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Lecture 8. Chemical Properties Of Soils

Lecture 8. Chemical Properties Of Soils. IMPORTANT DATE. Oct. 2. Forest Ecology 1st exam (in class) BRING A CALCULATOR. I. Soil Acidity. pH pH Range of forest soils. I. Soil Acidity. pH: a measure of the soil acidity, or H ion concentration. 1 pH = log ------ H+

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Lecture 8. Chemical Properties Of Soils

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  1. Lecture 8. Chemical Properties Of Soils

  2. IMPORTANT DATE • Oct. 2. Forest Ecology 1st exam (in class) • BRING A CALCULATOR

  3. I. Soil Acidity • pH • pH Range of forest soils

  4. I. Soil Acidity • pH: a measure of the soil acidity, or H ion concentration. 1 pH = log ------ H+ ii. Soil pH can range from 1 to 14, with 1 being acidic, 7 is neutral, and 14 is alkaline Most forest soils range from 4 to 7.

  5. I Soil Acidity iii. Natural sources of acidity During the “acid rain” days of the 70’s there was widespread lack of understanding that many natural ecosystems have acidic soils because of natural soil processes.

  6. Natural Sources of Acidity a. Organic acids (R-0H) R-OH R-OH <--> RO- + H+

  7. Natural Sources of Acidity b. Bicarbonate (HCO3-) Concentration of CO2 in the atmosphere ?? decomposition root respiration Concentration of CO2 in the soil ?? CO2 CO2 + H2O <--> H2CO3 < --> H+ + HCO3-)

  8. Natural Sources of Acidity c. Nitrate (NO3-) mineralization Organic N -->NH4 nitrification 2NH4 + 3O2 --> 2NO2 + 2H2O + 4H+ 2NO2 + O2 --> 2NO3 + energy

  9. I. Soil Acidity iv. Effects of soil acidity on plant growth • Altered nutrient availability • Adversely affects root membrane permeability • Decreased microbial diversity

  10. Effects of soil acidity on nutrient Availability. Why do farmers lime their soil?

  11. II. Ion Exchange • i. cation exchange capacity (CEC) • a. factors influencing CEC • b. base saturation • ii. anion exchange capacity (AEC) • iii. sources of exchange sites

  12. II. Ion Exchange • i. definitions: • Cation - a positively charged ion (K+, Ca++, H+, NH4+) • Anion - a negatively charged ion (NO3-, Cl-, HCO3, HPO4--) Recall similar charges repel and opposite charges attract

  13. II. Ion Exchange • i. cation exchange capacity (CEC): the capacity of a soil to attract and hold positively charged ions (cations) to the net negatively charged surface of clay or organic matter particle. Units are meq/100 gram dry soil. • 1 equivalent = 1 gram atomic weight of H+ or the amount of any other ion that will combine with or displace H. • CEC = (meq. Ca + meq. Mg +meq K + meq Na + meq Al + meq H) for 100 g dry soil.

  14. -OH R K+ K+ -O- K+ H+ -OH H+

  15. These two get the correct units of CEC II. Ion Exchange • 1 Eq H 1000 meq H 1g H • -------- x ---------- x -------- • 1 g H 1 Eq H 1000 mg H

  16. valence Atmoic wt. II. Ion Exchange • 30 mg Ca+ 1 eq Ca 1g 1000 meq • -------- x ---------- x -------- x ---------- • 100 g soil (40/ 2) g Ca 1000 mg 1 eq

  17. II. Ion Exchange • Examples of atomic weight and valence of elements used to calculate CEC • Element atomic weight valence • Hydrogen (H) 1 +1 • Calcium (Ca) 40 +2 • Magnesium (Mg) 24 +2 • Sodium (Na) 23 +1 • Potassium (K) 39 +1 • Aluminum (Al) 27 +3 NOTE: I will not require you to memorize atomic weights and valence -they will be provided.

  18. II. Ion Exchange a. Factors that influence CEC • % organic matter • Soil texture • Clay type • pH

  19. Comparison of CEC for different organic and mineral components

  20. II. Ion Exchange b. Base saturation (BS): the percentage of the CEC that is occupied by exchangeable bases (Ca+2, Mg+2, K+, Na+) relative to all bases (includes H+ and Al+3 ). As a general rule pH and soil fertility are positively correlated to % BS. %B.S. = (Ca + Mg + K + Na) ----------------------------- (Ca + Mg + K + Na + H + Al)

  21. III. Essential Nutrients i. macronutrients Macronutrients refers to the fact that plants require greater quantities of these nutrients than micronutrients - hey are NOT more important.

  22. III. Essential Nutrients i. macronutrients • Nitrogen: found in proteins and is an integral part of the chlorophyll molecule. Despite the fact that the earth’s atmosphere is 78% N, most forest are N limited. • Phosphorus: important in energy transfer, low abundance, derived entirely from weathering of parent material. • Potassium: constituent for many catylst for metabolic functions,largely derived from weathering of mica and feldspars

  23. III. Essential Nutrients i. macronutrients • Nitrogen: found in proteins and is an integral part of the chlorophyll molecule. Despite the fact that the earth’s atmosphere is 78% N, most forest are N limited. • Phosphorus: important in energy transfer, low abundance, derived entirely from weathering of parent material. • Potassium: constituent for many catylst for metabolic functions,largely derived from weathering of mica and feldspars. • Sulfur: important component of protein, derived from weathering of pyrite and gypsum

  24. III. Essential Nutrients Macronutrients continued 5. Calcium: integral constituent of the cell wall glue (structural), derived from weathering of limestone. 6. Magnesium: constituent of chlorophyll molecule, essential to photosynthesis, derived from weathering o dolomite. Serpentine soils have too high Mg;Ca ratio and it adversely affect tree growth 7. Sodium: important for controlling osmotic potential and membrane permeability.

  25. III. Essential Nutrients Micronutrients: all micronutrients are catalyst for required enzyme • Iron (Fe) and Copper (Cu): required for chlorophyll synthesis and catalyst for enzyme activities • Molybdenum (Mo): essential for nitrogen fixation. • Boron (B): required for N metabolism

  26. Plant Growth and Nutrient Availability Deficiency: growth increases with the addition of nutrient. Transition zone: growth response declines, nutrient generally is no longer limiting. Luxury consumption: excess uptake of nutrient, concentration. Increases. Toxicity: high concentration of nutrient adversely affect growth or may kill plant.

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