Soil science exam #2

Soils + the hydrologic cycle 

1. Water balance 

   1. SS = P - ET - D

   2. Evapotranspiration = evaporation + transpiration 

   3. Transpiration - loss of water from plants such as trees + grasses 

   4. Evaporation : loss of water from water bodies and land surface 

   5. Water balance over a year 

      1. Precipitation  > ET surplus can go to soil storage groundwater recharge runoff 

      2. ET> Precipitation Drawing down stores soil water
         
         

      3. Water balance over a year in an arid environment

      4. This is because the ET is pushing up against precipitation, no opportunity for excess precipitation

   6. Potential evapotranspiration

      1. Amount of water respired from a well watered, densely vegetated system

      2. Actual ET may not reach PET - because systems are not always well watered 

      3. Estimation techniques often incorporate multiple components of ecosystems (radiation, temp, wind, and atmospheric pressure)

Soil aeration 

The process by which air in the soil is replaced by air in the atmosphere 

• A well aerated soil the soil air is similar in composition to atmosphere above and poorly aerated contain more CO2 and less O2

• Soil aration in terms of gaseous composition of soil air 

• Soil aeration primarily controlled by 

  • Soil water content

  • Soil mnacroporosity 

  • Rate of O2 consumption in the soil 

• Influence of water on soil aeration

  • Soil water reduces soil aration

  • Because rate of O2 diffusion through water is many times slower than the rate of O2 diffusion through air

• Water filled pore space

  • % water filled pore space = volume of soil water / volume of soil pores

  • Soil is saturated when water filled pore space = 100%

  • R

• Rate of O2 consumption of soils

  • Micorbial activity involves microbial respiration which consumes O2 and produces CO2

  • Microbial activity increases with temp, up to a maximum 

  • Therefore, rate of O2 consumption increases with temp 

• Aerobic soil = has oxygen

• Anaerobic soil = lacks oxygen 

• Wetland soils 

  • Wetland soils are water saturated for prolonged periods, when soil temperatures and other conditions are such that plants and microbes can grow and remove oxygen, thereby assuring anaerobic conditions

  • Wetlands have hydric soils 

    • There are often Histosols 

  • PLant adaptation to waterlogged environments aerenchyma

    • Aerenchyma: large intracellular structures (pore spaces) which extend through the entire plant and allow for storage and transport if gas to submerged roots 

• CO2 in soil air 

  • soil CO2 concentrations are 

    • A. Higher in June than in November

    • B. Lower in June than in November

    • C. The same between June and November Range

  • Rasnge of possible soil temp and their implications for soil processes 

  • Daily variations in air temp near earth's surface is controlled mainly by the input of energy from the sun (yellow) and output of energy form the surface (blue)

  • Soil temp - dinural cycles 

    • Huge range near surface

      •  At 0.5 cm, > 20 C range

      • Max. temperature ~ 2pm

    • Dampened and delayed with depth

      •  At 10 cm, < 6 C range

      •  Max. temperature ~ 6pm

      •  Negligible diurnal cycle at 80cm

  • Soil temp - seasonal cycles 

    • Soil temps fluctuate more at surface than at depth 

2/19./24

 ALbedo  - proportion of radiation that is reflected by a surface 

• High albedo, low albedo

• Bare soil generally has a lower albedo (absorbs more heat  than a soil with crop residue

• Water will be cooler due to specific heat capacity 

• Specifc heat capacity: the amount of energy required to  raise the temp of a unit of a substance by 1 degree C 

• Specific heat capacity of water and sand are different

• A dry soil warms up easier than wet soil 

  • Due to higher specific heat capacity of water compared to soil minerals 

  • Soil temp in different tillage systems

2/22/24

Soil colliods - Organic and inorganic matter with very small particle size and a correspondingly large surface area per unit of mass 

• Sand silt clay are sometimes called soil separates 

• Soil texture is sometimes called soil particle size distribution

• Soils with fine particles have a greater soil surface area than soils with coarser particles 

• Crystalline 

  • Denotes a definite chemical compositions with planner surfaces and regular angles 

  • Atomic arrangement of quartz shows planar surfaces and regular angles

  • However crystalline silicate clays in soil are not formed from disintegration of large crystals into smaller crystals

  • Crystalline silicare clays are among new minerals  - pedogenic clay s

• SOil colloids, including crystalline silicate clays contribute enormously to soils cation exchange capacity (CEC)

  • In cation exchange, cations absorbed to negatively charged clays exchange with cations in soil solution

  • Basic structural components of silicate clay

• Tetrahedral and octahedral sheets stack in different configurations in the different types of crystalline silicate clays 

  • 1:1 tetra to octa hedral 

  • A loose metaphor 1:1 clays are a stack of break and peanutbutter, low filling diversity 

  • 2:1 tetra to octa 

  • 2:1 clays are a sandwich with lots of fillings   

• Different layer structures of silicate clays

• Biotite and muscovite micas are primary minerals, here we discuss secondary minerals also called micas

Crystalline silicate clays in context 

• Kaolinite - white gold - in industrial uses 

  • Minimal shrink, preferred fr ceramics

  • Iron impurities make it red

• Expanding 2:1 clays - smectite

  • Gives vertisols their shrink- swell 

  • Have industrial applications

  • Where swelling when wet is needed to create seal

NON crystalline silicate clays 

• Allophane is a non crystalline silicate clay  composed of Si, Al and O atoms, not arranged in crystalline sheets

  • Primary constituent of volcanic soils (andisols)

• Processes leading to soil charge

  • Constant charge due to isomorphic substitution

    • The process of replacing one structural cation for  another of similar size 

    • Net charge difference is -2 from a Si +4 to Mg2+

    • Often in soil cations are replaced with less positive cation, leading to net negative charge 

  • pH dependent change

2/26/25

Soil colloids 2 

• Review from last cass, what makes a soil colloid a soil colloid 

  •  Organic and inorganic matter with very small particle size and a correspondingly large surface area per unit of mass 

• WHat is a crystalline silicate clay 

  • Denotes a definite chemical compositions with planner surfaces and regular angles 

  • Atomic arrangement of quartz shows planar surfaces and regular angles

  • However crystalline silicate clays in soil are not formed from disintegration of large crystals into smaller crystals

• What features distinguish the types of crystalline silicate clay 

• Iron and alluminum oxides 

  • Here showing Gibbsite  

    • An aluminum oxide clay common in highly weathered soils

  • Octahedral sheets hydrogen bonded together 

  • Other oxide- type clays can have iron instead of aluminum, or be less crystalline structure 

• Organic colloids 

  • Also known as soil organic matter

  • Non crystalline structure 

  • -OH hydroxl groups 

• What happened to the dyes - yellow is more negative and bleu is more positive so blue sticks to the soil!!!!

• Processes leading to soil charge 

  • Constant charge - due to isomorphic substitution 

  • pH-dependent charge 

  • “Such as substitution [isomoprhic; reduction in charge ] commonly occurs in aluminum dominated dioctahedral sheets.”t

• Another view of isomorphic substitution 

  • Process through which structural cation and shapes are exchanged with similar cation that leads to a net negative charge 

• Processes leading to soil charge 

  • pH-dependent charge 

  • Hydroxl (-OH) functional groups exist on the edged of inorganic colloids and organic colloids alike 

• We can consider PH dependent charge by imagining the hydroxl group on the edges of clays and on organic compounds as a 2 car garage 

  • House is the oxygen atom 

  • Cars are hydrogen ions 

  • PH as a reflection of hydrogen ion concentration in  a substance 

  • Decrease in soil ph is like a football game happening in the neighborehood

  • WHen there is 2 the charge becomes +1

• Increase in soil ph is everyone is gone, charge becomes -1

• More acidic - low ph - positive charge - greater anion exchange capacity 

• More alkaline - high ph - more negative - greater ion exchange capacity 

• When we sum exchange sites in soil, we report units of charge per mass (soil or colloid) 

  • For CEC, the sum is of negative charges 

  • PH- dependent negative charge increases as soil pH increases although degree varies with soil colloid 

  • As a function of soil ph

• Key point 

  • Weathering of clays follows a general trend in which: 

    • 2:1 clays weather into 

    • 1:1 clYS WHICH WEATHER into 

    • Fe and Al oxides

  • Driven by leeching of silica and cations

  • Constant charge of soil colloid decreases with weathering →

  • Princinples of CEC that contribut to low CEC in southeastern US

  • Highly weathered clays developed under warm humic climate

  • ALso 

  • Lower organic matter high decomposition rates

  • More acidic soils → ph dependent charge leads to anion exchange capacity 

1. 4 rules gocerning cation exchange 

   1. 1. Reversabilty 

      1. WHat goes on may come off 

   2. 2. Charge equivalence 

      1. One +1 cation for another +1 cation or two +1 cations or one +2 cation etc

   3. 3. Ratio Law

      1. The ratio of two different cations in soil solution will equilibriate with those absorbed to exchange complex

   4. 4. Cation selectivity 

      1. SOme cations are held more tightly on exchange complex than other →

      2. The view of cations floating in a solution by themselves is a simplification because cations are usually hydrated

      3. hydrated radius describes the effective size of cation in solution 

2. Measuring cation exchange capacity 

   1. Additon of NH 4 to soil 

   2. Replaces other action son the exchange matric these cations are leached into beaker and excess NH is removed with organic solvent 

   3. Very high concentration K+ solution is used to replace and leach absorbed NH4

      1. NH4 and K+ have similar hydrated radii so ratio law comes into effect

   4. Amount of NH4 leached from osil can then be quantified representing total negative charges )CEC) fron soil 

   5. WHy do we use NH 4 to measure cation exchange 

      1. Small hydrated radius makes it more likely to:

         1. Replace larger more hydrated cations

         2. Not be displaces by larger more hydrated cations

         3. NH4 in solution can be easily measured

2/28/24

Mesuring cation exchange 

1. Additon of NH 4 to soil 

2. Replaces other action son the exchange matric these cations are leached into beaker and excess NH is removed with organic solvent 

3. Very high concentration K+ solution is used to replace and leach absorbed NH4

   1. NH4 and K+ have similar hydrated radii so ratio law comes into effect

4. Amount of NH4 leached from osil can then be quantified representing total negative charges )CEC) fron soil 

5. WHy do we use NH 4 to measure cation exchange 

   1. Small hydrated radius makes it more likely to:

      1. Replace larger more hydrated cations

      2. Not be displaces by larger more hydrated cations

      3. NH4 in solution can be easily measured

Soil organic carbon increases soil CECm and does this to greater extent in high pH soils 

Soild higher in CEC - whether due to organic colloids, inorganic colloids, or both- have greater capacity to prevent nutrients cations from leaching

SOIL ACIDITY 

• Features of a log scale

  • 1 each gardation of “1” on a pH scale represents a 10 fold difference in H+ ion concentrations

  • 2. Absolute change in H+ iron concentration for 1 unit pH change is much greater on the acidic side than on alkaline side of scale

  • PH range of soils and other materials 

• To understand how acidity develops over time, where does it come from?

  • Sources of soil H+ in soil 

    • Dissociation of carbonic acid from CO2 

      • Process if molecules splitting apart 

      • High concentrations of CO2 in soil are dissolve into soil solution which then forms carbonic acid, when then dissociates to bicarbonate and H+

    • DIssociation of acidic functional groups on organic matter (from plants)

      • Functional groups = specific groups of atoms within molecules that have their own characteristic properties regardless of the other atoms present in a molecule

    • Oxidation of ammonium (NH4) to nitrate (NO3-) releases two H+

      • Microbes oxidize NH4 as an energy source through process known as nitrification

      • This contributes to acidification of ammonium based fertilizers

    • Oxidation of sulfer

      • Either through organic matter that contains SH groups

      • Or through pyric oxidation FeS2

    • Input of acids in precipitation

      • Sulfur dioxide (SO2) and Nitrogen oxifes (NO) are released from fossil fuel combustion 

      • Undergo atmospheric reaction to form acid rain

      • When dissolved in rainwater and dissociate

      • Generating acidity 

    • Plant roots taking up cations then releasing H+ to maintain their charge balance

      • Plant roots cells need to maintain a charge balance across their cell membranes

      • Therefore if one positive charge (nutrient cation) goes in, one positive charge (H+ or other cation must go out

      • Plant roots taking up anions, then releasing bicarbonate HCO3 to maintain charge balance

      • Reduction of nitrate to nitrogen gas ( denitrification)

• Types of soil acidity 

  • Active: in solution

  • Exchangeable: held ner colloid surfaces

  • Residual: tightly bound to colloid surfaces 

  • Active acidity is a very small amount of acidity compared to exchangeable and residual acidity. 

• Acid cations = cations that generate and H+ aqueous solution in soil, thes are H+ and Al3+

  • AL3+ generates H+ by hydrolyzing water and combining with resulting OH 0 

  • One AL3+ can erelase up to three H+ ions 

Soil pH will decrease but to a lesser extent of that of water if you ad 3cMol of acid to soil on pH

• Because soil has a buffering capacity  

• Buffering: An addition of acidity will cause more acidity to move to exchangeable acidity in soil colloids so the addition of acidity is not fully reflected in active measured acidity

• Mechanisms of pH buffering

  • Protonation and deprotonation of organic matter functional groups (R-OH)

    • Gaining or giving protons H+

  • Protonation and deprotonation of pH- dependent charge sites 

  • Cation exchange reactions

  • Reactions of aluminum and carbonates 

• Soils become acidic when 

  • H+ ions are added to soil

  • Thes H+ solutions exchange with nonacid cations Ca2_ Mg 2+, K+ Na+ held on colloids

  • Noncaid cations are leached way (bc they travel with anions)

    • In arid regions nonacid cations remain in soil and re-exchange with acid cations, preventing a drop in pH level 

    • An acidic soil has an exchange complex dominated by acid cations

  • WHat about this soil propoerties might be different 

    • Organic matter of inorganic colloids could influence buffering capacity which is why they are different, orange is sandier

    • L

• L

Acidic organic material

High rainfall

Parent material low in nonacidc cations

Sandy soiuls (low buffering capacity)

3/3/25

• Hydration of cations influences their effective radii, and therefore how easily they are replaced in a cation exchange 

  • Larger hydrated  cations have weaker bonds and therefore are replaced easily 

• Sources of H+

  • Respiration- dissociation of carbonic acid 

  • Decomposition of organic matter

  • Oxidation of ammonium based fertilizers 

• Acid saturation 

  • Recall percent base saturation 

  • We calculate acid saturation using same approach 

  • Acidity throughout the soil profile 

• Given The sources of H+, which pH graph would you predict is more likely found in a humid climate 

  • Surface of soil is more subject to plant matter composition and weathering

  • More acidic in the higher soil and less acidic in the lower soil 

  • Solubility of aluminum declines rapidly at soil pH above ~5.0-5.5

• Inputs of acidic organic material mobilize AL3+

  • Fewer H+ sources; Al precipitates, contributing to formation of Bs horizon 

Soil pH and crops 

• Some crops prefer acidic soils, some prefer neutral, some prefer alkaline soils 

• Justus von liebig's law of the minimum published in 1873

• “If one growth factor/nutrient is deficient. Plant growth is limited, even if all other vital factors / nutrients are adequate…. Plant growth is improved by increasing the supply of the deficient factor /nutrient “

• Modified truog diagram which purports to show nutrient availability across the range of soil pH:
  Limitations:

  • Width of band is not actual amount of nutrient 

  • Even at widest part of band, nutrient may not be non limiting for plant 

  • Even at narrowest part of band nutrient may not limit plant growth 

  • Diagram implies that optimal soil pH is about 6.5, but crops can be highly productive outside this range

  • Even if topsoil pH is low, low ca, plants may uptake Ca from subsoils

• Limitations; more recent 

  • Plant roots and soil particles both have pH dependent charges and nutrient availability is mediated by both plant and soil charge 

  • Evidence of plant uptake and colloid resorption following apparently opposite patterns 

    • pH conditions with most absorption of colloid are same as pH conditions that make it best for plant uptake 

  • Many unknown remain regarding role of pH in nutrient availability 

• Contrary to statement that remain popular in agronomic texts the soil pH cannot be used to predict or estimate plant nutrient availability 

• What is well established regarding mechanisms of crop preference for soil pH labels 

  • Nutrient mineralization increases with pJ 

    • More in N cycling later classes

  • Aluminum toxicity at low pH

• Aluminum toxicity at low pH

  • At pH <5.5 aluminum is in the Al3+ form and competes with the essential nutrients like Ca 2+ Mg2+_ and K+ for negatively charged exchange sites

  • Plants can experience toxicity form taking up Al3+ and trying to use it in palace of Ca2+

  • Aluminum takes hydrogen and generates hydrogen irons and lowers pH

Why do we lime soils

• We lime soils because it helps us to neutralize soil acidity and increase soil pH

  • Acid cations in lime can replace cations in solution of soil

• The greater the buffering capacity of soil the more lime is needed to realize the pH

  • 

• Effect of limiting in raising pH is greatest in horizon is application

  • Evenbut dilute increase in pH

• Liming generally needs to be repeated over time

  • because water and effects can change the liming effects

• Alkaline = pH above 7 = more OH- ions

  • Alkaline soils are mostly found in arid reagions

  • Arid regions have limited sources of H+ due ot low biological activity

  • Arid regions experience limited leaching of Ca2+, Mg 2+ K+ and Na+

• Features of soils in arid regions

  • Water limitation

    • Potential evapotranspiration > precipitation

    • PET - potential could be greater than what is actually evaporating

    • In arid environments theres a larger demand for water in environment

    • demand for water is greater than water that is going into soil s

  • Island of fertility

    • Plants protect soil from erosion and promote water infiltration and storage

    • Grazing animals concentrate manure to grazed areas providing more organic matter

    • leading to fertility to suppport more plant growth (start over at protection of soil )

  • Used for grazing

    • Requires less water input than rainfed crops

  • In some areas people irrigate soils in arid regions which can increase the risk of soil salinization

Process of soils accumulating excess salt= soil salinization

• Salt affected soils:

  • ~7% of earths land area,

  • 23% of cultivates area

  • 50% of irrigated area

  • Can have an extremely bad effect on food

• Alkaline soils: pH above 7

• Saline soil: high concentration of soluble salts

  • in exchess of 4 deciSiements per meter

  • Salts commonly found in soils and natural water and their solubilty (mmolc L^-1)

  • Key point: carbonate and bicarbonate based salts are usually lower in solubilty than sulfate and chloride based salts

  • We can understand related process of saline lake formation ex: great salt lake

    • Due to inputs of water with dissolved salts

    • evaporation of water

    • absence of exit pathways for salts

    • repeat

  • Formation of saline soils through the addition of irrigation waer

    • Saline irrigated soils form from:

      • Inputs of water with dissolved salts

      • evapotranspiration of water

      • Absence of exit pathways for salts

      • repetition of this process

    • Even freshwater has small amounts of dissolved slats which are concentrated in the soil

  • Measuring salinity

    • Separately quantifying all the salts is too labor intensive and expensive

    • Therefore, we rely on bulk quantification of salts through

      • Total dissolved solids (TDS)

      • electrical conductivity (EC)

• Total dissolved solids extraction process

  • Extraction of dissolved salts in aqueous solution

  • filtration to remove soil particles

  • Evaporation of water (shown in diagram)

  • Weighing of remaining soilds

    

• Electrical conductivity, principle

  • More rapid than directly quantification of TDS

  • Based on principle of salt water a s a good conductor of electricity

  • More salts in solution —> greater electrical conductivity

• Conductivity, practice

  • Mix distilled water with soil until it flows slightly

  • allow salts to dissolved overnight or half an hour

  • extract solution and measure ec with electrode

  • report ec reported in deciSiemens per meter

    • Describes abilty of soil to conduct electrical current

SODIC SOIL

• The soluble salts are primarily sodium

• Sodic soils are high in sodium as the dissolved salt

  • Higher in sodium because its lower in calcium

  • Soil sodicity can be quantified with the exchangeable sodium percent (ESP) shown here

  • OR with the sodium absorption ration (SAR)

    • SAR= {Na+}/(0.5{Ca2+}+0.5{Mg2+})

• This quadrant of salinity and sodicity

  • Sodic soils can have particularly high pH levels

    • due to reactions of sodium with carbonate and bicarbonate in solution which calcium and magnesium undergo to much lesser extent )

  • AND sodic soils have particularly poor structure

  • 

• The charge to hydrated radius of cations influences soil structure

  • Sodium has a slightly smaller hydrated radius than calcium or magnesium but only half of the charge

  • Lets imagine a couple of soil colloid particles van der waals forces can contribute to their aggregation

  • 

• Sodic soils - consequences of poor structure

  • Forms a crust almost on top of soils

The charge of hydrated radius of cations influecnes soil structure

• Sodium has slighly smaller hydrated radius than calcium but only half of the charge

  • will increase or decrease soil aggregation?

• Sodic soils

  • COnsequences of poor structure

  • Flocculated (aggregated) vs dispersed strucure, flocculated can allow water to move, disperesed plugs soil pores and impede water movement

• 3 distinct causes of low permeability under sodic conditions

  • Dispersion

    • Clay particles seperate from one another rather than flocculating

  • Slaking

    • Aggregate disruption upon becoming wet —> clogging of soil pores

  • Swelling

    • Sodium enhances swelling expanding 2:1 clays

    • hich relationship would

Which relationship would you expect between ESP and Ksat

• ESP intereferes with Ksat

More ways salts can interfere with plant growth

• Osmotic effects

  

• Water moves from high to lowe

  • Higehr potential in non saline soil solution

  • lower potential in plant root due to solites lowering water potential

  • Water in soil and plant converges in potential

  • making it more difficult for plant rooots to remove water from soil

• Specific ion effects - what they are

  • Like mushroooms - some are harmless and some are deadly

  • Some ions are fine (CA2+ K+)

  • some ions cayse problems (Na+, CL-)

• Specific ion effects:sodium

  • Sodium is a quasi essential element

    • Required for some but not all plants

    • neededby corn, sorghum, and oter tropical grasses

    • Excess sodium in soil can become toxic because Na competes for K+ which is an essential element

• Are all saline soils also sodic soils?

  • False

• Reclaimation strategies - saline soils

  • Cannot be reclaimed by chemcial amendments, conditioners, or fertilizers

  • Field can only be recliamed by removing salts from plant root zone

• Opposing goals of irrigation

  • For refular irrigation: just apply enough water limitation on plant growth

  • for removing salts from root zone

    • Apply water in excess of what is needed for crop growth, so salts can move downward through soil profile and out of root zone

Efficacy of leeching

Reclaimation strategies - sodic soils

• Application of gypsum - which contains calcium

• calcium replaces sodium held in cation exchange on soil colloids

• then soluble salt, NASO4 is formed, which can be easily leached away

  

Exam review

• Aluminum toxicity

  • Aluminum is positively charged ion that can bind to the cation exchange capacity as soil becomes more acidic and the soil pH decreased

  • Aluminum displaces beneficial nutrients from the CEC

• Cation exchange capacity

  • A soils ability to like exchange cations and how many positively charged ions a soil can hold

  • Expanding 2:1 clays have higher capacities

• Protonation

  • The proces of adding protons (H+) to function groups on soil surfaces, which can change soil pH and charge, this occurs more often in acidic soils

• Alfisols

  • Soil is rich in aluminum and iron

  • Argillic, kandic, or natric horizion

  • found in more wet soils

• Ultisols

  • Strongly weathered acidic soils found in humic regions

  • HIgh in pH and Al3+

  • Found in more temperate areas

You uncover archives of ancient civilization

Instead of 12 soil orders, they group soils into 3 categories based on base saturation

• Low base saturation

• Medium base saturaton

• HIgh base saturation

• Describe extent of soil weathering for each of these three soil orders

• As soils beccome more weathered, base saturation goes down

• so a more sautrated soil will be less weathered.

Mollisols