Soil Cation & Anion Exchange Capacity: Terms, Factors, & Measurement, Lab Reports of Agricultural engineering

Download Soil Cation & Anion Exchange Capacity: Terms, Factors, & Measurement and more Lab Reports Agricultural engineering in PDF only on Docsity! Cation Exchange Capacity (CEC) Terms to Know • Cation • Isomorphous Substitution • Anion • Hydroxyl • Colloid • pH – power of hydrogen – neutral pH = 7 – acidic < 7 – basic > 7 • adsorption – electrostatic attraction to the outside of the colloid (think adhere) – NOT the same as absorption! Strength of Hold & Likeliness of Adsorption • How tight an ion is held is determined by 1) Size of ion’s hydrated radius • Smaller radius = tighter hold 2) Magnitude of ion’s charge • Higher charge = tighter hold Al3+ > Ca2+ > Mg2+ > K+, NH4+ > Na+ > Li+ • How likely an ion species is to be adsorbed is determined by its concentration in the soil solution • Higher concentration = more adsorption • High concentration of one ion species relative to another ion species can supersede the effect of radius and charge Balanced by Charge colloid K+ K+ colloid Ca2+ + 2K+ + Ca2+ Charge for Charge….. NOT ion for ion Also note: This is a reversible reaction Cation Exchange Capacity (CEC) • Total amount of exchangeable cations that can be held by a given soil mass • Total CEC impacted by 1) Soil texture (amount of clay) 2) Clay type 3) Soil organic matter 4) pH Anion Exchange Capacity (AEC) • When the pH drops below 6 and clays become positively charged – Positive charge on colloids attracts anions • Colloids with AEC: oxide clays, amorphous clays, 1:1 clays – Humus generally retains a negative charge regardless of pH, but will increase its negativity at higher pH values • Allows soils to adsorb anionic nutrients – e.g. NO3- , PO43- • Allows soils to adsorb anionic toxins and keep them out of groundwater – e.g. 2,4-D Cation/anion exchange capacity (emol,,/kg) CEC/AEC and pH 4 Cation exchange sat capacity 2 } Anion exchange s capacity ! i = oO | l 4 5 6 7 Sail pH Measuring CEC/AEC • Measure charge • Units are cmolc/kg (charge/soil mass) • 2 ways to estimate CEC: – Add up charge given by each ion species – Add up charge held by each soil colloid type