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Principles of Soils
Soil Chemistry
| Term/Question | Definition/Answer |
|---|---|
| Importance of soil chemistry? | chemical properties of soil determine soil fertility and nutrient availability; impacts soil pH; Determines availability of toxic chemicals; can influence physical properties and moisture, OM |
| What four things are essential to plant nutrients? | Carbon, hydrogen, and oxygen; Primary macronutrients - nitrogen, phosphorus, potassium; Secondary macronutrients - calcium, magnesium, sulfur; Micronutrients |
| Carbon, Hydrogen, Oxygen | obtained by photosynthesis, NOT soil nutrients |
| Primary macronutrients - Nitrogen, Phosphorus, Potassium | NPK is nature's source of nitrogen; Nitrogen and Phosphorus are most common fertilizers applied |
| Secondary macronutrients - Calcium, Magnesium, Sulfur | Sulfur is common fertilizer need in sandy soils; Calcium and magnesium are found in many soil parent materials |
| Micronutrients | Boron, Chloride, Copper, Iron, Manganese, Molybdenum, Zinc, Nickel |
| What element is essential for Nitrogen-Fixation by Legumes? | Cobalt - only N-fixation |
| What are three other important soil Elements? | Sodium - high levels cause soil crusting; Aluminum - becomes toxic to plants in acidic soils; Silicon - Abundant in soils, no benefit or harm |
| What are two sources of soil nutrients? | most come from SOIL MINERALS, but nitrogen comes from ORGANIC MATTER |
| What are the four pools of soils elements? | Soil minerals; Organic matter; absorbed on soil colloids; soil solution |
| Soil Colloids | clay and humus particles; carry slight negative charge; quantity of colloids in soil determined by soil texture/OM; interact closely with chemicals that have positive charge |
| What are positively charged ions called? | cations |
| Cations | Potassium - K+; Sodium - Na+; Ammonium - NH4+; Hydrogen - H+; Calcium - Ca++; Magnesium - Mg++ |
| Typical proportions of Cations in Soils | Calcium - 75-85%; Magnesium - 12-18%; Potassium - 1-5%; Sodium - 1%; Others - 1% |
| Soil Test = Cation Exchange Capacity | Total number of exchangeable cations a soil can hold; relative to negative charge, Clay and OM have greatest influence, measures as milliequivalents per 100g of soil |
| Negatively charged ions = | anions |
| Anions | Chloride - CL-; Nitrate - NO3-; Sulfate - SO4-2; Borate - BO4-3; Phosphate - H2PO4, H2PO4-2 |
| Anion Retention in soils | Phosphate held strongly due to quick formation of insoluble compounds; Nitrate and Chloride not held in soils and move freely with soil water; Sulfate held loosly in some low pH soils |
| What is Soil pH? | soil reaction (pH) is an indication of the acidity/basicity of the soil; at pH 7, Hydrogen ion concentration equals the hydroxyl ion concentration |
| Hydroxyl ion vs. hydrogen ion | Hydrogen causes acidity; Hydroxyl causes basic condition (alkaline) |
| pH | negative logarithm of the hydrogen ion activity (concentration); pH = -log[H+]; as pH decreases by one unit, acidity increases 10-fold |
| What is the importance of soil pH? | influences availability of plant nutrients; may influence toxicity of aluminum; influence activity of soil microorganisms; influence soil applied herbicide activity |
| Plant Nutrients Availability | direct effect on Fe, Mn, Ca, Zn, Mo; Neutral pH favors mineralization from organic matter |
| Aluminum Toxicity Influence | acidity releases aluminum from soil minerals |
| Soil Applied Herbicide Application | determines if able/available to kill weeds; determines the degradation rate |
| What are some of the main ways soils become acidic? | parent material is acidic (sandstone); application of nitrogen fetilizers; heavy precipitation; native vegetation = forested land; crops grown; burning fossil fuels |
| Form of nitrogen fertilizer application? | ammonium/ammonium phosphate |
| heavy precipitation = | leaching of bases |
| because crops grown, | basic compounds removed by harvest |
| What are some of the sources of soil acidity? | Active acidity - measured directly by pH meter to test Hydrogen in soil solution; Reserve acidity - hydrogen held on soil exchange sites/organic matter that is available to replace the hydrogen in soil solution: Al3+ + H+ |
| What element can be added to raise the soil pH? | lime or calcium carbonate: CaCO3 |
| What are some steps to managing soil pH and Lime? | soil test to determine lime required; plan ahead bc lime takes time; lime particles size determines how fast neutralization occurs; limes works best when applied early |
| Soil tests for lime? | soil pH and CEC |
| How can you manage to lower the soil pH? | apply elemental sulfur to naturally high pH soils; high pH caused by poor drainage |
| Poor drainage: | install subsurface till drainage |
| Elemental sulfur: | soil bacteria converts to sulfuric acid; not economical for field crops |
| Salinity | high concentration of total salts |
| What are some causes of salinity? | Poor Drainage; Shallow Water Table; Seepage Zones; Parent Materials |
| Where is salinity in soils a problem in Nebraska? | Lancaster and Saunders counties |
| What is salt? | and inorganic material/mutually charged molecule that can dissolve in water |
| How is salinity measured? | estimated by measuring the Electrical Conductivity (EC); mmhos/cm |
| What are two different ways to manage saline soils? | leach soils with enough non-saline water that salts are moved below root zone (adequate drainage required); grow salt tolerant crops |
| What are the minimum requirements for saline soil reclamation? | assess problem; make sure there's mechanisms for drainage; have clean water supply |
| Sodicity = | high concentration of sodium (Na) |
| What changes may sodic soils (sodium) make to soil physical properties? | soil aggregates are destroyed; causes poor drainage/crusting |
| What are two things you need to know when managing sodic soils? | the basic rule; may need to grow salt-tolerant crop until process in complete |
| The Basic Rule | good drainage - outlet to send sodium when it is displace; source of calcium/exchange process - gypsum (CaCO4 x 2H2O); source of water to flush sodium from system |
| In what ways is salinity stress similar to draught? | salts have pulling force on water; soil salinity = root salinity water less likely to enter roots; soil salinity is to high - water is pulled back into soil and plant wilts and dies; (most plants respond to salinity in same way as water deficiency) |
| What can an accumulation of Na+ do? | damage cell mechanisms |
| What can an accumulation of Cl- do? | inhibit photosynthesis |
Created by:
horktera
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