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 CaCO 3 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)

Lime material

• • • • • • CaCO 3 calcic limestone CaMg(CO 3 ) 2 Dolomite CaO: Quick lime CaOH calcium hydroxide Byproducts: ground shells, cement factory waste Consume H + and provide an alternative cation for the exchange phase (Ca or Mg)

Liming to increase soil pH

• Lime characteristics   cost purity   speed of effect (fine ground vs coarse) ease of handling • Lime requirement  depends on

pH

,

CEC

and

buffer capacity

of the soil • Lime Application: small amounts split and incorporated into the soil

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 http://wwwlb.aub.edu.lb/~webeco/SIM215acidsoilsandlimimg_files/image002.gif

• 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

Sources of Soil Salinity

Natural causes: • Weathering of parent material with little or noleaching • 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)

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 EC

Conductivity 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

Water moves from regions of higher water potential (pure water = 0) to regions of lower water potential (saline water = -x) across a semi-permeable membrane (e.g., plant roots) http://www.genomestudy.com/BIO196/Lab4/osmosis.gif

Atm = -20,000 kPa Leaf = -500 kPa Root = -70 kPa Soil = -50 kPa http://www.uic.edu/classes/bios/bios100/lecturesf04am/changesinwaterpotential.jpg

Sodium Hazard – dispersion and Na toxicity

ESP > 15% Dispersed soil Na saturated ESP < 15% Flocculated soil Good soil structure Ca saturated

Sodicity Measurement • Sodium Adsorption Ratio = “SAR” SAR = [Ca +2 [Na + ] + Mg +2 ] ½ units = mmol/L SAR = [Na + ] [(Ca +2 + Mg +2 ) / 2] ½ units = mmol c /L (old units = meq/L) The concentration of cations in the soil saturated paste extract (

solution phase

)

Sodicity Measurement • Exchangeable sodium as a percent of the total CEC = “ESP” ESP = exchangeable Na X 100 CEC units = cmol c /kg soil (old units = meq/100g) The concentration of cations on the soil

exchange phase

Low ESP High ESP http://www.terragis.bees.unsw.edu.au/terraGIS_soil/images/ec-ncps-soil_solution-4.jpg.jpg

Nomogram for estimating ESP to/from SAR (Handbook 60, U.S. Salinity Lab, 1954)

Saline and Sodic Soils Halophytes are plants which tolerate or even demand sodium chloride concentrations in the soil water they absorb.

Saline Soils • Most common salt problem and the easiest to correct • EC > 4.0 dS/m; SAR < 13 or ESP < 15 • May be called ‘white alkali’ because of the accumulation of salts on the surface • Typical ions: Ca +2 , Mg +2 , K + , Na + ; SO 4 -2 , Cl , HCO 3 -

Soil Chemistry of Saline Soils • pH is usually 7.8 - 8.2 but can also be acidic Soil Physical Condition • Soil physical condition is generally good (well aggregated with good internal fluid movement) • Crusting may be a problem

Plant Growth Problems • Osmotic potential contributes significantly to total water potential; inability of plant to extract water is the major plant growth problem on saline soils. • Toxic ions can be a problem (Na + , Cl , HCO 3 ) • Plants differ in their tolerance to salt

Increasing NaCl concentration  http://www.isb.vt.edu/news/2001/news01.dec.html

Chile pepper response to salinity

Sodic Soils • Less common problem and much more expensive to correct. • EC < 4.0 dS/m; SAR > 13; ESP >15 • May be called black alkali because of the accumulation of humic material (black color) on the surface (Na causes organic matter to disperse) • Too much Na is the problem • Typical ions are Na + , Ca +2 , Mg +2 , K + HCO 3 , CO 3 -2 ; Cl , SO 4 -2 ,

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 HCO 3 ) 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 Na 2 SO 4 .

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. CaSO 4 , S o , H 2 SO 4 The H 2 SO 4 the S o dissolves CaCO 3 are used. in the soil to produce Ca +2 and is converted into H 2 SO 4 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, H 4 BO 4 , HCO 3 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 = EC iw /EC dw EC iw is EC of irrigation water EC dw is maximum acceptable salinity of the soil solution Example: if EC of irrigation water is 2.5 dS/m and crop can tolerate an EC of 6 dS/m. What is LR?

LR = 2.5/6 = 0.4

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