Physical and Chemical Properties of Soils - A Living System | SoilS 201, Exams of Agricultural engineering

Download Physical and Chemical Properties of Soils - A Living System | SoilS 201 and more Exams Agricultural engineering in PDF only on Docsity! 3/17/2006 Soils 201 Exam II Review Packet 1 – Physical and Chemical Properties of Soils - This Soils 201 Exam II study guide contains both information and questions. You will be responsible for the questions posed here. Questions are drawn directly from course lectures and Exploratory Sessions. Clarifying material can be found in the appropriate book chapter. Symbols used are those from Brady and Weil’s texts General concepts covered: 1. Units 2. Density 3. Porosity 4. Aggregation 5. Factors that influence physical properties 6. Managing for good tilth 7. Soil Water a. Content (gravimetric, volumetric) b. Potential c. Hydraulic conductivity 8. Managing for water use efficiency 9. pH 10. Septic systems 11. Soil colloids and CEC 12. Exploratory Session Exercises Units Converting between • Length: mm, cm, m • Mass: g, kg, Mg • Volume: cm3 , m3; (remember, 1 m3 = 1 x 106 cm3 = 1,000,000 cm3) Density: mass/unit volume = Mg = g m3 cm3 Particle density: density of individual particles: mass particle Volume particle Why do substances have different particle densities? How would particle density compare for a local soil compared to a tropical soil? Which one has more silica and which one has more Fe/Al oxides? How does particle density of organic matter compare to particle densities of minerals? Bulk density: Db = mass solids Volume of complete soil (that is, solids + pores) Which is affected by management: bulk density or particle density? Why? • Rank Db from least dense to most dense for different soil textures. Answer: clay → silt loam → sand 3/17/2006 Soils 201 Exam II Review Packet 2 • What are typical values for each of the above textures? • Why does sand have a higher Db than clay? (Think about the pore space in clay particles.) • Which includes pore space: bulk density or particle density? Try drawing a picture of a 3-d box. One is filled to the top with soil, the other is compacted and only half fills the box. Which one includes pore space and which one is Db? Porosity = % pore space = volume pores Total soil volume • How much of total soil volume is taken up by pores? Total pore space includes pores that are filled with air and water. Answer: 50%. This average is optimal for plant growth. • What soil would have higher porosity: one with higher OM content, or a sandy clay? • There are three kinds of pores: 1) macropores (which includes 2) biopores), and 3) micropores. Which one is found between peds (i.e. which ones are interpeds?); which ones are actually between soil particles; which ones are caused by actions of soil macrofauna? • Which of the pore classes are generally filled with water? • Water-filled pores slow air movement. Aggregation Micro-aggregates are cohered dominantly by the chemical and physical forces Macro-aggregates are biologically bound by plant roots, fungal hyphae, slimes exuded by soil microbial life and roots Good and stable soil aggregation is a sign of soil tilth = good general soil physical characteristics (a non-scientific term) Factors that influence soil tilth 1. Compaction a. Know 2-4 factors that influence compaction b. Does compaction act mainly on micropores or macropores? c. Understand action of raindrops on soil surface crusting (silt loam soils experience the worst crusting) 2. Oxidation of soil carbon (conversion of OM-carbon to CO2) when bare soil is exposed to the air, including when soil aggregates are pulverized, exposing more surface area to react with the air 3. Organic matter content a. Understand relationships between aggregate stability and soil OM level b. Is a mineral soil with high or low OM best able to hold both water and air? c. Which retains its structure, which turns to mud? Why? Managing for for tilth 1. Understand management practices that enhance soil tilth; these include, but are not limited to the following. Be able to provide 2-3 management strategies for each. a. Minimize compaction b. Maintain and enhance soil OM 3/17/2006 Soils 201 Exam II Review Packet 5 Septic Systems 1. Septic systems rely on physical, biological, and chemical actions to treat household wastewater. 2. Pre-treatment occurs in the septic tank, “final” treatment occurs in the leachfield if it is constructed properly. 3. A variety of alternative types of septic systems, often involving expensive pumps, have been developed to sufficiently treat wastes where there is a high water table. • What occurs in each component of the septic system? • Explain the biomat formation and function. • What are the potential pollutants present in wastewater that we rely on a septic system to treat? How is the pollution potential of each type of pollutant treated in an ideally functioning septic system? Which of the pollutants are most easily treated? Least? Soil Colloids and CEC Cation exchange capacity • What is it? • Does it have a positive or negative charge? • What charge do cations and anions have? • CEC is determined by: o Soil texture o Kinds of silicate clay minerals present o Amounts of clay silicate minerals present o Amount of OM o Soil pH • You should have an understanding of how these factors influence CEC CEC • Know how to diagram the cation exchange process, in which cations on a clay can be replaced by incoming cations • CEC units are cmolc/kg. • Soil CEC depends on the relative proportions of the different CEC sources (clays and OM - we can ignore sand, silt because their relative surface area is so small) • Identify acid and non-acid (base) cations Calculating CEC • Note that cmolc = cmols of *charge* • One cmol Ca2+ requires *two* cmols of net negative charge to be adsorbed • Hence 1 cmol Ca2+ = 2cmolc • One way of calculating the CEC is to know the relative proportion of the different colloids, look up the table CEC value for each and weight it (multiply it) by the percent that colloid is in the soil overall • Ex. total CEC = (kg colloid / kg soil) x cmolc/kg for colloid x) + ....(add in terms for remaining colloids). • Alternately, you can sum up the cmolc of adsorbed cations present in a sample of soil • 1 cmol Al3+ = 3 cmolc 3/17/2006 Soils 201 Exam II Review Packet 6 Soil colloids • What are the four major types of soil colloid? • They have different inherent negative charge (so have different CEC); understand the general ranking of the major types • What is the difference between adsorption and absorption? Silicate clays • There are 1:1 and 2:1 type silicate clay minerals • Type of clay mineral is determined by octahedral and tetrahedral layers, and the particular sequence (arrangement) of these layers • This variation in silicate clay layer arrangement determines the CEC for each particular mineral • You should be able to describe 2 kinds of silicate clay minerals and why CEC is different for these • Think about internal and/or external charges, total surface area, interlayer spacing, ability of ions or water to get into these interlayers (there may be no interlayer spacing: kaolinite; or it may be entirely occupied by an ion like potassium: illite; or the silicate clay layers may expand when wet: vermiculate and smectite; these factors influence CEC (see the book; see your notes) • What effect does surface area have on CEC? Colloids and CEC • Know the *general ranking* of different colloids with respect to CEC - esp vermiculite, smectite, illite, kaolinite, oxide and hydroxide clays, and OM • Exact numbers will be given if you need them Exploratory Session Exercises 1. How to read soil surveys a. Identify locations on soil survey maps b. Calculate areas identified on survey maps c. Know what kind of information can be found in soil surveys d. Know how to read range and township locations 2. Calculations for measuring gravimetric and volumetric water content, bulk density, and porosity 3. Compare methods used to determine water potential, drawing from both book chapters and exploratory session 4. Compare methods used to measure soil pH; understand 3 methods of measuring pH a. Why does the pH meter need to be calibrated? b. What is the pH of the different solutions used to calibrate the pH meter? c. Why do we use two solutions to calibrate the meter? d. Why did I not pay attention to exploratory session explanations? 5. CEC and Soil Colloids a. What is cation exchange capacity? b. Relationship between pH and CEC c. What role does clay play in septic systems? d. What effect can hydrogen ions in soil solution have on Ca2+ ions adsorbed to clay or humus colloids 3/17/2006 Soils 201 Exam II Review Packet 7 Calculations Worksheets Calculation 1. Replacing Cations on Soil Colloids Remember: Cations are replaced on soil colloids on a charge basis. That is, we know the charge of the cation that we want to replace. We then know that it takes an equal amount of charge of another cation to replace it. If we have 2 cmol(+)/kg soil (centimoles charge per kilogram soil) of both hydrogen and aluminum ions, we have 4.0 total cmol(+)/kg that we want to replace. If we want to replace these charges with calcium, we will need 4.0 cmol(+)/kg of calcium cations. Study the example below. Question: Determine the weight of Ca2+ required to replace 2.0 cmol(+)/kg H+ and 2.0 cmol(+)/kg Al3+ with Ca2+ cations in a soil. Steps Calculations 1 Determine # of cmol(+)/kg that you need to replace 2.0cmol(+)/kg H+ + 2.0 cmol(+)/kg Al3+ = 4.0 cmol(+). Therefore, we want to replace 4.0 cmol(+)/kg of Al3+ and H+ cations with 4.0 cmol(+)/kg Ca2+ 2 Look up the atomic weight of the cation that will be used to replace the charge from step 1 40 g/mol Ca2+ 3 Determine the molar charge conversion for by converting the atomic weight of Ca2+ from g/mol to g/mol(+). Note that Ca2+ has two moles charge per its atomic weight of 0.40 g/mol. 40 g Ca2+ 1/100 0.40 g Ca2+ Mol x 1/100 = 2 cmol(+) 4 To determine g/kg Ca2+, multiply the charge (step 1) by the molar conversion for Ca2+ to get the weight of Ca2+ needed to replace this specified charge (step 1) on a g/kg basis 4 cmol(+) 0.40 g Ca2+ 0.8 g Ca2+ Kg x 2 cmol(+) = kg