Unit 6: Soil Chemical Properties

1. Unit 6: Soil Chemical Properties Chapter 4

2. Objectives • Definition & importance of soil colloids & their effect on the soil • Knowledge of humus and its role • Identification of CEC and the role of cation exchange • Effect of soil pH, soil acidity • How soil solution, buffering, & cation saturation percentage

3. Introduction • Soils can’t be managed & production can’t be optimized w/out basic knowledge of soil chemistry • Colloid – solid substance whose particles are very small, but have very large surface area • Primarily humus & clays • Colloids tend to stick together

4. Soil Clays • Most are crystalline in structure • Term clay actually carries three meanings • Particle size fraction >2 microns in size • Name for a group of minerals w/ specific composition • Soil textural class

5. Soil Clays • The Origin of Clays • Usually have specific composition • Newly formed crystals, usually different from primary minerals • Reform following dissolution of other minerals • Kind of clay formed determined by proportions of the different ions • Silica, alumina • If some materials leached away, clay types are changed or formation rates change

6. Soil Clays • Some clays form from slight alteration of primary minerals (micas) • Vermiculite, hydrous mica • Soils may have clays from ocean or sediment redeposit • Inherited clays – clay sediment formed in a different climate • Modified clays – changed by further weathering of original clays • Neoformed clays – new clays formed by crystallization of ions from solution

7. Soil Clays • Nature of Clays • Because of crystalline nature – composed of definite, repeating arrangements of atoms • Made up of O2 atoms, Si, Al held together w/ +/- charged ions • Clay particles may also be known as micelle • Clays have net negative charge, will attract and hold positively charged ions (cations) • K, Na, NH4, Ca, Mg, H, Al[OH]2, Al

8. Soil Clays • Amounts held vary w/ type of clay & vary w/ charge of the ion • Plant roots use some exchangeable cations as nutrients • Leaching may remove several cations • Cations can be replaced by other cations • As they are exchanged • If their charge is more positive

9. Soil Clays • Charge on Clays • Isomorphous substitution • Clays act like weak acids, release H ions from bonding sites (sites deprotonated) • Now has an open site to attract another element • Ions will be attracted that are of similar weight & charge • Amount of deprotonation depends on soil pH • Sites formed known as cationexchange sites • Other ions compete to be adsorbed to these sites • Amount of negative charge = soil’s CationExchange Capacity (CEC)

10. Soil Clays • Clays w/ layers spread apart allow soil solution to pass through the layers • Montmorillonite, vermiculite • Have accessible exchange sites along their surface • Nutrients can leach easier • Clays shrink/swell – not suited for building & construction • Tightly bonded layers, little room between micelles • Kaolinite, chlorite • Don’t swell when wet • Can use to make pottery, tile, etc. • May have relatively lower CEC

11. Soil Clays • Silicate Clays • Picture a deck of cards • Each card is layer of a clay • Each layer held together magnetically • Amorphous silicate clays – lack crystallinity • Typically occur where weathered products existed, but not sufficient time/condition for crystal dev. • Common in soils forming from volcanic ash

12. Soil Clays • Kaolinite & halloysite – residues from extensive weathering in high-rainfall, acidic soils • Net negative charge is low • What does that mean for CEC? • Strong H bonding layers together • Doesn’t allow H2O to penetrate • No swelling • Common in southeastern U.S.

13. Soil Clays • Montmorillonite & saponite • Swelling/sticky clays • Belong to group called smectites • Water easily penetrates clay layers • Shrink/swell is common • Bentonite – impure deposit of montmorrilonite used to seal earthen ponds/lagoons, thickens paints, ties up toxics in feeds, cosmetics • Found in soils w/ little/no leaching • Poorly drained soils, soils developed from limestone, flood plains of rivers

14. Soil Clays • Hydrous mica, illite – fine-grained mica, structure similar to montmorillonite • Tight bonds don’t letter water penetrate • Slight to moderate swelling • Named after state of IL • Vermiculite – occurs in different forms • Used for insulation, potting soil, packing material • What does this tell you about its structure? • Swells very little • Extremely high CEC • Most often an accessory soil, not dominant

15. Soil Clays • Chlorites – hydrated Mg & Al silicates • Similar to vermiculite • Restricts swelling • Actually has net positive charge • What effect does this have? • Sesquioxide Clays – form under extensive weathering, leaching in warm climates • Small amounts exist in many soils • Can be dominant, or accessory

16. Soil Clays • Usually predominant soil in humid, hot, well-drained soils • Usually shades of red & yellow colors • Don’t swell, not sticky • Have high P adsorption capacity • What does this result in? • What problems might it cause?

17. Organic Colloids • Humus – temporary, intermediate product left after decomposition of plant & animal remains • Continues to decompose slowly • Humus particle = organic colloid • Consists of various chains of carbon atoms • Has negative charge • What does this mean? • What effect does it have on the soil?

18. Organic Colloids • CEC is many times greater than clay colloids • What conclusion does this give you? • Humus exerts considerable influence on the soil • In what form?

19. Cation Exchange Soil colloids will attract & hold positively charged ions to their surface Replacement of one ion for another from solution = cation exchange Adsorbed cations resist removal by leaching, can be replaced by other ions by mass action Takes place on clay/humus colloids & on root surfaces

20. Cation Exchange Most commonly held cations – Ca, K, Mg, H, Na, Al, NH4 • Proportions of these cations change constantly due to leaching, plant absorption • Cation Exchange Mechanism • Secure cations & keep them available to the plants for potential absorption • Water moving through the soil may move/remove some cations

21. Cation Exchange • For every cation that is adsorbed, one goes back into soil solution • Some may precipitate out & form insoluble salts • Affects soil aggregates, and nutrient availability • Plants absorb soil N as it is made available • Well-vegetated soils lose less N than bare soils • Rate of movement decreases as strength of adsorption increases • Ex. Lead & cadmium from sewage • Held tightly to soil clays and allowed to filter slowly out rather than pollute water

22. Cation Exchange • What effect does liming soil have? • How does it work in the soil…specifically? • What changes might we expect after liming? • What else might it change in the soil? • Cation Exchange Capacity • CEC – quantity of exchangeable cation sites/unit wt. of dry soil • Measured in centimoles/kg of dry soil • Which soils will have higher CEC? Sand/Clay?

23. Cation Exchange • Amounts of exchangeable cations can be high (even at 24-36”) • CEC level typically constant – as long a soil humus/clay content is the same • Labs measure CEC w/ soil analysis • Can estimate, if you know soil clay & organic matter content

24. Cation Exchange • Importance of Cation Exchange • Plant nutrients Ca, Mg, K are supplied to plants mainly from exchangeable forms • Exchangeable pools of Ca, Mg, K are major sources of these nutrients for plants • Amount of lime required to raise pH of an acidic soil increases as the CEC increases • Cation exchange sites hold Ca, Mg, K, Na, & NH4 ions & slow their release by leaching • Adsorb many metals present in wastewater & prevent pollution to ground/surface waters

25. Anion Exchange & Adsorption • Anions – negatively charged ions – sulfate, nitrate, phosphate, chloride, etc. • Not held on CEC sites • Anion exchange sites – positively charged sites, or ligand exchange sites • Highest Anion Exchange Capacities (AEC) – occur in amorphous silicate clays • AEC’s generally low • Low pH relates to high AEC values

26. Soil pH Indication of the acidity/basicity of the soil • At pH 7.0 – H+ ions equal OH- ions • 10x change between each whole pH number • pH 5.0 is 10x more acidic than pH 6.0 • Typical soil pH ranges from 4.0 to 10 • Most plants grow well from 5.5 to 8.5 • Strongly acidic soils undesirable – develop toxic levels of Al & Mn, microbe activity greatly reduced • Strongly alkaline soils have low micronutrient availability, P may be deficient

27. Soil pH Soils can become acidic as rainfall leaches nutrients away What is more difficult to alter, soil acidity or alkalinity? • What do you alter each one with? • Chelates – fertilizer forms that can be added to protect soil nutrients

28. Soil pH • Importance of Soil pH • Affects solubility of minerals • More soluble in slightly acidic soils • Most crops do best at pH – 6.5 • Plants preferring acid soils • Azaleas, rhododendrons, blueberries, pineapple • Plants preferring basic soils • Barley, sugar beets • High Ca demand • Alfalfa – neutral/slightly basic pH

29. Soil pH • Also affects soil microbes • Decreased soil microbe activity w/ acidic soils • Slow/stop decomposition of beneficial materials • Decreased N availability • Basic Cation Saturation Percentage • Base Saturation Percentage – proportion of basic cations to the total cations • More acidic the soil, the lower the BSP • At pH 7.0, BSP is essentially 100% • Aids in the decision on how much lime to add

30. Equilibrium & Buffering Solution – solvent in which solubles are dissolved • Soil water w/ nutrients dissolved in it • Soil nutrients must be in solution to be absorbed by plants • Plants & microbes need Ca to thrive, absorb it from soil solution • Clay & humus adsorb Ca readily • Water can leach Ca away

31. Equilibrium & Buffering Most soils resist appreciable pH changes • Resistance to change – buffering capacity • Increases as CEC increases • Soils high in humus and/or montmorillonite or vermiculite clay – high buffering capacity • Organic & clay soils – much higher CEC, more strongly buffered than sandy soils

32. Assignment