5. Buffering capacity • Soils high in SOM and clay minerals are more resistant to change in pH • Sandy soils and highly weathered soils are least buffered • Base Saturation = exchangeable bases CEC BS = (exch Ca + Mg + Na + K) (exch Ca + Mg + Na + K + Al + H)
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5. Buffering capacity Soils high in SOM and clay minerals are more resistant to change in pH Sandy soils and highly weathered soils are least buffered.
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5. Buffering capacity
• Soils high in SOM and clay minerals are more resistant to change in pH
• Sandy soils and highly weathered soils are least buffered
• Base Saturation = exchangeable bases CEC
BS = (exch Ca + Mg + Na + K)
(exch Ca + Mg + Na + K + Al + H)
6. Lime Requirement
• Amount of CaCO3 needed to increase the pH of the soil to an optimum pH
• Depends on soil mineralogy, % clay fraction, % OM, cultivation practices (leaching, fertilization, etc)
• Variety of liming materials
• Only practical to raise pH to ~6 (KCl-extractable acidity is ~0)
To increase pH in a well-buffered soil requires much more lime than in a sandy or weathered soil; more lime required to go from 6 to 7 than
from 4-5
• a major threat to agricultural productivity in arid regions
• One-third of the world’s irrigated land is salinized
• More than one million hectares affected
• Salts cause both osmotic effects and specific ion toxicity
Natural causes:
• Weathering of parent material with little or no
leaching
• more salinity in hot, dry regions (climate + irrigation)
• Accumulation of salts in enclosed drainage basins
• Coastal spray and inundation
• High water tables (capillary rise brings salts to the
surface)
Sources of Soil Salinity
Anthropomorphic causes of Salinity
• Irrigation Not just with poor quality water Inadequate leaching and drainage
• Acid rain (enhances weathering; salt production)
• Application of fertilizers, manures, biosolids, composts which are often saline
Salt-impacted agricultural soils
Measurement of Salinity• Electrical Conductivity (EC) is an measure of the flow of electricity
through a material • Saline soils and salty water conduct more electricity than nonsaline
soils or pure water. It is the ions that pass or conduct electricity from one ion to the next. As salt concentration increases, EC increases. Acidic or low pH solutions also exhibit high EC Expressed in dS/m (SI units) or mmhos/cm (old unit) dS/m = mmhos/cm
• Use an EC ‘bridge’ or meter to measure how well water extracted from soil can conduct electricity:
Dissolved ions and two metal plates
Voltage is applied & ions move toward oppositely charged plates
EC values for common waters (dS/m)
• Deionized water: 0.0005 to 0.002
• NMSU tap water: 0.5 to 1.0 (rarely this high)
• Seawater: 40 to 55
• Good irrigation water: < 0.7 Rio Grande N of Las Cruces is good Quality decreases (EC increases) downstream
• Poor quality irrigation water: > 3
• Saturated Paste Extract EC of saline soils: ≥ 4
Relate I and EC
• Ionic strength is a parameter that estimates the interaction between ions in solution.
• Salts are ionic solids that dissolve in water
• Empirical relationships: I = 0.0127 EC (in arid and semi-arid regions) I = 0.014 EC (in humid regions) Easier to calculate because you don’t need full
composition of solution
Instruments to measure ECConductivity meter Electromagnetic induction
Time Domain Reflectometry
Measurement of Salinity – TDS
TDS – Total dissolved solids Cations + anions + anything <2 microns Good quality water has <500 mg/L or ppm TDS measure using gravimetry or EC
• Evaporate water off and accurately weigh the residue• Problematic due to hydration and volatilization
EC (dS/m) x 640 ≈ TDS (mg/L) • TDS ‘meters’ are really EC meters with conversion factor
Osmotic potential (OP)• That portion of the Total Soil Water Potential due to
the presence of solutes in soil water• Salts reduce the water potential by inhibiting the
movement of water molecules• OP (kPa) ≈ -0.40 x EC (dS/m)
Pure Water OP = 0
Water Diluted by a Solute (Red Spheres) OP is negative
Soil Chemistry of Sodic Soils • pH is usually 8.5 or greater because Na is high (Na is a
strong base-forming cation)
Soil Physical Condition • Soil physical condition is poor (Na disperses the colloids
resulting in the loss of aggregation) • Very slow or no fluid exchange
Plant Growth Problems
• Poor aeration and standing water
• Toxic ions (Na+, Cl-, and HCO3-) can be a problem
• Some plants may be tolerant to poor fluid exchange and high Na
Saline-Sodic Soil
• EC > 4; SAR > 13 ESP > 15
• Combination of problems found in saline and sodic soils
• Soil physical condition is more like a saline soil in that drainage is normal
15 year old pecans south of Las Cruces that are stunted by sodium
This saline-sodic soil near Vado is one of the worst in the Mesilla Valley. A heavy clay layer keeps water from freely draining.
The SAR of this soil is about 25 and the EC is about 15 dS/m.
The white salt is mainly NaCl and Na2SO4.
Reclamation of Saline and Sodic Soils
Saline Soils• Leach with good water • The leaching requirement (LR) can be used
Sodic Soils
• Exchange Na with Ca and leach. CaSO4, So, H2SO4 are used. The H2SO4 dissolves CaCO3 in the soil to produce Ca+2 and the So is converted into H2SO4 in the soil by microorganisms.
• Leach with good water• Growth of plants (barley, triticale, halophytes) that can
withstand poor aeration and high levels of Na. • Can take several years.
Problems caused by Salinity and Sodicity
• Osmotic effects: by lowering the osmotic potential and making it difficult for plant to extract water
• Specific Ion effect: Na, Cl, H4BO4, HCO3 can be toxic and can cause imbalances in the uptake and utilization of other cations
• Soil structure deteriorates and aeration decreases• Plants get stunted and exhibit small dark bluish green
leaves
Leaching Requirement:
Amount of water needed to remove excess salts from saline soils
LR = ECiw/ECdw
ECiwis EC of irrigation water
ECdw is maximum acceptable salinity of the soil solution
If root zone needs 15 cm of water to be fully wetted, then amount of water to be leached = 15*0.4= 6 cm
So supply 15 + 6 or 21 cm of water total to irrigate and leach
Example: if EC of irrigation water is 2.5 dS/m and crop can tolerate an EC of 6 dS/m. What is LR?