SL AA2 The world at your feet.

Silicates

·         An isolated silicate ion SiO₄^- is shown below – Each Si atom is covalently bonded to four O atoms at the corners of a tetrahedron. The central Si atom has an OS of +4.

·         Many minerals are silicates in which SiO₄ tetrahedra are linked by sharing O atoms. The simplest example of this is the Si₂O₇^6- ion shown in fig 10 below.

·         Note that each unshared oxygen atom carries a single negative charge.

·         The tetrahedra can join up as chains (fig 12), double stranded chains e.g. asbestos (fig 13), sheets e.g. micas or clays (fig 15) or in a giant three dimensional network e.g. quartz.

·         In sheets all the negatively charged O atoms point in one direction, upward out of the plane of the paper. One O atom is wholly owned by one silica and the other 3 are shared so the no. of O atoms per Si =¹/₂ + ¹/₂ + ¹/₂ + 1 = ⁵/₂ Hence ratio of Si:O = 2:5.

·         The net charge on each Si₂O₅ unit is –2 so formula can be represented as (Si₂O₅^2-)_n

·         Positive ions (cations) e.g. K⁺, Ca²⁺, Mg²⁺ and Al³⁺ are held to the silicate sheets to balance the negative charge. Some fit in the hollows in the rings of tetrahedra.

·         During weathering some Si atoms are replaced from the centre of a tetrahedron by Al³⁺ ions. This means more cations are needed on the surface of the sheet to balance the extra negative charges as Al is only +3 whereas Si is in Ox state +4.

·         This affects the physical properties of the mineral.

Clay minerals.

Clay minerals contain two kinds of sheets;

·         Tetrahedral sheets - based on Silicate tetrahedra with varying amounts of Al(lll) substituted for Si(lV).

·         Octahedral sheets – mainly Al³⁺  ions surrounded by 6 oxide or hydroxide ions in an octahedral arrangement. Some mg²⁺ ions may replace Al³⁺, and the octahedra are linked by shared O atoms.

The sheets form into layers differently to give different classes of clay;

·         In montmorillonite Mg²⁺ ions substituted for some Al³⁺ and Al³⁺ for some of Si(lV) so individual layers have high –ve charge. This is even more so in vermiculite.

·         A swarm of cations therefore attracted to internal and external surfaces.

·         Cations are hydrated and these surrounding water molecules give clay its sticky feel.

·         The cations on the surface of clay minerals provide a source of nutrients for plant roots – Ion exchange tales place between the surface of clays and the soil solutions.

Soil Organic Matter.

·         Made up of plant debris, animal remains, excreta and decomposition products of all these. Forms nutrient store for future plants.

·         During decomposition elements in organic compounds are converted into inorganic ions e.g. NH₄⁺, NO₃^-, SO₄^2- and PO₄^3- - this is called mineralisation.

·         Some of C in decomposing matter is converted to CO₂ and released to atmosphere.

·         New organic molecules made – Humus.

·         Humus contains v. large molecules and the following 2 functional groups are common;

·         Both these can lose H⁺ leaving  –ve groups which can form ionic bonds with metal cations, hence humus can also hold nutrients in soil.

How are cations released by soil?

·         Cation exchange occurs between inner and outer surfaces of clay minerals and soil solution.

·         E.g. NH₄ ions exchange with Ca²⁺. NB – balance of charge must be maintained so

Clay-Ca²⁺ (s)  +   2NH₄⁺ (aq)              clay-2NH₄⁺ (s)    +    Ca²⁺ (aq)

^·          Direction of exchange depends on concⁿ of ions involved.

^·          The ability of a clay mineral to exchange ions is measured by its cation exchange capacity – the amount in moles of exchangeable +ve charge  held by 1kg of the clay mineral. (NB 1 mole M⁺ º ½  mole M²⁺ º ¹/₃ mole M³⁺) See table 4 pg 190.

^·          The ions held by the clay or humus are in equilibrium with the free ions in the soil solution. Plant roots withdraw nutrients from the soil solution, which can then be replaced from this pool of exchangeable cations.

Controlling Soil Acididty.

^·          H⁺ from rainwater, plant roots and microbe activity displace Ca²⁺ and other ions from soil solids.

^·          This makes soil more acidic and depletes soils store of nutrients in form of exchangeable cations.

^·          Also under acidic conditions the rate of weathering of clay minerals ­ which means;

^·          More Al³⁺ released into soil.

^·          Al₂O₃ is formed (high [Al ] is toxic to plants!)

^·          Al₂O₃ attracts and binds H⁺, making it become +vely charged.

^·          This attracts anions (-ve ions)

^·          So below certain pH values plant growth can be restricted (see table 5)

·         To neutralise acidity, basic carbonates e.g. CaCO₃ or bases e.g. Ca(OH)₂ can be added – the amount required depends on;

·         Soil pH

·         Soils buffering capacity see fig 25 this shows the amount of alkali needed to raise pH of various clay minerals.

·         As lime is added to soil pH changes very little at first, but then slowly rises.

·         The soil acts as a buffer (resists change in pH) because H⁺ ions bound to soil solids  replace some of the H⁺ ions from soil solution as they are removed.

SL AA3 – Saving money and protecting the environment.

·        Fertilisers cost money which can be wasted if they are leached out of soil as NO₃^-.

·        Addition of fertiliser needs to be matched to needs of crop as it grows.

·        Leached nitrogen could get into drinking water – Concern over nitrate(v) levels has led to an EC limit of 50mg dm^-3.

·        To reduce risk of leaching winter crops can be grown.

·        See fig 41 page 199

·        Low [NO₃^-] in Jan/feb.

·        Mineralisation in spring causes [NO₃^-] ­ and fertilisers are added to ­ it further, but crop growth causes [NO₃^-] ¯.

·        After harvesting [NO₃^-] ­

·        Ploughing occurs ­ O₂ in soil, therefore microbial activity ­ , warm moist soil in autumn leads to rapid mineralisation and [NO₃^-] ­

·        As T ¯ in winter and soil becomes waterlogged denitrification causes [NO₃^-] ¯.

·        Leaching then further reduces levels from Dec onwards.

·        As a result of studies done fertiliser can be applied to match needs of crops, and less than 2% can be left in the soil to be at risk of loss by leaching after not being taken up by plants.

·        This also means high yields can be maintained without risk to the environment.

Now try assignment 10.