Soil pH

1. Degree of soil acidity or alkalinity
2. Determined by the comparative concentraion of H⁺ and OH^- ions
3. Negative logarithm for the conc. of H⁺ ions in a soil
4. Sum off the H⁺ and OH^- charges must add up to make 10^-14

K_w

Ion product of the concentrations of the H⁺ and OH^- ions is a constant

@ 25^oc is 1 x 10^-14

[H⁺] + [OH-] = K_w = 1 x 10^-14

Factors That Affect Soil pH

1. Balance between acid and non-acid cations on colloid surface
2. Balance between H+ and OH- ions in the soil solution
3. Toxicity of non-nutrient element aluminium

Sources of

Hydrogen Ions

in

Soils

1. Carbonic acid
2. Accumulation of OM
3. Oxidation of Nitrogen
4. Oxidation of Sulfur
5. Acid Rain
6. Plant uptake of cations

Carbonic Acid

• H₂CO₃
• Weak acid
• Formed when CO2 from soil gas dissolves in H2O

CO₂ + H₂0 --> H₂CO₃ <--> HCO₃^- + H⁺

• Contributes negliable H⁺ ions when pH is < 5

pKa

pH value where the conc. of protonated and deprotonated speices are 50/50

pH at which dissocation reaction occurs

Strong acid= low pKa value

Accumulation of OM

Tends to acidify soil:

1. OM forms soluable complexes with non-acid nutrients such as Ca and Mg - facilitating the loss of these cations by nutrient leaching²⁺²⁺
2. OM is source of H+ ions as it contains numerous acid functional groups from which these ions can dissociate
3. Ph increase= more functional groups undergo dissocation of H+ ions, leaving behind an increased number of negatively charged sites on the molecule

Functional Group

An atom ,or group of atoms, attached to a large molecule

Each has a charcteristic reactivity

Nitrification

Ammonium ions (NH4+) from OM or fertiliser are subject to microbial organisms that convert the N to nitrate ions (NO3-)

NH₄⁺ + 2O₂ <--> H₂0 + H⁺ + HNO₃

HN0₃

Dissociated Nitric Acid

NO3^-

Strong acid

In nitrification reaction- does not tend to recombine with H⁺ ions to reproduce NH₄^-

NH₄⁺

Ammonium nitrate

!Very important to plants!

Oxidation of Sulphur

Decomposition of plant residues

Oxidation of organic -SH groups to yield sulfuric acid (H₂SO₄)

Produce large amounts of acidity in certain soils in which reduced sulphur is plentiful and oxygen levels are increased by drainage or excavation

H₂SO₄

Acid Rain

1. Raindrops fall through unpolluted air and dissove CO₂, forming carbonic acid to lower pH of water to about 5.6.
2. Nitric acids form in precipitation due to sulfur and nitrogen gases produced from numerous sources.
3. Strong acids completely dissociate to form H+ ions and sulfate or nitrate ions

H₂SO₄ <--> SO₂^- + 2H⁺

HNO₃ <--> NO₃^- + H⁺

Plant Uptake of Cations

Plants must maintain balanced charges on ions they take up from soil solution

For every positive chargetaken in on a cation, a root can maintain charge balance either by taking up an anion or giving out an other cation

 Alkalinizing Processes

1. Reduction of nitrate to nitrogen gases under anaerobic conditions

2. Acid rain--> protons added--> to change the soil pH cation speices are lost and replaced with H⁺ protons--> acidity is reduced

e.g. 1 Ca²⁺ ion lost and replaced with 2H⁺ ions

Role of Aluminium

Always plays a role in soil acidity

H⁺ ions are adsorbed on a clay surface, they usually do not remain as an exchangeable cations for long, they attack the structure of the minerals, releasing Al³⁺ ions in the process

Dissolved Al3+ = bad!

Al undergoes corrosion and oxidation 

Hydrolysis Reactions of Al³⁺

Al³⁺ + H₂0 <--> AlOH²⁺ + H⁺  

(OH- from water binds with Al³⁺)

AlOH²⁺ + H₂0 <--> AlOH₂⁺ + H⁺

(OH^- from water binds with AlOH²⁺)

AlOH₂⁺ + H₂0 <--> AlOH₃ + H⁺ 

(OH^- from water binds with AlOH₂⁺)

Roles of Al³⁺

in Soil Acidity

Acid Cations

Pools of Soil Acidity

1. Active acidity

2. Exchangeable acidity

3. Residual acidity

Active Acidity

H⁺ ion activity in the soil solution

Pool is very small compared to the acidity in the exchangeable  and residual pools

Extremely important

Determines the solubility of many substances and provides the soil solution environment to which plant roots and microbes are exposed

Exchangeable Acidity

Exchangeable aluminium and hydrogen ions present in large quantities in very acid soils 

Released into the soil solution by cation exchange with an unbuffered salt, such as KCl

Aluminium hydrolyzes to form additional H⁺ 

Evn in moderatley acid soils, the limestone needed to neutralise this type of acidity is commonly more than 100 times that needed to neutralise the soil solution

At a given pH, exchangeable acidity is generally highest for smectites, intermediate for vemiculites, and lowest for kaolinite

Residual Activity 

Associated with the Al³⁺ and H⁺ that are bound in nonexchangeable forms by OM and clays

pH increases, the bound hydrogen dissociates and the bound aluminum ions are released and precipitate as amorphous Al(OH)₃ 

Changes free up negative cation exchange sites and increase the cation exchange capacity

Residual activity is commonly far greater than either the active or exchangeable acidity

Cation Saturation Percentage 

Proportion of the CEC occupation by a given ion 

100% saturation= (% acid saturation + % base saturation)

% acid saturation= cmol_c of exchangeable Al³⁺ + H⁺ 

                    cmolc of CEC

% nonacid saturation= % base saturation=

cmol_c of exchangeable  Ca²⁺ + Mg²⁺ + K⁺ + Na⁺

cmol_c of CEC

=100 - % acid saturation

Base/

Nonacid Saturation

Refers to the proportion of

Ca²⁺, Mg²⁺, K⁺ and Na⁺ on the CEC

Want a greater % of base cations as they act as reserves to be replaced with acid cations when acid rain occurs

Relationship between

Acid Cation Saturation

and pH

Soils with highest % acid saturation have the lowest pH

pH increases as the % acid saturation decreases, regardless of changes in the cations exchange capacity 

Ph is more closely to % acid saturation than to the absolute acid cations

E.g.

Even though a soil has twice as man mol_c of acis cations, a soil with a lower % acid saturation will have a lower pH

Buffering

Soils resist change in the pH of the soil solution when either acid or base is added

How is a

Titration Curve

Obtained?

By monitoring the pH of a solution as an acid or base is added in small increments

Soils are most higly buffered when aluminum compunds (low pH) and carbonates (high pH) 

Soil is least well buffered at intermediate pH levels where H+ ion dissociation and cation exchange are the primary buffer mechanisms

Buffering Mechanisms

1. Residual--> AlO associated with clay and OM
2. Exchangeable--> functional (hydroxyl and protonated) groups of OM and clay surfaces; pH dependent CEC sites
3. Active--> dissolved Al³⁺ and H⁺ in solution- avaliable in soil- buffering capacity

Residual Activity

(Buffering Capacity)

Al hydrolysis reactions contribute to buffering mechanisms at acidic pH (4- 5.5)

Al(OH)₃ +H⁺ --> Al(OH)₂⁺ + H₂O

acid added to soil   dissolution    formation of water-

                                                           no pH change

Exchangeable Activity

1. OM Reactions

(Buffering Capacity)

RCOOH + OH^- --> RCOO^- +H₂O

• At acidic pH
• Surface functional group of OM is protonated
• When base is added, the reaction produces H₂O
• No change in pH

Exchangeable Activity

2. pH Dependent Site of Clay

(Buffering Capacity)

≡Si-OH --- H⁺ --> ≡Si-OH + H₂O

• When base is added, H+ dissocate from the surface functional group
• Resulting in no changes in the hydrogen ion conc. in soil solution
• No change in pH

Exchangeable Activity

3. Cation Exchange

(Salt-Replaceable)

(Buffering Capacity)

≡Al-OH --- K⁺ +H⁺ --> ≡Al-OH --- H⁺ + K⁺

• When acid is added, H+ can displace an exchangeable K+ ion
• No change in pH

Active Acidity Pool

(Buffering Capacity)

H⁺ + OH^- --> H₂0

• When you add base, dissolved H+ ions form water with base

Human-Influenced Soil Acidification

1. Nitrogen Fixation

!!AMMONIUM SULPHATE FERTILISERS ACIDIFY!!

(NH₄⁺)₂SO₄ + 40₂ <--> 2HNO₃ + H₂SO₄ + 2H₂0

(Ammonium sulphate)    

2HNO₃ + H₂SO₄ + 2H₂O --> 4H⁺ + 2H₂0

• Soil pH decreases after N fertiliser application
• Surface soils show more pronounced acidification than sub-soils due to HNO3 production 

Human-Influenced Soil Acification

1. Nitrogen Fertilisation + Tillage Graph

Human-Influenced Soil Acidification

2. Acid Deposition Steps From Deposition

• Nitric oxides and sulphur dioxide gases are converted into H2SO4 and HNO3
• In rain, they readily dissociates into H⁺ and SO₄^2- or NO₃^-; makes pH as low as 4 or 2!
• Once exchangeable Ca²⁺ is depleted exchangeable Al³⁺ and H⁺ become dominant species in soil solution --> further lowering of pH of soil
• Low pH promotes leaching of Ca ions --> further pH decreases

Acid Rain In S.E. North America

• Rainfall= pH 4.88
• Class 1= most sensitive (base cations in soil= low)
• Poorly buffered
• Low CEC values
• High acid saturation
• Affected by down-wind current from major industrial centers

Effects of Acid Rain

On forests

• Forest trees often require high Ca to synthesis wood
• When soil acidification continues Ca/Al ration drops below 1 (i.e. threshold for the plant Al toxicity)
• Al3+ increase = pH decrease
• Aluminium hydrolysis reactions --> releases H+ ions

Effects of Acid Rain

(On aquatic ecosystems)

• Direct Al toxictiy to fish
• pH≈5 is a threshold for fish to survive

Biological Effects of Soil pH

(Aluminium Toxicity)

• Affects plants (e.g. roots growth) and microbial activites (e.g. N cycle)
• At pH <5.2, Al³⁺ and AlOH²⁺ species becomes more soluable --> prounounced Al toxicity to plants
• At acidic pH, most metals presents
• Symptoms: a stunned root system and chlorotic (yellowish) leaves

Biological Effects of Soil pH

(Managase, Hydrogen and Iron Toxicity to Plants)

• Mn and Fe are essential plant nutrients
• Excessive quantities cause toxicity at pH ≤5.6 for Mn and ≤4 for Fe
• Reduction of Mn⁴ --> Mn²⁺ and Fe³⁺ --> Fe²⁺ (e.g. in rice paddies) induce the toxicity
• At pH <4, H⁺ directly affect the root membranes

Biological Effects of Soil pH

(Nutrient Avaliability to Plants)

• In strongly acidic soils (pH<5), the avaliability of macronutrients (Ca, Mg, P, N and S) is low; avaliability of several cations (e.g. Fe, Mn, Cu and Co) increases
• At the moderately alkaline pH, most macronutirents are avaliable, but macronutrients are less avalaible
• P and Ca²⁺ precipates out at higher pH's
• Fe, Mg and Mn bind together at lower pH and precipiatate out- not avaliable to plants and bacteria etc.
• Ideal pH values for plants are at pH 5.5-6.5

Biological Effects of Soil pH

(Microbial Effects)

• Actinomycetes and bacteria are active at intermediate to higher pH values
• Fungal activity becomes dominant at acidic pH
• Legume crops (e.g. alfafa and clover) prefer near- neutral to alkaine pH
• Forest species (e.g. Azaleas- apprepiate macronuritents, do not like neutral soils, need a lot of ions) prefer acidic soils- need Fe. Also aspharagus and peaches
• Cultivated crops grow well at slightly acidic to near neutral pH

• Ca²⁺ lowers pH
• Bicarbonate increase pH
• Common lime materials- Marl (CaCO₃), Dolomite limestone (CaMg(CO₃)₂) and Wood ash (CaO)
• Carbonate (CO₃^2-) and Hydroxide (OH^-) ions are conguate bases which can consume H⁺ from weak acid like water

Raising Soil pH by Liming

(Chemical Equations)

!!Hydroxyl ions are produced in both cases!!

Case 1:

CaCO₃ + H₂O --> Ca₂⁺ + CO₃^2- +H₂0 --> Ca²⁺ + CO₃^2- --- H⁺ +OH^-

Case 2:

CaO + H₂O --> Ca(OH)₂ --> Ca²⁺ + 2OH^-

Calcium oxide readily dissolves in water, and the form calcium hydroxides

This further dissoiates into Ca²⁺ and 2OH^- ions

How Does Liming Work?

E.g. dolomite-limestone

CaMgCO₃ + H₂O --> Ca2⁺ + Mg²⁺ + 2CO₃^- --- 2H⁺ + 2OH^-

Ca2+ and Mg2+ are dissociated and carbonate ions gain two hydrogen ions from a water molecule, resulting in the production of two hydoxyl ions

1) First, Ca and Mg ions displace the exchangeable Al + H in clay humus surfaces

H⁺ and Al³⁺ on colloidal material + 2Ca²⁺ + 2HCO₃^- + 2OH^-

²⁾ Hydroxyl ions react with Al³⁺ to form a soluble Al(OH)3

Al³⁺ + 2OH^- --> Al(OH)³

3) A hydroxyl ion reacts with H⁺ to form water

OH^- + H⁺ --> H₂0

Lime

Factors Affecting Liming Requirements

1. Specific pH requiremtns
2. Buffering capacity of soils
3. Chemical composition of liming materials
4. Fineness of liming materials

Factors Affecting Liming Requirments

(Buffering Capacity of Soil)

• If soils are well-buffered, we need to add more lime to raise pH
• Especially in clayey soils and OM rich soils
• Sand--> greater SA= greater CEC = more exchangeable buffering capacity mechanims
• Clay--> finer texture requires more lime to change pH

Factors Affecting Liming Requirements

(Chemical composition of lime materials)

• Chemical composition affects the long term ph stability
• 100% CaCO₃

Factors Affecting Liming Requirements

(Fineness of Liming Materials)

• Dissolution of lime materials is SA driven
• Finer material= faster dissolution
• At least 50% of particles <0.25mm in diameter are good liming materials

Estimating Lime Requirements

(for soils with low CEC)

Exchangeable aluminum method:

Enough lime to eliminate the exchangeable alumium (instead of raising pH to 'target level')

E.g.

50% total CEC= 10cmol/kg= Al exchangeable acidity

so 5 cmol of Al³⁺ ions need to be displaced and would require 5cmol/kg of CaCO³

5cmol_c/kg x (100g/mol CaCO₃) x (1mol CaCO₃/ 2mol) x 10.0Lmolc x cmol_c)

= 2.5g CaCO₃/kg soil

To convert to kg/ha - x2x10⁶ kg/ha

=2.5g/kg x2x10⁶ kg/ha

=5000kg/ha soil

How to Lower Soil pH if

Over-Limed

Organic Material Amendments:

decomposed OM can be added- it contains organic acids that help to lower pH

Inorganic Material Amendments:

Ferrous sulphate (FeSO4) and Alum (ALl₂(SO₄)₃)

Abiotic--> Fe²⁺ + 2H₂O--> Fe(OH)² + 2H⁺

4Fe²⁺ + 6H₂O + O₂ --> 4Fe(OH)₂ + 4H⁺

Biotic--> 2S + 3O₂ + H₂O --> 2H₂SO₄ --> 2H⁺ + SO₄^2-

microbial oxidation of elemental sulfur

Causes of Soil Alkalinity

(i.e. high soil pH)

Causes of Soil Alkalinity

(Reaction Sequences)

Reaction chain sequences-->

1. CaCO₃ <--> Ca²⁺ + CO₃^2-

2. CO₃^2- + H₂0 <--> HCO₃^- + OH^-

3. HCO₃^2- + H₂O <--> H₂CO³ + OH^-

4. H₂CO₃ <--> H₂O + CO₂

When CO₂ is abundant, the reaction goes back from 4-1 --> consumption of OH^- results in lowering pH

Common ion effect: when the system is saturated with Ca²⁺, precipitation of CaCO₃ is favoured in the system (overall reaction does back from 4-1)

Reaction sequences of sodium carbonate- carbonic acid- CO₂:

1. Na₂CO₃ <--> 2Na⁺ + CO₃^2-

2. CO₃^2- + H₂O <--> HCO₃^- + OH^-

3. HCO₃^- + H₂O <--> H₂CO₃ + OH^-

4. H₂CO₃ <--> H₂O + CO₂

Na based carbonates= very soluable!!

Role of Na in Alkaline Soils

• Na₂CO₃ is more soluable than CaCO₃
• NaCO rich soils show pH as high as 10 or higher

Characteristics and Problems of Alkaline Soils

1. Mirconutirent deficency
2. Boron deficency
3. Phosphorus deficiency
4. Macronutrient availability
5. Cation Exchange Capacity
6. Calcium Rich Layer

Characteristics and Problems of Alkaline Soils

(Micronutrient deficency)

Zn, Cu, Fe and Mn are not available because of the hydrolysis products and metal-carbonate precipitates

Zn²⁺ +2(OH^-) --> Zn(OH^-)₂ ↓

Zn²⁺ + CO₃ --> ZnCO₃ ↓

Characteristics and Problems of Alkaline Soils

(Boron deficency)

Boron strongly sorbed on soil mineral surafces at alkaline pH- it is not readily avaliable

Clay loam- strongly retains boron, due to high SA

Sandy loam- retains less boron, due to low SA

Characteristics and Problems of Alkaline Soils

(Phosphorous deficency)

Characteristics and Problems of Alkaline Soils

(Macronutrient avilability)

Ins trongly acidic soils, the availability of macronutrients is reduced, yet the availability of cations increases

At moderately alkaline pH, most macronutrients are available, but micronutrients are less available

Characteristics and Problems of Alkaline Soils

(Cation Exchange Capacity)

CEC generally ↑ with pH ↑

The presence of 2:1 clays in alkaline soils is a good source of permenant charge

pH dependent charge of other minerals also contribute to high CEC

Characteristics and Problems of Alkaline Soils

(Calcium rich layer)

• Calification occurs in warmer, semi-arid environments, usually under the grassland vegetation
• Soils tends to be rich in OM and high in soluable bases
• The B horizon of the soil is enriched with calcium carbonate precipitates from water moving downwards through the soil, or upward by capillary action of water from below

Development of

Salt-Affected Soils

Natural process:

› In most cases, soluable salts are originated from weathering or primary and secondary minerals in soi

› In the dry regions, water evapourates fast, and leaves salt behind (gypsic horizons)

Human activity:

› Result of lateral encorachment of seawater

› Salts usually become a problem when too much water is supplied, not too little--> increased input of salt-bearing water, more than increased draiange water

Irrigation water, over time, drops large quantities of salt- even from best quality pure freshwater, which accumulate over time

Pure water is lost through evapouration, but the salt stays and accumulates

Accentuated in arid regions:

1. Water available from rivers or underground is relatively high in salts as it has flowed through dry-region soils that typically contain large amounts of weatherable materials

2. The dry climate creates a relatively high evapouative demand, so large amounts of water are needed for irrigation

If irrigation water contains a significant proportion of Na⁺ ions comapred to Ca²⁺ and Mg²⁺ ions (especially if HCO^3- is present) sodium ions will come to saturate a major part of the colloidal exchange sites, creating an unproductive site, with sodic soil

Salinization

How Do we Measure Salinity and Sodicity?

Total Dissolved Solids (TDS)

• To determine the total amount of dissolved salt in a sample of water
• Heat the solution to at least 180°^c  until only a dry residue remains
• Measure the residue
• TDS= milligrams of solid residue/ L of water (mg/L)

Electrical Conductivity (EC)

• Conductivity increases as more and more salt is dissolved in water
• Indirect measurement of the salt content
• Measured either on samples of soil or on the bulksoil insitu
• deciSiemens per meter (dS/m)

Expressions of

Sodium Status

(Exchangeable Sodium Percentage) (ESP)

 Identify the degree to which the exchange complex is saturated with Na

ESP= Exchangeable Na+, cmolc/kg

         Cation exchange capacity, cmolc/kg

>15- associated with severly deteriorated soil physical properties

-ph values of <8.5

Expressions of

Sodium Status

(Sodium Absorption Ration)

(SAR)

Comparative concentrations of Na⁺, Ca²⁺ and Mg²⁺

SAR (mmol of charge per L)=                 [Na+]               

                                         (0.5[Ca²⁺] + 0.5[Mg²⁺]) ½

 SAR value of 13 for the solution extracted from a saturated soil paste is approx. equivalent to an ESP value of 15

Draw Graph Showing

How to Classify Soils Using

pH, EC, ESP and SAR

Classes of Salt-Affected Soils:

Saline

• Contain sufficent salinity to give EC values >4dS/m
• ESP <15
• SAR <13
• Exchange complex of saline soils is dominated by Ca²⁺, Mg²⁺ and Na⁺
• pH is usually <8.5
• Soluable salts help prevent dispersion of soil colloids, plant growth on saline soils is not generally constrained by poor infiltration, aggregate stability or aeration

Classes of Salt-Affected Soils:

Saline-Sodic Soils

• Soils that have both detrimental levels of neutral soluables salts and a high proportion of Na⁺ ions
• ESP >15
• SAR >13
• Physical properties intermediate between those of salie and sodic soils
• High conc. of neutral salts moderates the dispersing influence of the soil
• Salts provide excess cations that move in close to the negatively charged colloidal particles, reducing their tendancy to repel or disperse each other

How Do Salt-Affected Soils Influence Plant Growth?

Osmosis

• Soluable salts lower the osmotic potiential of the soil water making it more difficult for roots to remove water from the soil
• Rarely results in wilting or even reduced water uptake, does require that plants expqand more energy making osmotic adjustments
• Accumulating organic and inorganic solutes to lower the osmotic potiential inside their cells to counteract the low osomotic potiential of the soil solution outside
• Plants are most susceptible to slat damage in the early stages of growth
• Young seedlings may be killed by saline conditions that older plants of the same species can tolerate
• The radicle appears to be particularly sensitive to salinity
• Young root cells encounter a soil solution high in salts, they may lose water by osmosis to the more concentrated soil solutions
• Cells collapse
• Salinity delays or prevents germination

How Do Salt-Affected Soils Influence Plant Growth?

Specific Ion Effect

• Kind of salt makes a big difference in how plants respond to salinity
• Na⁺, Cl^-, H₃BO₄^- and HCO₃^- are quite toxic
• Even strains within a species differ widely in their sensitivity to these ions
• High levels of Na⁺ competes with K⁺ in transport across the cell membrane during uptake, making it difficult for plants to obtain the K⁺ they need from saline-sodic to sodic soils
• Adequate Ca²⁺ helps the plant discriminate against Na⁺ and K⁺

How Do Salt-Affected Soils Influence Plant Growth?

Physical Effects of Sodicity

• Deterioration of physical properties may be a factor in determining which plants can grow in sodic soils
• Collodial dispersion can harm plants in at two ways:

1. O₂ deficency due to breakdown of soil structure and the very limited air movement that results

2. Water relations are poor due to the very slow infiltration and percolation rates

Plant Sensitivity:

Four general salt-tolerance groups- sensitive, moderately sensitive, moderately tolerant and tolerant

Plants that can grow on salty soils=

• Halophytes (salt-loving plants_
• Salt-tolerant plant varieties developed by plant breeders

Genetic Improvements:

• Plant breeders able to produce plant strains with salt-tolerance greater than that possessed by conventional varieties
• Discovery of a single gene that enables halophytes to sequester high amounts of Na⁺ in their cellular vacoules
• In the vacoule, the Na⁺ ions can act to the plants advantage by contributing to low internal osmotic potiential, whilst remaining isolated from cellular systems that are susceptible to Na⁺ toxcity

Salt Problems Related to Non-Arid Climates

Deicing salts--> used to keep roads and sidewalks free of snow and ice during winter months

--> recently switched from NaCl --> KCl or MgCl₂, to avoid the specific chemical and physical problems associated with Na⁺ salts

Containerised plants--> salts build-up over time if care is not taken to flush them out

Good

Irrigation Management

Good management=

salt entered system= salt removed from system

1. Irrigation Water Quality
2. Drainage Water Salinity

Good

Irrigation Management

(Irrigation Water Quality)

• Monitoring the chemical quality of irrigation water to salt affected soils is a prime management stategy
• Salty water can be used as long as we carefully manage the salt input and output

Good

Irrigation Management

(Drainage Water Salinity)

• Drainage water contains more slats than the irrigation water applied to the same field
• Common for there to be downstream environmental damage
• Most efficent approach is to collect the drainage water, keep it isolated and reuse it to irrigate a more salt-tolerant crop. This approach provides high-quality canal-water and low-quality drainage water for use on appropriate crops
• After several cycles of reuse, the drainage water must be disposed of, as it will be too saline for irrigating
• More common approach is to route the drainage water back into the canal, mixes poor-quality drainage with the high-qualirt canal water
• Overly saline is channeled into small ponds that allow the water to evapourate and the salts to be collected
• Toxic elements such as molybdenum, arsenic and selenium accumulate

Soil Reclamation

Reclamation of Saline Soils

Reclamation of saline soils = minimise leaching

• Largely dependant on the provision of effective drainage and availability of good quality irrigation water
• Leaching requirement

Leaching Requirement

• The amount of water required to remove the excess salts from saline soils
• Is determined by the characterisitvs of the crop, the irrigation water and the soil
• Ratio of salinity of the irrigation water (EC_iw) to the maximum acceptable salinity of the soil solution for the crop to be grown (EC_dw- EC of drainage water)

LR= EC_iw

       EC_dw

If Ec_iw is high and a salt-sensitive crop is chosen (i.e. high LR value), an extensive leaching program is realised

Goal is to minimise LR and the amount of drainage water that requires disposal

Management

of

Soil Salinity

• Must minimise drainage water and protect root zone

Irrigation timing:

• Young seedlings and germinating seeds are 'salt-sensitive'
• Irrigation right are planting to move the salts away from the root zone

Spatical Variability:

Lower EC level- less irrigation needed

Higher EC level- irrigate more often than lower EC- want to get rid of salt

Accurate EC value for whole system needed to give precise irrigation requirements--> called precision agriculture

Management

of Soil Salinity

Reclamation of Saline-Sodic and Sodic Soils

Attention must be given to reducing the level of exchangeable Na ions and then to be problem of excess salts (solution to dispersion)

1) Gypsum amendments-->

Removing Na+ and replacing them with either Ca2+  or H+ ions

Provide Ca2+ in form of gypsum

Reclamation of Saline-Sodic and Sodic Soils

(Gypsum Amendments)

• Removing Na⁺ and replacing them with either Ca²⁺  or H⁺ ions
• Provide Ca²⁺ in form of gypsum (CaSO₄)

2NaHCO₃ + CaSO₄--> CaCO₃ + Na₂SO₄ + CO₂↑ + H₂O

Na₂CO₃ + CaSO₄ ↔ CaCO₃ + Na₂SO₄

Colliod with 2Na⁺  + CaSO₄ ↔ Ca²⁺ replaces 2Na⁺ + Na₂SO₄

Na₂SO₄= soluable salt easily leached

Treatment must be supplemented later by through leaching of the soil with irrigation water to elach out most of the sodium sulfate

Reclamation of Saline-Sodic and Sodic Soils

(Sulfuric Acid Application)

• Sulfur upon biological oxidation, yields sulfuric acid
• Sulfuric acid changes sodium bicarbonate to less harmful and more leachable sodium sulfate and decreases pH

2NaHCO₃ + H₂SO₄ --> 2CO₂↑ + 2H₂O + Na₂SO₄

Na₂CO₃ + H₂SO₄ --> CO₂↑ + H₂O + Na₂SO₄

2Na⁺ on colloidal surface + H₂SO₄ ↔ 2H⁺ on colloidal surface + Na₂SO₄

Decomposition

Breakdown of large molecules into smaller and simpler components

When organic tissue is added to soil 3 general reactions take place (mechanical shredding by soil fauna must occur before these reactions can occur)

1. Enzymatic oxidation of carbon compounds to produce CO2, H2O, energy and decomposer biomass-

R(C,4H) + 2O₂ --> CO₂↑ + 2H₂O + energy

2. Release and/or immobilization of the essential nutrient elements, by a series of specific reactions that are relatively unique for each element

3. Formation of compounds very resistent to microbial action

To speed up decomposition add heat -38^oc- room temp. and break it up

Rates of Decomposition

↑ Rapid Decomposition ↑

Sugars, starches and simple proteins- 27%

Crude protiens- 8%

Hemicellulose- 18%

Cellulose- 45%

Fats and waxes- 2%

Lignins and phenolic compounds- 20%

↓ Very Slow Decomposition ↓

Decomposition:

An Oxidation Process

In well-aerated soils, all the organic compounds found in plant residues are subject to oxidation

R- (C, 4H) + 2O₂ --ENZYMATIC OXIDATION--> CO₂↑ + 2H₂O + energy (478kJmol_c)

Important side reactions that involve elements other than C and H accompany it

Breakdown of Cellulose and Starch

Cellulose and starch are polysaccharides

Enzymatic degradation proceeds in steps:

1. Long chains are broken down by rather specialised organisms into short chains, then into individuals sugar (glucose) molecules
2. COC chemical bonds linking the sugar molecules of cellulose together are much more difficult to break than those of starch, the intial step in cellulose decomposition reuires a specialised organism that produces the enzyme cellulase

Methanogenic Bacteria

Contribute to...

4C₂H₅COOH + 2H₂O --> 4CH₃COOH + CO₂↑ + 3CH₄↑

CH₃COOH --> CO₂↑ + CH₄↑

CO₂ + 4H₂ --> 2H₂O↑ + CH₄↑

Process of Organic Decay

1. Small populations of autochthomous (native) organisms are slowly digesting very reistent OM --> low respiration
2. As decomposable materials accumulate, zymogenous organisms take over --> CO₂ production increase exponientially
3. Microbial biomass reaches 1/6 of OM, some resistant materials start to breakdown known as priming effect
4. As food sources run out, some zymogenous organisms die out and then decompose. Mineralisation occurs
5. As food sources run out, it goes back to stage 1 until new raw materials are depositied

Priming Effect

Mineralisation

Zymogenous Organisms

Autochonous Organisms

Organic Decay

How C/N Ratio Influences OM Decomposition

• Organisms must obtain sufficent N to synthesise nitrogen- containing cellular components (i.e. amino acids, enezymes and DNA)
• Soil microbes must incorporate into their cels about 8 parts of carbon for everyone part of nitrogen (C/N ratio 8:1)
• Only 1/3 of carbon metabolised by microbes incorporated into their cells

High N= low C/N ratio= rapid decomposition

When C/N >25/1, microbes seek out soluble N in soil solution--> N deficiency for plants- we wants soluble N (nitrates) to be avaliable to plants

Low N Material

High C/N Ratio

(i.e. C/N >25)

As added readily decomposable OM residues contacts the soil, the microbial community responds to the new food supply

Heterotrophic zymogenous micro-organisms become active- multiple rapidly and yield CO₂ in large quantities

Because of the microbial demand for N, little or no minerla N (NH₄⁺ or NO₃^-) is avaliable to higher plants during this period

Nitrate Depression

• A.K.A. Nitrate Depression period
• Persists until the activities of the decay organisms gradually subside due to lack of easily oxidisable carbon
• As their numbers ↓, CO2 formqation drops off and N demand by microbes becomes less acute
• As decay proceeds, C/N ratio of remainign plant material decreases as C is being lost (by respiration) and N is being conserved (through incorpation into microbial cells)
• Mineral N begins to be released when the C/N ratio of the material drops below 20- nitrates appear again in quantities and original conditions prevail, soil is richer in N and humus

High N Material

• OM with low C/N ratio, more than enough N is present
• After decomposition begins, some N from the OM is released into the soil solution, augmenting the level of soluble N avaliable for plant uptake

Nitrogen-rich materials decompose quite rapidly--> period of intense microbial growth and activity, no nitrate depression

Influence of Lignin and Polyphenol Content of OM

Lignin= difficult for microbes to digest it; hence high lignin content means very slow decomposition

Polyphenol compounds= may inhibit decomposition; often water soluble; form highly resistant complexes with protiens during residue decomposition, slowing the rates of N mineralisation and C oxidation

Residues high in phenols and/or lignin are considered ppor quality resources for the soil organisms that cycle C/N

Influence of

Lignin and Polyphenols

on OM

Soil Organic Matter

(SOM)

Biomass + Detritus + Humus

Biomass

Intact plant and animal tissues and microorganisms

Soil Humus

Largely amporphous (non-crystalline constituents of soil) and colloidals mixture of complex organisc susbstances no longer identifiable as tissue

Soil Organic Carbon

(SOC)

C component of SOM

Humus

Make up of Soil Organic Matter

Humic Substances

Consists of large organic molecules

Large net negative charge- CEC= 150-300cmol/kg- average soil= 3-50 cmol/kg

High surface and many hydrophillic and some hydrophobic sites

Unclear/ dark black matter

Resistant and complex polymers

Humin Substances

Highly condensed, complexed with clays

Highest molecule weight

Insolubale in both acid and base

Most resistent to microbial digestion

Fulvic Acid

Lowest molecular wight

Yellow to red

Soluble in acid and base

Most susceptible to microbial attack

Non-Humic Substances

• 20-30% of humus
• Less complex and less resistent to the decomposition
• Among the specific biomolecules with definite physical and chemical properties, polysaccarides (sugar-like) are important in soil aggregate formation
• C_n(H₂O)_m
• n and m are variable

Stability of Humus

• Humus is resistent to microbial attack, resulting in conserving plant nutrients such as N
• Interactions with clay minerals provide the stability of humus (i.e. stored soil N longer)

How to Define SOM

Based on the pools of SOC that are varied in their susceptibility to microbes

Active SOM- high C/N ratio and short-half life- includes detritus (particulate Om) 1-2 years

Slow SOM- stable and last about decades; associated with colloidal properties of soil humus (15-1000 year) C/N= 10-25

Passive SOM- very stable and last long time; associated with collodial properties of soil humus

• Productive soils managed with conservation-oriented practices contain relatively high amounts of OM fractions associated with the active pool, including microbial biomass, particulate OM and oxidisable sugars
• Existence of active, slow and passive pools have proven very useful in explaining and predicting real changes in SOM levels and in attendent soil properties
• Different pools of SOM play quite different roles in the soil system in the carbon cycle
• Presence of a resistent (structural) pool of carbon in plant residues, as well as an easily decomposed (metabolic) pool, explains the initially rapid but decelerating rate of decay that occurs when plant tissues are added to a soil

1. As soon as undisturbed soils are cultivated, decomposition of active and slow SOM pools begins. But the loss from passive fraction is slow
2. This results in the loss of stability and nutrient cycling
3. After the introduction of native vegetation, soil OM starts to recover

Carbon-Balance in Natural Ecosystems 

Agroecosystems

Ideal to increase C input to soil and decrease loss

Increasing more plant materials (e.g. crop production)

Control erosion, the use of conservation tillage (no-till)

Carbon-Balance in Natural Ecosystems

Forest Floors

Forest floors standing biomass is much greater than agroecosystems

The rate of OM decomposition is very slow due to:

1. Absense of physical disturbance (e.g. tillage practice)
2. Production of phenolic compounds and lignin that are resistant to the decomposition and C losses
3. Small C loss by erosion

Overall net gain of C

Sources of

Soil Carbon Gain

Green manures (plant material incorporated with the soil while green or soon after for improving soil)

Conservation tillage

Return of plant residues

Low temperatures and shading

High soil moisture

Surface mulches

Application of compost and manures

Appropriate levels

High plant productivity

High plant root: shoot ratio

Effects of

Conventional Tillage

on Soil Carbon

Short-term effects= aeration= increased air= increased D_b

Long-term effects= increased D_b--> expose SOC--> fine collodial materials that compact

Studies show that less tillage= more soil OC (in top 5-10cm)

Effects of

Conservation Tillage

on Soil Carbon

Techniques that protect the soil from erosion and also discourage the rapid discourage rapid decomposition of crop residues

Help maintain or restore high surface SOC levels

Complex rotation achieved higher SOC levels than mono-cropping- uses less tillage less frequently and produced more root residues

Systems that maintain soil fertility with manure, lime and phosphorous stimulate much higher OC levels --> greater additions of OM in the manure and in the residues from higher-yielding crops

Application of lime and fertilisers to previously unfertilised and unmanured plots, noticeably increased SOM levels--> production and return of larger amounts of crop residues and addition of sufficent N to compliment the carbon in humus formation

Sources of

Carbon Loss

from Soil

Erosion

Intensive tillage

Whole plant removal

High temperatures and exposure to sun

Fire

Higher plant productivity in the well watered environemtn yields in SOM

Finer textured soils accumulate more SOM because:

1) Produce more biomass

2) Lose less SOM due to poor aeration

3) Clay- humus complex tends to retard the decomposition of SOM

Poorly drained soils promote the plant matter production and poor aeration inhibits the OM decomposition

Distribution of SOC across North America

GHGs cause the Earth to be much warmer than it would be otherwise be

Allow short-lenght radiation in, but trap much of the outgoing long-lenght

• CO₂
• CH₄ (Methane)
• N₂O (Nitrous oxide)
• CFCs

CO₂

Average ≈400ppm in atmosphere today

Need to gain C in soils--> to improve the soil quality and plant productivity

Active fraction--> passive fraction that can sequester C for hundreds or thousands of years

CH₄

• Small amount of CH₄- yet its efficency at shielding means it is becoming as much as an issue as CO₂
• Conc. is rising at ≈0.6% a year

Major sources are:

Anaerobic decomposition of carbon sources in wetlands and rice paddies

Propionate + 2H₂0 --methanogenic bacteria--> acetate + CO₂  + 3CH₄

Anaerobic decomposition in ruminant animals

Landfills

Biomass and fossil fuel combustion

N₂O

Sources:

Soil microbiological processes largley contribute to the global N₂O emmissions

Denitrification process in anaerobic environments

In waterlogged soils- N is reduced and becomes N₂O

Denitrification

Biochemical reduction of nitrate to gaseous nitrogen

2NO₃^- --> N₂O

CFCs

Sources:

Chemical reactions among ozone precursors in the atmosphere...

1) Fuel combustion from cars, trains, equipemnt, power plants, oil refineries and factories

2) Organic compound evapouration from consumer products such as paints, cleaners and solvent

Good O₃

Helps shield from UV rays

i.e. that is why we care so much about hole in O₃ layer

Bad O₃

Too much is bad

 O₃ in air that we breathe in is toxic and harmful

Practices to Decrease GHG Production from Soils

• Reforestation of denuded areas
• Switching cropland to conventioanl tillage to no-tillage- sequesters 0.2- 0.5 Mg/ha of C during the first decade
• Conversion of cultivated land may sequester C at twice these rates
• Increasing SOM to near precultivated levels

Biofuel Production and Effect on Soil D_b

E.g. corn stover

• Controversy over whether such fuels will actually produce more energy than is consumed in the sossil fuels used to grow and process the crops
• Decreased Db with production of corn stover
• In long term the C will decrease in soil

Types of Composting

Vermicomposting

Thermophilic Composting

Vermicomposting

High quality compost made at ambient temperatures by a slow decomposition process

Epigeic (litter-dwelling) earthworms are added to help transform the material

Consists of the casts made by earthworms eating the raw OM in moist, aerated piles

Piles are kept shallow to avoid heat build-up that would kill the earthworms