Week 3 - soil chemistry

https://www.saskoer.ca/soilscience/chapter/soil-chemistry/

What are soil colloids? What does it do?

• very small inorganic (clay) or organic (humus) particles

• net negative charge in temperate soils

• determines soil chemical properties

What type of exchange occurs with soil colloids?

the primary site for nutrient exchange in the soil + plant nutrient uptake

• chemical cation/anion exchange, chemical reactions, adsorption of water 

  • adsorption: molecules, atoms, ions gathering on surfaces 

• large surface area to help stick substances together 

What is role of clay in soil?

• active mineral portion of soil 

• colloidal, crystalline, amorphous, morphous 

What is clay in soil terminology?

• <0.002 mm diameter particle 

• group of similar minerals 

• soil textural class 

Why is clay important to humans and nature?

• most important chemical weathering product of soil 

• many uses: construction, tiles, pottery 

• contribute to the exchange of ions 

  • helps soil fertility in forestry + agriculture 

• shrinking/swelling clays also used in construction/art/agriculture 

What are different categories for determining clay type?

• different material (silica, alumina) during formation 

• proportion of minerals + ions 

  • amount of silica vs alumina 

  • presence of ions 

• degree of acidity 

• pH of leaching (dissolution) water 

How does the tropical biome affect the formation of clay?

• hot, humid climate 

• highly leached soil 

  • formation of different types of clay vs non leached soil 

  • contains many dissolved primary minerals that recrystalize the clay that are different vs non leached soil 

• clay types: 

  • kaolinite, allophane, smectite, sesquioxide 

How does the temperate biome affect the formation of clay?

• cold, moist-dry climate 

• leached-less leached soil 

  • less dissolving and recrystallization 

• clay types: 

  • vermiculite: clay formed from mica 

    • similar properties to primary minerals (slight alteration) 

    • selective/incomplete solubility 

  • montmorillonite, illite, bentonite 

What are the 3 sources of clay?

1. inherited clay 

• deposited as clay in sediment from another location 

  • clay could be from different geologic period 

2. modified clay 

• weathering/degradation of original clays 

3. neoformed clay 

• new clay formed from leaching of primary materials 

• does not inherit any structure from a pre-existing mineral 

What are the 2 crystalline natures of clay?

morphous clays: definite, regular, repeating arrangement of atoms 

amorphous clays: irregular structure 

morphous + amorphous clay:

• composed of planes of oxygen bonded by silica + alumina (ionic bonding) or other ions 

• type of ion = type of clay 

What are the types of silicate clays?

1. amorphous silicate clay: allophane, imogolite

2. morphous silicate clay - 1:1 silicate clay: kaolinite, halloysite

3. morphous silicate clays - 2:1 silicate clay: smectite clays, illite, vermiculite, chlorite

amorphous silicate clay: name clay, chemical properties, location, ionic bonds

• allophane (most common) + imogolite 

  • allophane = Al₂O. ₃· (SiO₂) 

• mix of Al + Si = no well formed crystals + repetitive structure 

• highly weathered conditions w/ insufficient time to develop crystal growth 

• common in soils from volcanic ash 

• unusual = high affinity for P 

morphous silicate clay – 1:1 silicate clays: name clay, chemical properties, location, ionic bonds

• kaolinite (most common 1:1 clay) + halloysite 

• residues from extensive weathering 

  • in high rainfall areas, typically leached, well drained + acidic soils 

  • found in humid warm climate (florida, subtropics, tropics) 

• classified as kandite clays (for pottery)

• does not allow water between layers 

  • strong H bonding 

  • kaolinite = non swelling clay 

What clays are commonly used?

kaolinite

• clay saturated with water → molded → hardened 

  • non expanding nature = can fire dry without cracking 

• pottery, tiles, bricks

• oxidation changes clay from grey to red 

vermiculite

• expands 20-30x size

• insulation, plant potting material, packing, fireproofing 

smectite: chemical properties, location, ionic bonds

• montmorillonite, saponite, bentonite

• swell when wet, shrink when dry 

• little or no leaching 

• dry soil

  • arid + semi arid regions (poorly drained) 

  • develop from alkaline bedrock 

illite: chemical properties, location, ionic bonds

• k ions holding layers together 

  • layers held tight + little water penetration between layers 

• slight to moderate swelling 

• found in soils not extensively weathered + high in primary minerals 

• occurs in similar environment as montmorillonite 

vermiculite: chemical properties, location, ionic bonds

• worm like structure 

• expands 20-30 times its original size

  • clay layers held together weakly 

• in warm/dry climate + well drained soil 

chlorite: chemical properties, location, ionic bonds

• classified as phyllosilicate clays 

• similar to vermiculite in structure 

• dominant cation = Mg 

• 2 layers of silica, 1 layer of alumina, 1 bonded interlayer of Mg (2:1:1) 

  • interlayer restricts swelling + has positive charge 

• common in soil formed from sedimentary rock 

  • serpentine soils: green, brown or spotted minerals (in northern california) 

non-silicate clays: name clay, chemical properties, location, ionic bonds

gibbsite + geothite

• classified as sesquioxide clays

• extensive leaching in tropics

• yellow, red, brown soils

• silicates washed away → leaving Al + Fe oxides remains

  • aluminum hydroxide, iron oxide + hydroxide

• not sticky, does not swell

  • can absorb a lot of water, very stable in soil

• high surface adsorption of P = less P for plants

What are organic colloids? Chemical properties?

• humus: intermediate product of highly decomposed plant and animal remains 

• most stable form of organic matter in soil (low solubility, low reactivity) 

• overall neg charge (like clay) 

• amorphous

• separated by molecule size 

  • fulvic acid + humic acid 

• higher cation exchange capacity vs clay 

What is the role of soil colloids in cation exchange?

net neg charge that attract pos charge ions (cations) to the surface

cations in soil solution approach soil colloid → exchange of cations between soil solution + soil colloid = cation exchange

Where does cation exchange occur?

clay colloids, humus colloids, plant roots

Properties of absorbed cations

• absorbed cation to soil colloid resist leaching (but can be removed by mass action)

• competition for negative site by a large number of ions present in soil solution

• strength of the adsorption depends on which ion is exchanged  

Common cations

Ca2+, Mg2+, H+, Na+, K+

How does the proportion of cations on soil colloid surface change?

• addition by: dissolving minerals, liming, gypsum, fertilizers 

• losses by: plant adsorption, leaching 

How is cation exchange mechanism affected by different ions?

• ions move at different speeds 

• held with different forces of attraction to the exchange site 

• different soil types = influence the strength of ions held on to the exchange site 

What does the strength of ion attraction depend on?

1. increasing valence (the combining power with other atoms when it forms chemical compounds/molecules) 

2. cation’s hydrated size decreases 

3. strength of site’s negative charge

Heavy metals and cation absorption strength

• heavy meatals stay absorbed to the exchange sites 

• contaminated water percolating through soil becomes clean as contaminants are absorbed to the soil 

• can use microbes to clean the contaminants 

What is the cation exchange capacity CEC? When will it change?

• CEC = measure of the quantiy of exchangeable cation sites per unit weight of dry soil 

  • cmol_c/kg  or centimols of cations per kg of dry soil 

• depends on the type of soil 

  • sands = 1-5 very small 

  • clays = >30  

  • inc in CEC = inc clay content in soil

• will change when: 

  • soil pH, humus content, clay content changes 

Why is cation exchange important?

• key for soil fertility 

• cause or correct soil acidity 

• alters soil physical properties 

• purifies percolating water (contaminants) 

• losses of nutrients via leaching 

• supplies nutrients to growing plants 

• hold fertilizer (K+ and NH4+) 

What is anion exchange

• anions = negatively charged nutrients 

• sulfate, nitrate, phosphate, molybdate, bobrate, chloride 

• not held on to cation exchange sites 

The highest anion exchange capacity (AEC) occurs in soil with:

• amorphous silicate clay (volcanic origin) 

• aluminium and iron hydrous oxide clays 

• to a lesser extend in kaolinite 

• mostly tropical and subtropical soil 

  • strong weathering conditions, low pH 

• soils with high % of iron oxides 

AEC vs CEC

AEC = 1/10 of centimole (1/10th of CEC’s size) 

How strongly are anions held?

• phosphates = strongly held to the soil (strong attraction) 

• common phenomena in tropical soils with low pH 

• strong attraction of phosphates to soil colloid = P unavailable for plants + dec in crop prod 

variable charge soil

• CEC + AEC of soil = function of pH 

• soils with a variable chage 

  • tropical/subtropical - acidification 

  • arid zones – alkalinization 

• soil pH changes when SOM + mineral content are altered 

• pH rises = strong trend to CEC + weak trend to AEC (pH 5.0 to 5.2) 

Basic vs acid soil and cations

acid soil – leaching of cations 

• areas of high rainfall 

• replacement of cations by H+ 

• H+ replaced by Al(OH)2+ 

basic soil – no leaching 

• areas of low rainfall 

• soil high in Ca = basic 

• 0 = acidic, 14 = basic