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2nd most important property of a soil and indicates degree of acidity/alkalinity; influences root uptake availability of both nutrients and toxins
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
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
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
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
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+
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
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
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
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+
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
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
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
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
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
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
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
Know the formal definition of saline soils with respect to EC and SAR/ESP.
EC > 4.0 dS/m; SAR < 13 or ESP < 15
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
Know at least 2 reasons why salinity impairs plant growth.
Osmotic potential; toxic ions; plants differ in their tolerance to salt
Describe how a saline soil is reclaimed.
leach with good water
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
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
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)
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
Know the formal definition of a saline-sodic soil with respect to EC and SAR/ESP.
EC > 4; SAR > 13 or ESP > 15
Describe the general physical and chemical conditions of a saline-sodic soil.
more like saline soil since fluid exchange normal
Describe how a saline-sodic soil is reclaimed including the use of amendments.
treat to replace Na with Ca, proper irrigation with good water