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Soil pH
2nd most important property of a soil and indicates degree of acidity/alkalinity; influences root uptake availability of both nutrients and toxins
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Understand that it is the difference in concentrations of H+ and OH+ ions that causes changes in pH.
pH = -log [H+] ; [H+] * [OH-] = 10^-14
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Understand the ranges in pH of certain ecosystems
<5.5 – very acid ; between 5.5 and 6.9- mildly acid ; occur in humid regionsbetween 7.1 and 8.4- alkaline; >8.5- very alkaline; occur in dry regions
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Be able to describe and discuss the two main causes of soil acidity
- 1.production of H+ ions from the decomposition of organic debris
- 2.Leaching of basic cations; H+ preferred on exchange complex over basic cations
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Understand that it is the CEC of soil colloids that governs soil pH and how this occurs on the soil exchange complexes
effective CEC increases as pH increases; due mainly to binding and release of H+ ions on pH dependent charge sites; substantial amount of variable charge is supplied by organic matter and weathered edges of clay minerals
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Understand the differences in acidic cations (H+, Al3+, Fe3+) and non-acidic cations, how the non-acidic cations are displaced and removed via leaching
Al+3/ Fe 3+ solubilized as pH decreases, knocking basic cations off the exchange complex; reacts with water to produce H+
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Understand the role of Al3+ in increasing acidity and how the processes are effected by increased precipitation.
the greater the weathering (from precipitation and temperature), the greater the amount of Al+3 and H+ that are produced and the lower the soil pH; Al+3 has strong tendency to hydrolyze, combining with the OH- molecules and leaving the H+ to decrease pH
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Understand how soils become alkaline and why they occur in arid systems
basic soils dominate exchange complex because they have not leached out; with less vegetation, there is less productions of H+ for exchange with basic cations; weathering of nonacid cations from minerals, production of base-producing anions
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Know the three pools of soil acidity and be able to describe each-be able to expand on the ideas of how soil organic matter and texture play roles in exchangeable and reserve acidity.
- 1.active acidity: H+ measured in soil solution, determines solubility of many substances
- 2.exchangeable acidity: H+ on exchange complex; associated with exchangeable aluminum and H+ present in soils; smaller texture, more CEC for them to be present on
- 3.residual acidity: H+ and Al+3 bound in soil but can react to produce active and exchange acidity, can dissociate as pH increases; easier bound with smaller texture and higher CEC
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What is buffering capacity of soils? And what roles do the pools of soil acidity affect the buffering capacity of soil?
Ability of a soil to resist pH change; dependent on amount and type of clay and organic matter content; the amount of aluminum hydroxide on an exchange complex leads to low exchanges and more H+
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Why is buffering of soils to pH important?
- 1.ensures some stabliliy in the soil pH
- 2.influences amount of amendments required to bring about a desired change in soil pH
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What are the anthropogenic factors that influence soil acidity and alkalinity?
- 1.Acid forming fertilizers
- 2.Organic matter
- 3.Acid deposition- acid rain
- 4.Mine tailings
- 5.Irrigation practices
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What are the effects of changes in pH on plants, microorganisms, and nutrients?
Availability of nutrients through chemical binding (concns of Fe, Mn, Zn, P, N decrease as pH increases); toxicity of elements (low pH Al, Mn, Fe; high pH B, Mo); CEC reduced at low pH; bacterial activity inhibited at pH <5
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Understand the difference between an alkaline pH, a calcareous soil, a saline soil, and a sodic soil.
- Alkaline pH: >7.0; Calcareous soils have CaCO3;
- Saline soil: level of slat high enough to interfere with the growth of most plants
- Sodic soil: EC< 4.0dS/m, ESP>15
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Know how salinity develops under natural and anthropologic conditions.
- Natural: weathering of parent material with little or no leaching, accumulaiton in enclosed drainage basins, high water tables in river valleys
- Anthropogenic: irrigation with salty water, poor drainage (particularly in river valleys), poor irrigation practices
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Know how to measure soil salinity (EC) and soil sodicity (SAR or ESP) including the units.
Soil salinity: electrical conductivity bridge used to determine amount of ions in soil (dS/m)Soil sodicity: ESP = exchangeable Na/ CEC * 100; exchangeable sodiun as a percentage of total CEC
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Describe the plumbing system of a typical irrigated river valley such as the Mesilla Valley and know why it relates to salinity.
water drained from farmland increases in salinity as it goes further downstream
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Know the formal definition of saline soils with respect to EC and SAR/ESP.
EC > 4.0 dS/m; SAR < 13 or ESP < 15
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Describe the general physical and chemical conditions of a saline soil.
white alkali- accumulation of salts on surface; pH usually 7.8 – 8.2 because Na is low compared to Ca and Mg
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Know at least 2 reasons why salinity impairs plant growth.
Osmotic potential; toxic ions; plants differ in their tolerance to salt
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Describe how a saline soil is reclaimed.
leach with good water
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Know the formal definition of a sodic soil with respect to EC and SAR/ESP.
EC < 4.0 dS/m; SAR > 13 or ESP >15
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Describe the general physical and chemical conditions of a sodic soil.
black alkali because of accumulation of humic material on surface; contains too much Na- disperses colloids resulting in loss of aggregation; pH 8.5 or greater; low or no fluid exchange
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Know at least 2 reasons why sodicity impairs plant growth.
poor aeration and standing water; toxic ions can be a problem; some plants may be tolerant to poor fluid exchange and high Na (ex. Barley, Four-Wing Saltbush)
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Describe how a sodic soil is reclaimed.
Exchange Na with Ca and leach; leach with good water; growth of plants that can withstand high levels of Na- may take several years
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Know the formal definition of a saline-sodic soil with respect to EC and SAR/ESP.
EC > 4; SAR > 13 or ESP > 15
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Describe the general physical and chemical conditions of a saline-sodic soil.
more like saline soil since fluid exchange normal
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Describe how a saline-sodic soil is reclaimed including the use of amendments.
treat to replace Na with Ca, proper irrigation with good water
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