Unit 11: Acidic Soils & SaltAffected Soils
Chapter 8
Objectives
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Impacts/Causes/Effects of soil acidity
& salinity
Action of lime in the soil & products
available
Application methods for lime
Reclaiming & managing salt-affected
soils
Introduction
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Many adverse affects from acidic &
saline soils
Some research says ¾ of humidregion soils need lime
~ 2.5 b ac affected by saline conditions
Salinity can occur from various
reasons, various regions
Salinity much harder to manage than
acidity
Why Some Soils Are Acidic
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Most soils become acidic due to
leaching
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Soil cations leached through soil profile
Favorable soil cations replaced by Al on
CEC sites when pH < 4.7 – Al toxic to
most plants
Areas receiving >30” rain/yr high risk for
acidity, if not managed
Why Some Soils Are Acidic
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Must have centuries of leaching of
cations to acidify naturally
Most acidic soils in the U.S. found:
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East of Mississippi River
Pacific coastal soils
Mountain areas
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Avg 35” rain/yr – soil pH’s 5-6
Ecological Relation of Soil
Acidity
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Acidic soils usually leached
Strongly acidic soils have:
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Few basic cations (Ca, K, Mg, etc.)
available for absorption
High amounts of Al, Mn, etc.
Low contents of micros
Toxic levels of Al, Mn
Severely slowed microbial process & N
fixation
Ecological Relation of Soil
Acidity
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Acid-tolerant plants have adapted to
these conditions well
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Don’t require high levels of nutrients
Able to lock up Al
Composition of Lime
Lime standard treatment for acidic soils
Liming materials:
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Calcic Limestone (Ag Lime) – fine ground
Dolomitic Limestone – lime w/ Mg
Quicklime – burned limestone
Hydrated Lime – reaction w/ water to
hydroxide form
Marl – lime from bottom of freshwater
ponds
Composition of Lime
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Chalk – soft limestone from ocean
deposits
Blast surface slag – byproduct of iron
industry – has higher P content
Ground oystershell, wood ash – from
paper mill, sugar beet plants, fly ash, etc.
Fluid lime – suspension containing any
form of usable lime
Gypsum – not lime, but does supply Ca,
can help alleviate Al toxicity
Composition of Lime
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Chemical Guarantees of Lime
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Limestone seldom pure calcium
carbonate
More impurities, lower level of true
CaCO3 available
Lime purities can be expressed w/ a
CaCO3 equivalent – ex. 85%
Composition of Lime
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Reactivity of Lime
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Neutralizing power of lime determined by
rate of solubility of the material used
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Different forms more/less soluble
Fineness of grind also has great affect
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Why?
Reactions of Lime Added to
Acidic Soils
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Addition of lime to an acidic soil
eliminates two major (among others)
problems
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Excess soluble Al (toxic levels)
Slow microbial action
Other benefits to liming
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Raised pH reduces excess soluble Mn, &
Fe
Reactions of Lime Added to
Acidic Soils
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Ca & Mg (deficient in many acid soils)
can be added in one operation w/
Dolomitic lime
Increases availability of P
Makes K usage more efficient
Increases N availability by promoting
microbe growth, decomposition of
organic matter
Increases plant-available Mo
Keeping pH above 6.5 reduces solubility
of heavy (toxic) metals
http://www.spectrumanalytic.com/suppor
t/library/ff/Soil_Aluminum_and_test_inter
pretation.htm
Crops, Lime, & Soil
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How Much Lime to Apply?
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Soluble & exchangeable acidity need to
be neutralized to change pH
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Especially exchangeable
Acid tolerance
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Least: alfalfa, sweet clover
Low: corn, wheat
Moderate: oats, strawberries
High: blueberry, Lespedeza
Crops, Lime, & Soil
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Increased levels of clay/organic matter,
increase amount of lime needed to
change pH
Our soils typically <10% organic matter –
our target pH should be ~6.5
Soil nutrients more/less available at
varying pH’s
Crops, Lime, & Soil
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Methods of Applying Lime
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Most effective – apply lime each year
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How many do?
More common – add lime when needed
in large enough amounts to justify cost
Definitely should apply 4-12 mos before a
legume seeding, or few mos before high
value crop planting
Crops, Lime, & Soil
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Surface applied
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Most effective if incorporated
Liming No-till Fields
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No-till fields:
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Microbial action is much shallower
Acid layer at/near surface
Typical build-up of fertilizers near surface
(top 1-2”)
Liming raises that shallow soil pH,
increases effectiveness of fertilizers &
chemicals
Crops, Lime, & Soil
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Lime Balance Sheet
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Ammonium fertilizers may neutralize
100# of lime/yr
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N fertilization most common reason for soil
acidification in cropping soils
Can have ~330-500#/ac lost lime each
year
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Calls for 1 t/ac addition of lime every 5 yrs (in
addition to lime needed to neutralize N
fertilization)
Acidifying Soils
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If growing crops preferring acid soils
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Use fertilizers w/:
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S, Fe, Al compounds, sulfuric acid
Seldom attempted to acidify a soil, unless
for specific production purpose
Soluble Salts & Plant Growth
Excess salt kills growing plants
High salt levels can render a soil
unproductive for decades, centuries
 Soluble Salts
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Not restricted to table salt – many
different salts can be formed due to
chemicals available
In some soils, salt concentrations higher
than seawtaer (>3-4% total salt)
Soluble Salts & Plant Growth
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Irrigation can speed a soil salt problem:
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All irrigation water contains salt
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If a farmer adds 4” of water w/ 1000mg salt/L adds
890 lb/ac salt/yr
Raises naturally salty groundwater level
closer to surface
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Groundwater can rise to surface through capillary
action & evaporate – leaving salt behind
Soluble Salts & Plant Growth
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Measuring Soluble Salts
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Electrical Conductivity (EC) – conductivity
directly proportional to salt concentration
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Higher EC reading = more electricity
conducted = higher soil salinity
Effects of Salt Concentration
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Usually, just reduce plant growth due to
osmotic effect – interferes w/ plant’s
ability to extract soil water
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High saline soils can actually rod water back
from plant roots
Soluble Salts & Plant Growth
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Plants have varying tolerance to soil salts
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Not all affected at same time/same way
Effects of Specific Ions
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Na & Cl can be toxic to woody
ornamentals & fruit crops
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Some plants can be injured by <5%
exchangeable Na for some fruits, other
woody ornamentals <.5% Cl & .25% Na
Soluble Salts & Plant Growth
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Salt-Affected Soil Classification
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Saline Soils
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Enough salt at some position w/in the root
zone to interfere w/ plant growth
Causes:
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Unleached products
Salty irrigation water
Upward movement of groundwater
Soluble Salts & Plant Growth
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Sodic Soils
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Salt imbalance caused by Na is the dominant
cation, rather than Ca
Water infiltration problems
Toxic levels of Na
pH >8.5
Causes:
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Irrigation water
Weathering of parent materials
Upward migration of salty groundwater
Contamination from oil/gas well production
Soluble Salts & Plant Growth
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Can have a sodic horizon
Saline-Sodic Soils
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High in salinity & high in Na
Affect plants by osmotic effect & toxicity of Na
Good water infiltration
pH <8.5
Attempts to improve condition by leaching
results in sodic soil
Salt Balance
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23% of world’s cultivated land saline
39% sodic
Australia – many soil salinity problems
Irrigation & land clearing – primary
causes
Salt buildup existing/potential hazard
on 42m ac of irrigated land in U.S.
Salt Balance
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Salt balance – outgoing salt =
incoming salt
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Managed leaching to help wash away
any salt buildups
May call for a leaching requirement to
remedy & keep crops productive
Reclaiming Salty Soils
3 Rules:
1.
Establish internal drainage
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2.
Replace excess exchangeable Na
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3.
If not already adequate
May require tile installation, ditching
Can be impractical/costly
Necessary for sodic & saline-sodic soils
Extent varies w/ soil texture, clay, quality of available
water extent of damage
Leach out most of soluble salts
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Especially in root zone
Use good quality irrigation water
Reclaiming Salty Soils
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Reclaiming Saline Soils
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Can be easy, if:
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Low-salt irrigation water is available
Internal & surface drainage is adequate
Disposal areas for salt available
Difficult when:
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High water table
Fine-textured soils
Reclaiming Salty Soils
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Add organic mulch – slows movement of
water to the soil surface
Quantity of water required to help leach:
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Depends on depth needed to leach
% of salts to be removed
How its done (constant/intermittent sprinkling)
Reclaiming Salty Soils
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Reclaiming Sodic & Saline-Sodic Soils
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Sodic soils
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Downward movement of water can’t leach out
excess Na
Must first replace Na on CEC sites
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Use gypsum
Can then leach out excess Na
Can also use S to reduce soil pH
Managing Salty Soils
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Water Control
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Maintain high water content in soil
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Keeps salts diluted
Plants more able to tolerate higher salt levels
Leach soil before planting to move salts
below root zone in early plant
development
Managing Salty Soils
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Planting Position
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Salt moves w/ water
Plant on side of ridges where salt buildup may be avoided
Use sprinkler irrigation to keep salt
washed into soil profile
Managing Salty Soils
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Saline Seeps
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Changing topography of soil to create a
low point where water (w/ dissolved salts)
can seep out of soil & be collected
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Add plantings to help utilize the water
Assignment