AGRI 51 Principles of Soil Science Lecture 5C PDF

AGRI 51 PRINCIPLES OF SOIL SCIENCE  Soil pH 2 Chemical Nature of Soil Constituents Soil pH Soil pH is the single most important chemical property of the soil. Soil pH is a master variable  it affects many chemical processes.  affects plan...

AGRI 51 PRINCIPLES OF SOIL SCIENCE  Soil pH 2 Chemical Nature of Soil Constituents Soil pH Soil pH is the single most important chemical property of the soil. Soil pH is a master variable  it affects many chemical processes.  affects plant nutrient availability by controlling the chemical forms of the different nutrients and influencing the chemical reactions they undergo. pH of the soil determines suitability of plant growth 3 Chemical Nature of Soil Constituents Soil pH pH is the acidity or alkalinity due to relative concentration of H+ and OH- ions; maximum value of pH is 14. [H+] is expressed in Molarity (M or moles/L) At neutral pH = 7, the concentration of H+ = OH- pH = - log [H+] pH = log 1/[H+] 4 Chemical Nature of Soil Constituents Soil pH pH = - log [H+] pH = log 1/[H+] If the H+ concentration is 0.000001 mol/L or 1.0 x 10-6 M, the pH of the soil is computed as follows: pH = - log (1.0 x 10-6 M) = - (-6) pH = 6.0 at pH 6.0, there are 10x more H+ ions than at pH 7.0 and there are 100x more H+ ions at pH 5.0 than at pH 7.0 5 MSBuladacoII [email protected] Chemical Nature of Soil Constituents Soil pH  If [H+] > [OH-], pH < 7.0 = soil is acidic  If [H+] < [OH-], pH > 7.0 = soil is basic or alkaline  If [H+] = [OH-], pH = 7.0 = soil is neutral 6 Chemical Nature of Soil Constituents Soil pH 7 Chemical Nature of Soil Constituents Soil pH Effects of Soil pH: Microbial activity - Fungi: unaffected - Bacteria and Actinomycetes: inihibited at pH 5 Availability of nutrients -N decrease at pH 15 < 8.5 > 13 sodic EC – electrical conductivity ESP – exchangeable sodium percentage SAR – sodium adsorption ratio 30 Problem Soils Salt-Affected Soils Saline Soils - Soils whose concentration of soluble salts is high enough to hamper plant growth - Salts are mostly chlorides and sulfates of Na, Ca and Mg - pH 15% of their cation exchange sites occupied by Na+ ions - pH > 8.5 - soil is highly dispersed - dispersed humus carried upwardly by capillary gives the soil black color (black alkali) 32 MSBuladacoII [email protected] Problem Soils Salt-Affected Soils Reclamation of Saline Soils - Leaching of excess salts out of the root zone (internal and surface drainage and salt disposal dump areas) - Retardation of evaporation (use of sulfate mulch) - Use of salt tolerant crops 33 Problem Soils Salt-Affected Soils Reclamation of Sodic Soils - Replacement of excess Na on the exchange sites by Ca and leaching of exchanged Na out of the root zone - Gypsum (CaSO4.2H2O) cheapest and most commonly used Na Ca + CaSO4 = + Na2SO4 Na 34  Thanks! ANY QUESTIONS? Message me. 35