E7: SOIL

7.1 Discuss salinization, nutrient depletion and soil pollution as cause

of soil degradation

Where does it come from? Physical means

Weathering of rocks○ CaCO3 + H2O + CO2 -> Ca2+ + 2HCO3

-

Chemical meansBacteria and fungi in the soil breakdown

organic matter which then makes more soil

Components of soil Inorganic matter

Largest component From weathering of rocks Contains

○ Silica○ Silicates○ aluminosilicates

Water/ Air The amount affects how life forms survive and then decay

Organic matter Bacteria and fungi convert organic matter into humus

○ A water insoluble mixture○ Half it’s mass is carbon and approx. 5% nitrogen

Inorganic Matter

SilicatesSilicon is covalently bonded to four oxygens

Aluminosilicates Al3+ has a similar size to siliconCan replace the silicon to form

aluminosilicates

Inorganic (cont’d) If Al3+ replaces a Si4+ then in every four tetrahedrons

there is 1 electron deficient These ions + electronegative oxygen = ability to hold

onto plant nutrients Plant nutrients

K+

○ Controls the amount of water that enters via osmosis NH4

+

○ Synthesis of amino acids and proteins Ca2+

○ plant cell wall synthesis Mg2+

○ Synthesis of chlorophyll

7.1 Salinization

Accumulations of water soluble ions or salts in the soil due to continual irrigation to crops

How?Small amounts of salts ( CaCl2, MgSO4,

NaCl) are in waterWhen irrigation doesn’t wash them away

they build up in the soil

salinity

Salinity is the total amount of dissolved salts in water; grams of salts per kilogram of water (g/kg) or as parts per thousand (ppt).

Seawater has 11 major constituents that make up more than 99.99% of all dissolved materials.

Although salinity may vary, the major constituents are well mixed and present in the same relative proportions.

Salts

Chloride (Cl-) 55.07% Sodium (Na+) 30.62% Sulfate (SO4

2-) 7.72% Magnesium (Mg 2+) 3.68% Calcium (Ca 2+) 1.17% Potassium (K+) 1.10% Bicarbonate (HCO3

-) 0.40% Bromide (Br -) 0.19% Strontium (Sr 2+) 0.02% Boron (B 3+) 0.01% Fluoride (F-) 0.01%

Salinity

Affects other properties of seawater, such as its density and the amount of dissolved oxygen.

Salinity Significant Values

The average salinity of the world’s oceans is 35 ppt.

Freshwater has a salinity of <1 ppt.

Inshore waters with salinity values between 1 - 25 ppt are called brackish.

Waters with salinity greater than 40 ppt are called hypersaline.

Salinity

Addition of salts to pure water causes an increase in density. Salinity can be calculated by measuring the specific gravity of a water sample using a hydrometer, correcting for the effect of temperature and converting the readings to salinity by using conversion tables.

Specific Gravity = density of sample Density of pure water

Salinity

Be sure hydrometer is clean Fill 500 mL graduated cylinder with sample water Determine the temperature of your sample Place the hydrometer in cylinder and let settle. It

should not touch the cylinder walls, and should be read from the bottom of the meniscus.

Read the specific gravity from the hydrometer scale

Using the specific gravity and temperature values, determine salinity from salinity table in Teacher’s Guide

Read three times. The values should be within 2 ppt of the average. Discard outliers.

Calibration35 ppt standard:

○ Measure out 17.5 g NaCl (table salt) and pour into a 500-mL graduated cylinder.

○ Fill the cylinder to the line with distilled water and carefully swirl the solution to mix the standard, until all salt crystals have dissolved.

○ Pour the solution into a 1-quart plastic bottle and label.

Prepare a blank using 500 ml of distilled water.Follow the directions for a water sample.Check technique every six months

.

Salinization

How?

Bad irrigationSmall amounts of salts ( CaCl2, MgSO4,

NaCl) are in waterWhen irrigation doesn’t wash them away

they build up in the soil Rise of the water table

Removal of native plants which leaves to reduction in evapo-transpiration○ Soil water gets close to the surface and

evaporates leaving behind the salts

Salinization Effects

Reduces plant growthIncrease in ions means a decrease in plants

ability to take up waterOsmosis is affected ---Suggested experiment: effect of time, temp., size of seed, type of soil, water

Nutrient Depletion

Decreases soil quality = decline in crop yield

How?crops that take nutrients and minerals from

the soil and are continually harvested reduce soil quality

Countering Nutrient Depletion

Replenishing minerals and nutrients Crop rotation Nitrate/ Phosphate fertilizers

Chemicals end up in water supply○ Leads to eutrophication (excess growth of

aquatic life)

Soil Pollution or Contamination

Due to pesticides, herbicides, fungicides Pesticides = kill insect pests

○ Lower crop yields – destroy flora/ fauna○ Pollute ground water

Herbicides = kill unwanted plants or weedsFungicides = controls fungi

Soil Contamination

Nitrogen/ Phosphate fertilizers Domestic/ industrial waste

Contain heavy metals (chromium, copper, mercury, lead, zinc)

Organic contaminants (PCBs, PAHs)

PAH

7.2 SOM

Soil organic matterThe mixture of non-living

organic components present in the soil in various stages of decomposition

arising from the chemical and biological action on plant

matter and organisms

SOM

Amount of carbon in the soil is a measure of the organic matter presentCan be tested through titration

using a dichromate(VI) solution

SOM (cont’d) Contains

Humic substances ○ Contain phenolic acid (ArOH) and carboxylic

acid (RCOOH) functional groups; weak acids RCOOH RCOO- + H+

The anion RCOO- binds to plant nutrients

Sugars Amino acidsHigh molecular mass polysaccharides and

proteins

Carboxylic acid Anion: Chelate to plant nutrients

Ca 2+ Fe 2+ Mg 2+ Ca 2+

Zn 2+ Mn 2+ Co 2+

Al 3+ > Mg 2+ > K

Higher the charge density, stronger the binding

Chelate

Chelate

Polysaccharides

polysaccharides

Commercial astragalus extracts have been standardized to 40-50% polysaccharides;

some sources claim ability to provide 70-90% polysaccharides.

Phenolic acid

Carboxylic acid

Carboxylic acid

Nylon

Relevance of SOM in Preventing Soil Degredation Plays important role in soil quality Deserts (SOM= Sweet soil)

Organic matter + sand = improved plant growth

Holds onto water Improves soil structure Reduces soil erosion E12.4 chemical function

SOM: Biological

Binds to nutrients – used by microbes and for plant growth

Bacteria and fungi decompose SOM SOM Has large amounts of

Nitrogen 5%○ Synthesis of proteins and chlorophyll

Phosphorus○ Synthesis of enzymes and storage of energy

Sulfur ○ Synthesis of amino acids

SOM: Mineralization /ongoing process Organic matter that contains nitrogen

and phosphorus is broken down or mineralized to release nitrogen and phosphorus to the root

N, P, K, S

SOM: Physical

Allows stable forms of soil clusters Reduces soil erosionIncrease water retention propertiesIncreases ability of air/ water to move

through soil More water affects thermal properties

Moist soil has a higher heat capacity than dry soil

Aka more water = smaller temperature change