Soil Chemistry

1. Soil Chemistry Chapter 5

2. 5.1 Introduction • basic chemical composition of a soil is less useful than a knowledge of its component minerals and organic materials. • these dictate: • reactions that occur in the soil • availability of nutrients Soil Analysis Ch5

3. Decrease uptake by plants leaching conversion into insoluble forms Increase addition of fertiliser decomposition of plants animal poo dissolving of rock Exercise 5.1 Soil Analysis Ch5

4. 5.2 Clay Minerals • naturally occurring inorganic compounds • form initially in the crystallisation of molten rock material • known as primary minerals • eg olivine, quartz, feldspar and hornblende • not stable when exposed to water, wind and extremes of temperature • break down physically and chemically • reform and crystallise in a different structure Soil Analysis Ch5

5. Clay minerals • called secondary minerals • eg vermiculite, montmorillonite and kaolinite • tend to be much smaller in particle size than primary minerals • most commonly found in the clay fraction of soils • only the youngest and unweathered of soils will not contain mainly secondary minerals Soil Analysis Ch5

6. The Earth’s crust Soil Analysis Ch5

7. oxygen is negatively charged • the other major elements are positively charged • oxygen bonds with one or more of the cations, producing a chemistry of oxides • silicon oxides (silicates) • aluminium oxides (aluminates) • generally in combination as aluminosilicates • these dominate the minerals • low levels of other elements account for the differences in minerals Soil Analysis Ch5

8. Si binds to four oxygens in a tetrahedron • Al has six oxygens (often as OH) in an octahedron • not a matter of individual SiO4 or Al(OH)6 units • some Os are shared between the silicate or aluminate units • most common structure in clay minerals is the formation of sheets • “flat” layers of silicate tetrahedra or aluminate octahedra • these sheets stack on top of each other • held together by hydrogen bonding or electrostatic attraction Soil Analysis Ch5

9. 1:1 2:1 2:2 Common sheet arrangement in clay minerals (tetrahedrons in grey) Soil Analysis Ch5

10. real clay crystals are not pure silicates or aluminates • some Si or Al atoms are substituted during the crystallisation process • creates spare charges which give the overall crystal a charge • balanced by loose cations or anions Soil Analysis Ch5

11. these cations generally are held on the surface of the clay • are not strongly held • can be exchanged for other cations in an equilibrium process • measured as the cation exchange capacity (CEC) • soil pH has no effect on the exchange capacity from the clay minerals Soil Analysis Ch5

12. as minerals weather, they lose silicon • this leads to increasing proportions of aluminate in weathered clays • Al-OH species are amphiprotic • soils dominated by oxides of aluminium (and other metals) can have positive sites in acidic soils • this allows anion exchange Al-OH + H+ <=> Al-OH2+ + X- Soil Analysis Ch5

13. 5.3 Ion exchange in soils • when the loosely held cations or anions on the mineral surfaces are replaced by ions of the same charge (sign and magnitude) in solution • cation exchange is by far the most common • necessary for soil fertility • as soils weather, they lose cation exchange capacity and lose fertility Soil Analysis Ch5

14. Cation Exchange • clay minerals have negative charge due to substitution of aluminium or silicon in the crystal lattice • humus also contributes negative charge, due to the presence of dissociated organic acids • humus-COOH  humus-COO- + H+ Exercise5.2 • What effect would soil pH have on the amount of cation sites from humus? • low pH, less dissociated acid, less sites Soil Analysis Ch5

15. a cation in solution replaces an adsorbed cation on the soil particle • eg soil-Na + K+ (aq)  soil-K + Na+ (aq) • charges that are balanced, not number of charged species. Class Exercise 5.3 • Write an equation for the exchange of adsorbed sodium with solution calcium. • soil-Na + soil-Na + Ca2+ (aq)  soil=Ca + 2Na+ (aq) Soil Analysis Ch5

16. exchange is equilibrium • reversible and dependent on the levels of each of the species, particularly the solution species • eg if a soil solution becomes depleted in calcium, then some calcium will desorb from an exchange site into solution • known as buffering • in all but the most leached and infertile of soils, there will be a balance between adsorbed and dissolved ions Soil Analysis Ch5

17. Exercise 5.4 • What do you think would happen to a soil which is treated with lime (calcium hydroxide), in addition to a pH change? • high concentration of Ca in solution • this would be partly reduced by exchange with the soil cations Soil Analysis Ch5

18. Cation exchange capacity (CEC) • the moles of exchangeable positive charge per unit mass 100 g of dry soil • usually mmole/100g or cmole/kg (the same value) • Ca & Mg contribute twice as much to the CEC as an equivalent number of sodium and potassium ions because of their 2+ charges Soil Analysis Ch5

19. Class Exercise 5.5 • Comment on the trend in CEC in Table 5.1. • CEC increases with higher clay levels Soil Analysis Ch5

20. Significance of CEC • uptake of nutrient ions from plant roots occurs from solution only • as cations are absorbed into the roots, they are replaced in the soil solution by H+ ions • when the exchange equilibrium is disturbed, some of that ion will desorb from the soil particles • replaced by another ion • if the nutrient is a weakly adsorbed one, such as K, there may not be enough adsorbed to replenish the soil, presenting a fertility problem • K is the most likely cation to be in short supply Soil Analysis Ch5

21. Anion exchange • the important soil anions, nitrate and phosphate, behave very different at exchange sites • nitrate and chloride are only weakly held at positive sites • more likely to be found in soil solution • phosphate and sulfate are very strongly bound to the exchange sites • phosphate can become covalently and irreversibly bound Soil Analysis Ch5

22. Soil pH • one of its most important properties • it affects so many other soil properties, (eg ion exchange and nutrient availability) • soil pH comes about from a balance between acidic and alkaline species • reflects mainly the levels of dissolved H+ and OH-, but also the adsorbed H+ on cation exchange sites • normally ranges from 4-9 Soil Analysis Ch5

23. Sources of soil acidity • rain - polluted or fresh will be slightly acidic due to dissolved gases • microbial and root respiration – this produces CO2, which is slightly acidic in solution • oxidation of organic matter – this produces organic acids known as humic acids, together with nitric and sulfuric acids Soil Analysis Ch5

24. Sources of soil alkalinity • carbonate minerals – calcium and magnesium carbonate are common materials in minerals • they are slightly soluble in water, and produce OH- as they dissolve • these cations and Na & K are known as bases because of their association with alkaline soils • mineral weathering­ – many primary minerals as they weather release hydroxide salts of the basic cations Soil Analysis Ch5

25. Trends in soil pH • as soils age by weathering and leaching, they tend to become more acidic • primary minerals that release alkaline materials are replaced by neutral or slightly acidic secondary minerals • leaching removes the carbonate minerals • weathering occurs from the surface downwards so that the A and B horizons will tend to be more acidic than the C horizon Soil Analysis Ch5

26. Significance of soil pH • nutrient availability – the ability of plants to take up nutrients is very much dependent on the soil pH Soil Analysis Ch5

27. Significance of soil pH • effect on soil organisms – soil organisms prefer different pH levels • acid-sulfate soils - soils that are rich in inorganic sulfide minerals, such as pyrites, • can lead to the formation of excessive levels of sulfuric acid through oxidation • soil pH dives to very low levels • causes solubilisation of toxic levels of aluminium, manganese and iron from soil minerals • plant preferences – most alkaline soils; a few which need acidic soils Soil Analysis Ch5

28. Soil pH management • soils tend towards lower pH values as they age • the main need for pH management is to making the soil more alkaline • most common method by liming • agricultural lime is a mixture dominated by CaCO3, but also containing MgCO3 and Ca(OH)2 • comes from ground limestone, • add the nutrients calcium and magnesium to the soil • dolomite lime has a higher proportion of magnesium carbonate • to reduce pH , add Fe, S or peat Soil Analysis Ch5

29. Exercise 5.9 • What factors will affect the amount of liming required? • buffering capacity • pH Soil Analysis Ch5

30. Redox potential (Eh) • a measure of its ability to produce oxidation or reduction of chemical species in it • the most important soil property indicated by the soil Eh is whether it is aerobic or anaerobic • aerobic soils give a positive value • the lower the value the more anaerobic the conditions • a value that is affected by soil pH Soil Analysis Ch5