Untitled Flashcards Set

• 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 

• 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

• Reclamation strategies - saline soils

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

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

• Opposing goals of irrigation

  • For regular 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

Reclamation 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

3/10/25

pH range of soils and other materials 

Soil organisms - grouped by size 

• Macro-organisms  (>2mm) > MESO - organisms (>0.1-2mm) > MICRO organisms (<0.1mm)

• Worms, termites, mice > springtails, mites > tardigrades, nematodes, fungi, bacteria, archaea

• Soil organisms - grouped by metabolism 

  • Metabolic grouping of soil organisms based on source of energy  and carbon 

  • Source of carbon - combined organic carbon - biochemical oxidation 

    • Chemoheterotrophs , all animals, plant roots, fungi, actinomycetes and most bacteria

      • Earthworms, fungi, water bears

  • What are most of these organisms getting their combined organic carbon from?

    • Both chemoheterotrophs and Photoautotrophs 

  • Carbon dioxide or carbonate - solar radiation

    • Photoautotrophs plant shoots, algae, cyanobacteria

• CARBON can be cycled through an intermediate consumer before it is consumed by Chemoheterotroph

• Chemoautotrophs that use carbon dioxide and carbonate

  • Ammonia oxidizers and sulfur oxidizers 

  • Are doing it as an energy source transformation

  • Getting carbon from inorganic sources 

• Trophic levels and energy transfer 

  • Primary consumers in soil 

    • Herbivores : eat live plants

      • Larvae of cane beetle which feeds on living sugarcane plants in all stages of life cycle

    • Detrivores: eat remains of dead plants and microbes on them

    • Saprotrophs: microorganisms that consume detritus, corpses and feces

  • Secondary consumers in soil 

    • Carnivores : eat other animals

    • Microbivores feeder : eat microbes 

      • Protozoa, which graze on soil microbes 

• Trophic levels and energy a( and carbon transfer) of belowground communities 

• Other microbes exist in soil that arent as involved in the soil organic matter 

  • Process of transformation 

• Trophic cascade of aboveground communities 

  • 10% of energy is lost every time 

  • SMall part of period is small itty bitty animals compared to plants as largest energy source