Soil Chemistry
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| 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
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| What four things are essential to plant nutrients? | Carbon, hydrogen, and oxygen;
Primary macronutrients - nitrogen, phosphorus, potassium;
Secondary macronutrients - calcium, magnesium, sulfur;
Micronutrients
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| Carbon, Hydrogen, Oxygen | obtained by photosynthesis, NOT soil nutrients
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| Primary macronutrients - Nitrogen, Phosphorus, Potassium | NPK is nature's source of nitrogen;
Nitrogen and Phosphorus are most common fertilizers applied
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| Secondary macronutrients - Calcium, Magnesium, Sulfur | Sulfur is common fertilizer need in sandy soils;
Calcium and magnesium are found in many soil parent materials
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| Micronutrients | Boron, Chloride, Copper, Iron, Manganese, Molybdenum, Zinc, Nickel
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| What element is essential for Nitrogen-Fixation by Legumes? | Cobalt - only N-fixation
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| 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
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| What are two sources of soil nutrients? | most come from SOIL MINERALS, but nitrogen comes from ORGANIC MATTER
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| What are the four pools of soils elements? | Soil minerals;
Organic matter;
absorbed on soil colloids;
soil solution
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| 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
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| What are positively charged ions called? | cations
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| Cations | Potassium - K+;
Sodium - Na+;
Ammonium - NH4+;
Hydrogen - H+;
Calcium - Ca++;
Magnesium - Mg++
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| Typical proportions of Cations in Soils | Calcium - 75-85%;
Magnesium - 12-18%;
Potassium - 1-5%;
Sodium - 1%;
Others - 1%
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| 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
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| Negatively charged ions = | anions
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| Anions | Chloride - CL-;
Nitrate - NO3-;
Sulfate - SO4-2;
Borate - BO4-3;
Phosphate - H2PO4, H2PO4-2
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| 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
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| 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
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| Hydroxyl ion vs. hydrogen ion | Hydrogen causes acidity;
Hydroxyl causes basic condition (alkaline)
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| pH | negative logarithm of the hydrogen ion activity (concentration);
pH = -log[H+];
as pH decreases by one unit, acidity increases 10-fold
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| 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
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| Plant Nutrients Availability | direct effect on Fe, Mn, Ca, Zn, Mo;
Neutral pH favors mineralization from organic matter
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| Aluminum Toxicity Influence | acidity releases aluminum from soil minerals
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| Soil Applied Herbicide Application | determines if able/available to kill weeds;
determines the degradation rate
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| 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
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| Form of nitrogen fertilizer application? | ammonium/ammonium phosphate
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| heavy precipitation = | leaching of bases
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| because crops grown, | basic compounds removed by harvest
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| 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+
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| What element can be added to raise the soil pH? | lime or calcium carbonate: CaCO3
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| 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
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| Soil tests for lime? | soil pH and CEC
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| How can you manage to lower the soil pH? | apply elemental sulfur to naturally high pH soils;
high pH caused by poor drainage
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| Poor drainage: | install subsurface till drainage
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| Elemental sulfur: | soil bacteria converts to sulfuric acid;
not economical for field crops
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| Salinity | high concentration of total salts
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| What are some causes of salinity? |
Poor Drainage;
Shallow Water Table;
Seepage Zones;
Parent Materials
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| Where is salinity in soils a problem in Nebraska? | Lancaster and Saunders counties
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| What is salt? | and inorganic material/mutually charged molecule that can dissolve in water
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| How is salinity measured? | estimated by measuring the Electrical Conductivity (EC);
mmhos/cm
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| 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
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| What are the minimum requirements for saline soil reclamation? | assess problem;
make sure there's mechanisms for drainage;
have clean water supply
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| Sodicity = | high concentration of sodium (Na)
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| What changes may sodic soils (sodium) make to soil physical properties? | soil aggregates are destroyed;
causes poor drainage/crusting
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| 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
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| 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
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| 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)
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| What can an accumulation of Na+ do? | damage cell mechanisms
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| What can an accumulation of Cl- do? | inhibit photosynthesis
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