Soil pH, Salinity, and Lab Procedures

• Soil pH measures acidity or alkalinity on a logarithmic scale

Soil pH Values

• pH 7 is neutral

• Values below 7 are acidic

• Values above 7 are alkaline

• Soil salinity is the salt content of the soil

Laboratory Procedures for Soil Analysis

• pH H2O (actual or active pH)

  • Mix 10 g soil with 20 ml H2O, do not shake
  • Use a 50 ml centrifuge tube, without a cap

• pH KCl (potential pH)

  • Mix 10 g soil with 25 ml KCl, shake for 30 minutes
  • Use a 100 ml oval flask

• Salinity (electrical conductivity)

  • Mix 10 g soil with 50 ml ethanol, shake for 10 minutes
  • Use a 100 ml rectangular flask

Lab Rules for Using Centrifuges

• Ensure the centrifuge is balanced
• With an even number of samples, place them symmetrically around the axis
• For an odd number of samples, balance by adding water to a centrifuge tube

Understanding Sorption

• Sorption is the removal of a compound from a solution to a solid phase
• Adsorption is the accumulation of chemicals at the solid-liquid interface
• Absorption is the accumulation of molecules within existing solids

Soil Acidity/Alkalinity and Ion Balance

• Soil solution acidity/alkalinity depends on the balance between H+ and OH- ions
• This balance is dictated by acid and non-acid cations on colloidal surfaces

Soil pH Variability and Effects

• Natural factors like parent rock, topography, time, and weathering processes influence soil pH
• Soil pH determines the solubility of nutrients and chemicals
• Nutrient availability varies, with some nutrients more available in acidic conditions and others in alkaline conditions
• Most mineral nutrients are available when soil pH is near neutral, between 5.5 to 7.5

Impact of Soil pH on Plant Nutrients

• Strongly acidic soils can cause aluminum/manganese toxicity and deficiencies in calcium/magnesium and essential nutrients like phosphorus and molybdenum
• Contributors to acidic soils include high levels of ammonium-based nitrogen fertilizers and leaching of nitrate nitrogen
• Harvesting plant materials can increase soil acidity

Soil pH for Various Plants

• Apples, basil, carrots, cauliflower, corn, cucumber, eggplant, garlic, melon, pumpkin, pepper, radish, and sweet potato prefer a pH near 6 -7
• Raspberry, blueberry, blackberry, cranberry, peanuts, and potatoes prefer a pH near 4-5
• Artichoke, arugula, asparagus, broccoli, cabbage, cauliflower, celery, horseradish, mustard, onion, and lettuce prefer a pH between 6 -7

Modifying Soil pH

• Soil pH can be lowered in alkaline soils by using sulfur/ammonium-based fertilizers
• Lime or dolomite can be used to increase soil pH in acidic soils
• Adding organic matter or clay can make soil more resistant to pH changes

Methodology for Measuring Soil pH

• Actual pH (pH H2O)

  • Weigh 10 g of soil and mix with 20 ml distilled water
  • Mix the soil and water for 5 minutes
  • Measure the pHH2O value using a pH-meter in soil suspension

• Potential pH (pH KCI)

  • Weigh 10 g of soil and mix with 25 ml 1M KCl
  • Shake the soil for 30 minutes
  • Measure the pHKCl value using a pH-meter

Soil Salinity Impact

• Soil salinity involves the accumulation of soluble salts
• It contributes to land degradation, affecting soil fertility
• It affects global biogeochemical cycles, nutrient cycling, and global fluxes
• It modifies soil electrical conductivity, impacting soil microorganisms

Causes of Soil Salinity

• Primary causes:

  • Primary minerals (rock weathering)
  • Fluctuating groundwater depth
  • Evaporation exceeding rainfall
  • Coastal soils

• Secondary causes:

  • Irrigation waters containing soluble salts
  • Chemical fertilizers

Soil Salinity Effect on Pant Growth

• Osmotic effects reduce water availability to crops
• Specific ion effects can lead to toxic levels of sodium, chloride, and boron

Saline vs Sodic Soils

• Saline soils:
  • High levels of soluble salts (Ca, Mg, etc.)
  • Electrical conductivity > 4 dS/m
  • pH < 8.5
  • Reduced water uptake by plants
  • Disturbed soil respiration and nitrogen cycle
• Sodic soils:
  • High amount of sodium
  • Low levels of neutral soluble salts (EC < 4.0 dS/m)
  • pH values > 8.5, sometimes reaching 10 or higher
  • Reduced soil infiltration and increased erosion

Soil Salinity Management Strategies

• Establish drainage to remove salty water
• Leach soils with water to remove salts
• Apply scraping, flushing, mulching, and deep tillage techniques

Methodology to measure Soil Salinity

• Weigh 10 g of soil into plastic flask
• Add 50 ml of 50% ethanol
• Let shake for 10 min
• Measure soil ethanol extract conductivity (by conductometer)

Evaluation of Ethanol Extract Conductivity:

• < 30 µS/cm: Indicates most agricultural soils with normal fertilization and low soil salts
• 30 - 60 µS/cm: Denotes mineral-rich soil with moderate to high intensity fertilizing and liming without negative effects of salinity
• 60 - 120 µS/cm: Suggests high fertilization and increased soil salts in mineral-rich or acidic soils without adverse effects
• > 120 µS/cm: Signifies high soil salt load with potential negative impacts on plant growth, especially in drought conditions