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Transcript
Soil Chapter 4 Soil Chemical Properties Pages 97 – 123 An acre . . . 1 acre = 43, 560 sq.ft. So what’s an acre-foot? 1 acre by 1 foot deep When does this come up? Hectare . . . What’s an hectare? How many hectares in an acre? An acre is about 0.4 hectares 1 hectare ≈ 2.47 acres Clays & Colloids & Soil Chemistry Rock breaks down into gravels . . . then into sands . . . then silts . . . but NOT to clays Clays . . . Are a product of minerals leaching into solution and precipitating out of solution to form the crystalline clay particles Clays . . . Clay minerals are mostly newly formed crystals, reformed following the partial dissolution of other minerals . . . producing . . . Secondary minerals called clay. Chapter 4, pg. 98 Clays . . . So we refer to clays as secondary minerals . . . begins life in another form . . . dissolves . . . settles out as clay particles . . . called colloids . . . Collides . . . Surface area and characteristics are more important than their mass Large surface-to-surface contact sticks to other colloids Greek for glue Collides . . . Small particles that tend to dominate soil chemistry Smaller than a few micrometers (microns) in diameter (1 micron = 0.001 mm = 10-6 m) Itty bitty! Origin of clays . . . Clays form over thousands of centuries Erosion moves and redeposits clays . . . to bodies of water or on land Origin of clays . . . Origins can include . . . Inherited clays . . . deposited as clays perhaps formed in different climates eons in the past Origin of clays . . . Origins can include . . . Modified clays . . . original clays changed by further weathering Origin of clays . . . Origins can include . . . Neoformed clays . . . newly formed clays formed by crystallization of ions in solution Silica . . . Silicon dioxide (SiO2) an oxide of silicon Component of clay soils Silicon atom bonds with 4 oxygen atoms forming a tetrahedral arrangement Alumina . . . Aluminum oxide (Al2O3) an oxide of aluminum Component of clay soils aluminum atom bonds with 6 oxygen atoms forming an octahedral arrangement Clay crystals . . . Are formed by layers of silica sheets and layers of alumina sheets Montmorillonite clays . . . 2:1 lattice clay 2 sheets of silica tetrahedra per one sheet of alumina Referred to as an ‘expanding lattice’ or ‘expansive clays’ Montmorillonite clays . . . Silica Alumina Silica Silica Alumina Silica This arrange does bind well between silica, alumina, and silica This arrangement doesn’t bind well between the 2 silicas Montmorillonite clays . . . Analogy: bread-peanut butter-bread-bread-peanut butter-bread Montmorillonite clays . . . Montmorillonite clays . . . Water infiltrates between the 2 silicas and causes the clay particles to separate and expand Montmorillonite clays . . . Bentonite – a form of montmorillonite has commercial uses . . . used to seal earthen ponds thickens paints slurries for fire fighting cosmetics, etc. Hydrous mica or ‘illite’ . . . Has a layer of potassium (K) between the 2 layers of silica Potassium layer reduces expansion Hydrous mica or ‘illite’ . . . 2:1 lattice clay 2 sheets of silica tetrahedra per one sheet of alumina but with a layer of potassium between the two layers of silica Hydrous mica or ‘illite’ . . . Silica Alumina Silica Potassium Silica Alumina Silica This arrange does bind well between silica, alumina, and silica Potassium reduces expansion between the 2 silicas Hydrous mica or ‘illite’ . . . Analogy: bread-peanut butter-bread-jelly-bread-peanut butter-bread Kaolinite clays . . . 1:1 lattice clay . . . 1 sheet of silica tetrahedra per one sheet of alumina octahedra residues from extensive weathering, high-rainfall and acidic soils does not expand excellent for pottery Kaolinite clays . . . Silica Alumina Silica Alumina Silica Alumina Silica No expansion between silica and alumina Hydrous mica or ‘illite’ . . . Analogy: bread-peanut butter-bread-peanut butter-bread In the tropics . . . Clays can dominate in the tropics High rainfall and temperatures increase leaching of rock minerals Leachates precipitate to form clay crystals Shifting Gears! pH . . . What is pH? pH is a measure of acidity or alkalinity (basicity) soil pH = pH of water in equilibrium with soil pH . . . It’s a measure of the free hydrogen ions in a solution It’s measured on a scale of zero to 14 pH . . . Less than pH 7 being acidic Greater than pH 7 being alkaline or basic pH 7 being neutral pH . . . In alkaline soils Iron (Fe2+), Zinc (Zn+) and Manganese (Mn2+) can become unavailable Calcium (Ca2+) and Magnesium (Mg2+) become unavailable in acidic soils pH & effects on plant growth Plants in soil tend to do best in soils with a pH range of 6.5 to 7.2 Nutrients are in readily available forms to plants in this range pH & effects on plant growth Container or ‘artificial’ organic soils should range one full point lower – 5.5 to 6.2 At the correct pH all nutrients are at their highest level of availability pH . . . pH is a negative logarithm of hydrogen ion activity in solution pH = -Log[H+] ex. pH 5 = 10-5 =, pH 6 = 10-6, pH7 = 10-7, etc. H2 O [H+] = [OH-] = 1 x 10-7 = pH 7 pH is measured on a 14 point logarithmic scale pH . . . Every point of change is a 10x increase or decrease in concentration of acidity ex. pH of 6 is 10xs more acidic than a pH of 7 pH of 5 is 100xs more acidic than a pH of 7 Modification of pH . . . Addition of lime to acid soils increases pH to ‘sweeten’ the soil Addition of sulfur to alkaline soils reduces the pH to ‘sour’ the soil Modification of pH . . . Fertilizers components can affect pH depending upon content Irrigation water can affect pH of soils – probably increasing pH Modification of pH . . . New concrete construction can increase pH in soils adjacent to the new concrete Why? lime leaches into surrounding soils increasing the pH Modification of pH . . . So with that in mind . . . We can add lime to soils to increase the pH Soil sulfur can be added to soils to reduce the pH Peat moss can be added to soils to reduce the pH Modification of pH . . . Constant additions of organic matter to soils can reduce the pH High rainfall can reduce the pH pH’s affect of solubility and availability of plant nutrients or . . . What affect does pH have on plants? pH and Solubility Outside given ranges of pH nutrients become unavailable tightly bound to soil particles pH and Solubility At high pH nutrients can become deficient iron, zinc and manganese become unavailable in alkaline soils pH and Solubility At low pH some nutrients can become toxic aluminum (Al3+) can be toxic in soils with a low pH Eastern US soils . . . Tend to be lower pH Higher precipitation leaches soils Soils are chemically weathered Alkaline forming minerals are leached out Eastern US soils . . . Ca2+ and Mg2+ are leached leaving iron (Fe) often resulting in red clay soils Which are often acidic Typical of the tropics Eastern US soils . . . These areas tend to have lusher vegetation as a function of higher rainfall When the vegetation dies and decomposes . . . It serves to further acidify the soils Red Clay Soils of Virginia Western US soils . . . Lower rainfall in the southwestern US leaves soils with a higher pH Less vegetation in the more arid climate Less rainfall to leach the alkaline forming calcium and magnesium Often lighter in color Charges on Clays Some of the charge comes from ionizable hydrogen Some charges come from isomorphous substitution Isomorphous Substitution During the growth of a crystal lattice one atom is replaced with another atom of similar size within the crystal lattice The crystal’s structure does not change The main source of charge on most clay particles Cation Exchange Sites The excess negative charge from isomorphous substitution and ionized hydrogen atoms create excess electron sites on clay crystals These negatively charged excess electron sites become cation exchange sites Cation Exchange Sites These sites attract positively charged cations from the surrounding soil solution The cations do not become part of the clay structure The cations are held somewhat loosely at the cation exchange sites Cation Exchange Sites Cations in solution often swap places with other cations at these cation exchange sites The total amount of the negatively charged cation exchange sites is referred to as . . . The soil’s ‘Cation Exchange Capacity’ (CEC) Cation Exchange Sites Cation exchange sites hold mostly Ca2+, Mg2+ and K+ in soils with a pH greater than about 6 Cation exchange sites of more acidic soils can hold Al(OH)2+ or AL3+ ions Smaller amounts of Na+, NH4+, Zn2+, etc. can be held as well Cation Exchange Capacity (CEC) Is a measure of soil quality by the soil’s ability to exchange ions Cations are positively (+) charged ions Anions are negatively (-) charged ions Minerals required by plants form cations in the soil Cation Exchange Capacity (CEC) The sum total of exchangeable cations that a soil can adsorb Expressed in centimoles per kilogram of soil Cation Exchange Capacity (CEC) Clay particles adsorb cations Why? Cations are positively (+) charged and clay particles are . . . ? Negatively (-) charged Organic Matter The source or sources . . . Plant and animal residues . . . Manures – animal wastes including human waste Dead animals including insects, worms, etc. Organic Matter Plant wastes including . . . Leaf litter, branches, bark, plant roots, etc. All in various stages of decomposition Organic Matter The decomposition of organic matter results in humus Humus has a cation exchange capacity (CEC) much greater than clay Humus Considered a temporary and intermediate product Temporary because remaining organic compounds continue to decompose slowly Resulting from considerable decomposition of plant and animal remains Humus Humus particles are organic colloids or small particles 1 to 1nm 1 x 10-6m = 1/1,000,000 meter to 1 x 10-9m = 1/1,000,000,000 meter They’re like really small . . . Totally! Humus Humus consists of different chains and rings of linked carbon atoms Humus is negatively charged – kinda like clay particles! Unlike clay – the negative charge results from hydrogen ionization As opposed to isomorphous substitution Humus H+ ions are released by acids Negatively charged sites remain where H+ ions were Humus is amorphous (without specific form) Humus is separated into categories of molecules based on solubility Humus Fulvic acids . . . smaller molecules soluble in both acids and bases Humic acids . . . larger molecules soluble in base are called Benefits of Organic Matter Chemical and physical effects of organic matter on soils Reduces and maintains lower pH in soils High CEC Source of humus Source of nutrients Benefits of Organic Matter Affects soil porosity restricts large pores in sands slowing drainage increases large pore spaces in clays increasing drainage Benefits of Organic Matter Increases the ability of gas exchange in soils Increases soil biota – worms, nematodes, bacteria, beneficial fungi, etc. Reminder . . . Adsorption . . . Bonding of ions to the surface of a solid Ex. calcium bonding to clay particles Reminder Adhesion . . . Molecular attraction that holds the surfaces of two unlike molecules together Ex. water to a rock Reminder Cohesion . . . The force holding two like molecules together Ex. water to water
