OM, and soil pH

Estimate of CEC of pure SOM

200

Estimate of CEC of pure vermiculite clay

150

Estimate of CEC of pure montmorillonite clay

100

Estimate of CEC of illite clay

30

Estimate of CEC of kaolinite clay

10

Relationship between CEC and pH

As pH increases CEC increases (AEC decreases)

Explain how K and NH4+ can become fixed in soils

Some clay minerals have "holes" on their surface about the size of these molecules. When these ions enter those spaces the clay collapses around the cations "fixing" them.

What soil properties are determined by parent material and mineralogy

pH, CEC, AEC, soluble salts, clay mineralogy, and OM

What soil properties influence background fertility

pH, CEC, AEC, soluble salts, clay mineralogy, and OM

Definition of saline soils

Have an electrical conductivity greater than or equal to 0.4 Siemens per meter in the saturation extract. They have sufficient soluble salt to impair crop growth

Definition of sodic/natric soils

13-15 percent or more of the CEC occupied by sodium. Poor structure and accompanying poor plant growth

Definition of saline-sodic soils

ECs>0.4 Siemens per meter and from 13-15 percent of the CEC occupied by sodium. Good physical properties until salt is removed

Definition of calcareous soils

Contain free calcium carbonate

Define acidic soils

pH less than 7

Define alkaline soil

pH over 7

Particle size of gravel

>2 mm

Particle size of sand

2-0.05 mm

Particle size of silt

0.05-0.002 mm

Particle size of clay

What defines soil texture

Percentages of sand, silt, and clay

What are the soil separates

Sand, silt, and clay

What is the relative surface area of silt compared to sand

250x

What is the relative surface area of clay compared to sand

17,000x

Reason why clay size particles hold more water and retain more nutrients than sand and silt

They have a much higher surface area

What soil properties decrease as the amount of clay in soil increases

Bulk density, particle size, and pore size

What soil properties increase as the amount of clay in soil increases

Pore volume and surface area

What soil textural class has the highest bulk density

Sand

What soil textural class has the highest porosity

Clay

What properties of soils make them more reactive

Ones with higher surface area

Why are soils with more surface area more reactive

They have a higher charge and more surfaces upon which reactions take place

Soil drainage and plant available water are influenced by

Distribution of pore sizes

What is plant available water

Water that can be extracted from soil by plants

Definition of field capacity

How much water the soil can hold after most free drainage has occurred

Definition of permanent wilting point

Amount of water in a soil when plants will wilt and not recover even if water is added

Equation for the maximum value of available water

Field capacity - permanent wilting point

Which soil texture has the highest field capacity

Clays

Which soil texture has the highest permanent wilting point

Clays

What soil textures tend to have greatest ability to store available water

Intermediate textures (loam, clay loam, silt loam)

How can the amount of water in a soil be measured

Weight percentage, volume percentage, height of water, or energy retention

Soil structure definition

Arrangement of soil particles into larger units of varying degrees of coherence

Name for the structural units of soil structure

Pods

Two types of structureless soil structure

Massive and single grain

Types of structured soil structures

Granular, platy, blocky, and prismatic/columnar

Describe massive soil structure

Soil particles cling together, but do not break into similar structural units

What is single grain structure

Sand

Describe granular soil structure

Small round aggregates. Aggregates are porous and common to the plow layer

Describe platy soil structure

Aggregates are thin and form like a stack of plates

Describe blocky soil structure

Irregular, six sided aggregates

Angular vs subangular blocky soil structure

Angular has sharp edges and subangular has rounded edges

Describe prismatic/columnar structure

Like a column of soil with well-defined edges along edges of the column

Difference between prismatic and columnar structure

Prismatic has no rounded top and columnar has a rounded top

What soil structure is often preferred for a seedbed

Granular

Good structure in the topsoil affects what other soil properties

Aeration, water relations and soil tilts

Good structure in the subsoil positively impacts what other soil properties

Aeration, water relations, and resistance of the soil to root penetration

What do macropores provide to the soil

Aeration and drainage

What do micropores provide to the soil

Water holding for plant use

How do soil microbes affect soil structure

Decomposition of organic materials increase aggregation because the products are "glue" that hold them together. Over the long term this can decrease OM levels and aggregation

How do macro organisms affect soil structure

They mix the soil and create large channels

How do earthworms affect soil structure

Their excretions have a granular structure. They create macropores that improve aeration and drainage

Bulk density definition

Weight of dry soil in grams per cubic centimeter of soil

Bulk densities of mineral soils

1.7 to 1.1

Bulk density of organic soils

0.5

Particle densities of mineral soils

2.65

Equation for % porosity

= 100 - 100*Bulk Density/Particle Density

Range of % porosity

36% in sandy soils to 58% in clayey soils

What increases bulk density

Compaction, SOM content decreases, tillage

What determines bulk density

Soil texture and soil structure

What causes bulk density to decrease

Adding SOM

Sources of OM

Plant/crop residues, green manures/cover crops, animal manures, composts, biosolids, soil animals

Stable form of SOM that decomposes slowly

Humus

What are the results of the breakdown of OM

80% CO2, 15% humus, 5% biomass

Relationship between CEC and pH

As pH increases the CEC increases

Relationship between AEC and pH

As pH decrease the AEC increases

How does CEC relate to AEC at normal pH

CEC is larger than AEC

SOM generally improves what soil properties

Aeration, drainage and water holding

What can cause soil aggregates to be more water stable

Increase in OM

What nutrients are held by OM/humus

N, P, S, K, Ca, Mg, Zn, Cu, Mn, Fe

Relationship between plant available water and humus

As OM increases PAV increases

Ways to increase carbon sequestration

Cover crops, adding crop residues, increasing crop rotation complexity, and reducing tillage

Factors that influence soil microbial activity

Temperature, moisture, soil pH, organic matter, salinity, nitrogen application, and tillage

Optimum temperature for microbial activity

Above freezing to 35 C

Optimum water holding capacity for microbial activity

40 to 60 percent

Relationship between pH and microbial activity

Fungi are more active in acid soils and bacteria are more active in neutral to basic soils

How does tillage affect aerobic soil microbial activity

Tillage increases aerobic soil microbial activity

N content of high C:N ratio materials (wheat straw and sawdust)

Low N content, immobilization decreases N availability

Soil health definition

Holistic approach to evaluating how well a soil preforms at supporting various functions

O soil horizon description

Accumulation of organic matter

A soil horizon description

Mineral horizon at the surface or below O horizon. Has lost much or all of the original rock structure and has OM. Has been acted on by man

B soil horizon description

Could have any of the following:

Illumination of clay, iron aluminum, OM, carbonates, gypsum, silica

Removal of carbonates

Residual sesquioxides

Clay weathering

Structural development

Brittleness

C soil horizon description

Little has been affected by soil forming processes, but is not hard bedrock

Parent material definition

Rock, unconsolidated material, or OM that forms soil after physical, chemical, and/or biological pedognic processes have occurred

How to convert area from square feet to acres

Total square feet divided by 43,560

Three ways to calculate field area

Break into rectangles, squares, and triangles and add the area of them all

Weigh an aerial photo

Computer software

How to calculate slope

%slope = change in elevation in feet for a given horizontal distance multiplied by 100

List characteristics of well-drained soils

Oxidized

Iron and manganese are insoluble

Good aeration

Good available soil water

Aerobic microbial processes are active

Good root and top growth

Decomposition of most organic materials