Soil chemistry

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L29 How do substances flow through soil?

Lecture 29 How do substances flow through soil? The importance of soil chemistry (Phil Haygarth

Soil represents an amazing buffer and regulator of chemicals flows through the environment. In this lecture we will consider the basic range of processes that control soil flows through soil and the key chemical reactions that are important in controlling this.

the importance of soil chemistry essentially what this is about

- soil substances for good or bad can be regulated by soil - soil holds catbon which is a great buffer to the carbon flows around the earth and climate issues. soil also holds nutrients for plant production very important- key role determining how the plants get it and if we get fed

- soil has an ability to determine whether or not it releases things to the environment which may be polluting

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Basic concepts: soil chemistry (book)

soils multiphases symptions solids, liquids, gases and colloids collectively interact.

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Aim: to understand the chemical mobility of subst

Aim:  to understand the chemical mobility of substances through the soil,  for good or for ill

 - Plant / crop production      (good)  

- Pollutant mobility              (bad if things move out of soil and create sollution broski)

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Fundamentals of soil chemistry

Key reactions and processes 


Ion exchange and adsorption/desorption

Cation (and anion) exchange

Precipitation & dissolution



101 chemistries good descriptors of key chemical processes in soils- potential for soil to hold onto things or releasing things - important how we think about wider environment

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Plant nutrients

macronutrients soil or atmosphere- carbon

parent material cations  potassium magnesium or anions sulfate, nitrate

key pointers key reactions key substances- these are key macronutrients 

macronutrients e.g. calcium, magnesium,
- atmospheric sources oxygen, carbon 

these lesser common nutrients exist in soil

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Soil pH

most important master variable- most important chemistry is pH pH is the indicator of everything tat follows through in terms of soil chemistry

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random pH thing fam

degree of hydrogen ion concentration which is reflected in pH indicator

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Soil Acidity and Alkalinity

· In soil science often simply expressed as soil pH,   a master variable

· Many chemical and biological reactions in the soil  are dependent on pH

· · pH influences the solubility and hence availability

  of many plant nutrients and pollutants,   particularly metals

· Soil pH can be manipulated

critically we can control soil pH to some degree

e.g. sprint time farmrers add lime to acidify soil- add lime - lime helps to raise pH reduce acidity

poor drainage over time soil pH can increase

basic high soil pH fix - tile- deep tillage 

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Soil pH - some BASICS

soil compact water can't permeate through and sits on top compaction issue, plant roots released organic acid trying to make nutrients available for plants in top soil. Get roots going down deeper to spread out organic acid (Dilute) 

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substance movement

diagrams sums up substance movement e.g. phosphorous favourite element really key for nutrition - phosophorous mobility - during acid conditions pH 6 phosphorous tends to be less mobile- for phosphorous mobilisation in soil we must keep the pH neutral .aluminium is mbilised in vey acid conditions.

different elements have different responses to pH 

phosophorous tends to be less mobile- acidic conditions 

for the functions we want from soil - plant production and movement of key nutrients phosophorous - if you get pH to 6-7 that is ideal and optimmum to plant nutrition

keep Ph neutral for P, N, K - pH around 6-7 ideal for plant nutrition. 

mobility shows solubility 

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Solution speciation e.g. Fe and pH

illustrates the solubility of iron as a function of pH in pure water

chemistry of  different subtances will vary depending on their different propertieis and  different ways that they react with different compounds.

all elements vary depending on their properties and the way they react w/ compounds

when pH is hgh- limit

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Ion exchange in the soil

ion exchange - idea that plants acquire substances from the soil particularly nutrients from exchange process between soil particiles the colloids- clays that exist in the soil and the  epidermal cell and root hair which is part of the rhisophere in the soil

interface between soil particles and biological plant matter is the reaction zone for the idea of contact change where different substances move towards the soil and plant

but the key thing is what is going on the surface of that piece of soil clay or colloid- what ype of chemistry exists so that contact exchange can be optimised to move the material around.

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Soil ion exchange

•Exchange (i.e. mobility) of ions in minerals with ions in solution

•Cation exchange capacity: propensity for adsorbing cations

•Anion exchange capacity: propensity for adsorbing anions

soil is basically negatively charged positives and negatives

any cation around that surface will be good because it means the soil can hold onto the cation - ammonium, calcum - all stick to negatively charged soil clay -

soil clays and particles are negatively charged

organic hummus can be positive or negatively charged more complicated things around this surface

measure CEC:
In order to know how much often/fertiliisation or herbicide is needed 

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Cation exchange capacity

in simple terms the '+s' like to stick to the soil

contact exchange between the soil surface and the chemicals is basic equillibrium theory- think of the soil as a chemical reaction surface

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Factors Influencing CEC

• Soil pH

• Humus content

 • Clay type and content of the soil

what can we do as scientists to manipulate and affect that CEC
- can affect the soil pH which has an effect and we can determine the amount of humus in the soil

- keeping  the organic matters topped up in the soil are important for manipulating the soil condition and they can affect the cation exchange capcity because if it has organic matter it can change things a bit - it does iff you add organic matter

- rocks and minerals that make up the soil can be different minerals and these have different surface soils and consequently different types of CEC vemiculite high CEC capacity whilst kalonite is much lower- different types of minerals that inherent parent material gives us affects CEC but also we can manipulate pH and the humus input to affect that ability 

- how returns of C are important to manipulating soil conditions 

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Factors Influencing CEC

Soil pH Humus content Clay type and content of the soil

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Soil Buffering – An analogy

cation exchange or anion exchange capacity is ability for soil to hang onto cations or anions

exchange capacity is the ability of the soil to hold in transit cations and anions ready for exchange for plants to take up. a greater cation exchange capacity makes the soil more fertile-- like a cloakroom more to hold onto to

CEC ability to hold onto substances cation and anion from plant

The greater capacity, the grreater performance of fertility (hold onto more nutrients) 

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Element adsorption

Adsorption definition: Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.

Two important mechanisms

Outer-sphere complex: In soil science this is often referred to as “exchangeable cations”. Generally associated with electrostatic interaction, with permanent negative charge on the basal sites of clays. e.g. calcium on a phyllosilicate clay.

soil exchange a little more complicated - exchangably cations is the one we've been talking about - positive and negative

Inner-sphere complex: Sometimes called specific or chemisorbed, involves a chemical interaction. Much stronger than an outer-sphere complex. e.g. phosphorus on Fe oxides  - microversions of that go around the surface- not as simple as a clay particle with lots of pluses, very complex in reality have lots of little pluses and negatives 

v complex have little +ives & -ives

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Element adsorption

Adsorption model for soils 

reaction for ferric oxide- iron - reactions start to become more complex

smaller particle size more surface area clay have more surfac area than sand, silt, gravel 

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Preciptitation - dissolution

•Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid

•When the reaction occurs in a liquid, the solid formed is called the precipitate

a few more ways the soil can buffer and control the way substances move through soil either down to water or out the environment or up to the plant

basic chemistry - precipitation salt or sugar is a precipitate in other words it is in a solid form which will not be mobile - baecause salt is a physical structure it wont move well through the soil  - but put the sugar in your tea it dissolves- when dissolution occurs, when it becomes soluble then those reactions mean that they can be more mobile and the chemical substances can flow better

solid not mobile in water (because of physical structure)

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Solubility and minerals

Common Precipitates




when chemical substances form precipitates in soil they tend to be wrapped up and bound up therefore less mobile

whenever precipitation occurs it will not help mobility

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Solubility and minerals

Dissolution of Hydroxides 

shows pH dependence on the movement of some soils - hydroxide salts dont move well solubles do

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A complex is a structure consisting of a central atom or ion (usually metallic), bonded to a surrounding array of molecules or anions (ligands, complexing agents). 

Polydentate (multiple bonded) ligands can form a chelate complex. A ligand donates at least one pair of electrons to the central atom/ion.

•When substances are complexed they tend to be less mobile through system (difficult to move through soil) 

phosphorous ligands - another word for complex once things get wrapped up in complicated structures they becme less mobile become wrapped up - more difficult to see these things move through soil -  Basically point is when substances are complex they tend to be less mobile- all you really need to take from that

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Organic acids e.g. oxalic acid

Organic acids e.g. oxalic acid (H2L), form soluble complexes and alter speciation

organic acids are a great example of complexes really common in soil and complicated

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Oxidation and reduction

Oxidisation - lots of  oxygen around 
oxidised red bits where well drained lots of oxygen around 

reduction - reducing conditions lack oxygen

forms peat plant material doesn't break down, extreme rganic matter forming not breaking down to form humus so doe snot mix with the rest of the soil - often reducing conditions related to relief/topography at top of mountain or valley botttoms 

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Oxidation and reduction

•Oxidation is an important reaction in well-aerated soil material where oxygen supply is high and biological demand is low

During reduction (the reverse of oxidation), when the supply of oxygen is low but the biological demand is high, the ferric ion is reduced to the ferrous ion, which being more mobile , may leach downwards.

Reduced conditions often occur in water-logged soils while aerated soils are under oxidizing conditions.

Soils under reduced conditions have a high Eh value, and oxidized soils have a low Eh value. Fixation (and mobility) of elements in the soil depends on the Eh and pH of the soil.

Eh-pH diagrams can provide information on the potential fixation of elements in the soil.

Eh is measured in millivolts (mV, or milliV), or volts (V) and like specific conductance

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Oxidation and reduction

reduction-  peats in film are reducing conditions when you get a lack of oxygen it changes the chemical balance

Eh value is a  measure of the electrostatic condition that relates to oxidation/reduction

higly reduced soils - highly saturates, little oxygen around have a high Eh that affects chromium that exists in system - determines the fate of the chromium in the soil

highly reduced soil have high Eh (saturated,little O2 around)

affects precipitation or equilibrium of Cr in solution

By plotting can predict different forms of existing Cr in system (Soil 

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Fundamentals of soil chemistry

Key reactions and processes

–pH - basically negatively charged

–Ion exchange and adsorption/desorption

–Cation (and anion) exchange 

–Precipitation & dissolution - salts form dont move well , when become dissolved they do

–Complexation - things like to form complexes but when they do less mobile


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Essential soil science chapter 3

plants are immobile organisms- cannot move to find food - rely on soil to provide them nutrients and protection. 

as well as benefits soil can also act as a store of acidity and pollution.

potential for soil to store chemicals depends upon its texture (especially amount of clay it contains and its organic matter content. 

ability of clay minerals to store chemicals determined by two properties:
1) surface area
2) surface chemistry 

how are chemicals stored and released from soil?
soil scientists interested in ones that have beneifical effect - plant growth (nutrients)  & toxins

soil chemistry mainly controlled by the soils 'colloidal material' 

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what are colloids and why are they important

'colloidal particles' refers to clay minerals and soil organic matter e.g. the humus fraction

soil is made up or a mineral and an organic matter fraction. Both fractions contain colloidal material, these colloidal  materials have a great influence on the chemical properties of the soil. 

many sol colloids carry an electrostatic charge. clay minerals, hydrous oxides of iron and aluminium, and soil humus have an electrostatic charge.

This electrostatic charge gives them the ability to hold onto certain chemicals so they can store nutrients, acidity and toxins.

It is the presence of these collloidal particles that largely controls the storage and release of chemicals in the soil. 

Clay mineral structure 

Soil mineral colloids in the clay-size fraction conssit of clay minerals, hydrous oxides of iron and aluminium. we will concentrate on clay minerals because they are the most important mineral colloids in the majority of the world's soil. 

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variable charge

All soil colloids have variable charge
variable charge can be +ive, -ive or 0 depends on the acidity of the soil solution.

clays, hydrous oxides of iron and alluminium and soil humus all have a variable charge.

  •  variable charge can be positive, negative or zero
  • variable charge is strongly influenced by soil pH.

    How do soil colloids affect the adsorption and release of chemicals?

    at the colloid's surface ions are held tightly. for most soils which are not too acidic or alkaline the soil will be negatively charged.

    This attracts cations (+ive) to collect in a tighlty held layer at the surface of the colloid and a less tightly held 'cloud' of cations further away.

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Cation exchange capacity

Soil ability to adsorb cations is referred to as its 'cation exchange capacity' (CEC)

this property v. important when trying to measure a soil's ability to supply nutrients such as, K, CA & Mg.

soils that have high CECS tend to have high concentrations of colloidal particles

For example, a clay-rich pasture soil will have a far higher CEC than an acidic sandy-textured soil.

Anions such as NO, are not attracted to colloidal surfaces because like charges repel. This is why many anions are susceptible to leaching. (However exceptions occur)

What determines the availability of acids and nutrients?

soil colloids have the ability to act as a chemical store because they 'buffer' chemical changes by soaking up and then releasing ions from the soil solution

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What is soil acidity?

All acids have high concentrations of H + ions, and the greater the concentration of H + ions, the stronger the acid.

Acidity determines a whole range of soil characteristics,

  • such as the nature of the variable charge
  • nutrient availability
  • microbial activity  
  • release of certain toxins such as metals.

    Soil becomes acidic through several mechanisms

    Natural:  acidity produced by the breakdown of soil organic matter

     Human cause:  pollution from industrial sources that then creates acid rain

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Problems associated with acid soils

Acid soils can also be referred to as aluminium soils

Aluminium ions can cause plants nutritional problems because it can stick firmly to colloidal exchange sites, thus reducing the ability of the soil to retain base cations.

Aluminium is toxic to plants and animals because it can inhibit cell divi- sion and the elongation of plant roots.

Since soil fertility is so closely linked with acidity, it is good soil manage- ment to measure it before crops are sown.

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What are soil nutrients?

 What are soil nutrients?

any substance used by an organism as food.

soil nutrients two groups: Macronutrients and micronutrients
important macronutrient  potassium: 
Plants use a lot of potassium, and may assimilate more potassium than they actually need.
The ability of a soil to buffer against potassium shortages will depend on its CEC and pH


  • Phosphorus is essential for photosynthesis, nitrogen fixation, crop growth, produce quality and root development.
  • It is phosphorus reactions with other soil chemicals that reduce its availability to plants.
  • In most soils the concentration of plant-available phosphorus is low,
  • This is why  important to maintain the pH of agricultural soils at around 6-7, ensure  maximum amount of plant-available phosphorus.
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most cases, the majority of soil nitrogen is contained within organic matter. The small fraction of inorganic nitrogen is mainly made up of ammonium and nitrate. Ammonium can become fixed by certain 2 : 1 clay minerals whereas nitrate, because it is an anion, is susceptible to leaching.

 Phosphorus occurs as the phosphate anion, but, unlike nitrate, potential leaching losses are reduced because it forms insoluble compounds when the pH is outside the narrow range of 6-7 and ligand bonds with other soil chemicals that modify its anionic character.

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