Slide 1 Soil Reaction: pH Chemical Properties of Soil Slide 2 Soil pH (Reaction) pH is a measure of the Hydrogen ion concentration of the soil solution Slide 3 Soil Reaction (pH) Water DISSOCIATES into H+ and OH- ions In pure water, these concentrations are EQUAL (neutral) At neutral the pH is 7 pH LESS THAN 7 indicated a greater concentration of the H+ (acid) pH GREATER THAN 7 there is a higher concentration of OH- (alkaline/basic) Number is negative logarithm of hydrogen concentration pH of 4 is a hydrogen concentration of 0.0001 mol/liter Slide 4 Soil pH is a MASTER VARIABLE that affects soil chemical and biological properties Schill Slide 5 pH is correlated with: the SOLUBILITY of nutrient compounds and hence, their availability to plants Slide 6 pH is correlated with: organism populations and the levels of their activity the rate and degree of organic matter decomposition Slide 7 pH is correlated with: nitrogen mineralization (the nitrogen in organic matter is converted into inorganic material: ammonium NH4+ Slide 8 pH is correlated with: base saturation: the extent to which the cation exchange sites are saturated with cations other than H+ Slide 9 Cations on Clay Surfaces the proportion of bases to the TEC (Total Cation Exchange Capacity) is termed BASE SATURATION this is expressed as a % the GREATER the base saturation the HIGHER the pH value (more alkaline) Slide 10 pH of 7 cannot assume that a pH of 7 infers 50% base saturation the PROPORTION of bases to hydrogen for a pH of 7 will vary from one soil to another The pH at a given base saturation is also dependent on: the kind of clay present the adsorbed cations on the clay Slide 11 Alkaline Soils Soils will become alkaline if… they are over limed They are irrigated with alkaline water The parent material is calcium rich CaCO3 and MgCO3 develop (in arid conditions) Slide 12 Acid Soils Soils become acidic for one of 4 reasons Rainfall and leaching of base cations Organic matter decay resulting in organic acids Harvest of crops Acidic parent material Slide 13 Factors that Lower pH Lower pH results from the formation of carbonic acid from the dissolved carbon dioxide in soil water Slide 14 Factors that Lower pH the deposition of sulfur compounds through “acid rain” Slide 15 Plant Nutrients Primary Macro Nutrients Nitrogen NH4+, NO3 - Phosphorous H2PO4-, HPO4 -2 Potassium K+ Secondary Macro Nutrients Calcium Ca +2 Magnesium Mg +2 Sulfur SO4-2 Micro Nutrients Iron Fe +2, Fe +3 Copper Cu+2 Zinc Zn +2 Manganese Mn +2 Molybdenum MoO4 -2 Boron BO3-3,H3BO3 Chlorine Cl - Slide 16 Significance of Soil Reaction(pH) AVAILABILITY OF NUTRIENTS The most desirable pH for maximum solubility (availability) is about 6.5 to 7.5 Slide 17 Significance of Soil Reaction(pH) AVAILABILITY OF NUTRIENTS as pH increases the availability of calcium, magnesium, and potassium in the soil solution increases. Slide 18 Significance of Soil Reaction(pH) AVAILABILITY OF NUTRIENTS Nitrogen and Sulfur reach a maximum availability above about 5.5 and nitrogen starts to lose availability at a high pH. Slide 19 Significance of Soil Reaction(pH) AVAILABILITY OF NUTRIENTS Micronutrient most soluble at lower pH values less soluble at higher pH ( except Molybdenum ) Slide 20 Significance of Soil Reaction(pH) AVAILABILITY OF NUTRIENTS Phosphorus gets tied up with calcium in soils about 8-8.4 At 8.5, where sodium is the dominant cation it is available again. Loses availability below 5.5 Slide 21 Significance of Soil Reaction(pH) SOIL ORGANISMS AND THEIR ACTIVITIES Soil pH near 7 is best for the widest range of soil microorganisms Bacteria, algae, fungi and actinomycetes are all quite active Slide 22 Significance of Soil Reaction(pH) SOIL ORGANISMS AND THEIR ACTIVITIES As pH DECREASES the fungi increase in dominance as the bacteria and actinomycetes decrease in abundance and activity Result: decrease in organic matter decomposition Slide 23 Significance of Soil Reaction(pH) SOIL ORGANISMS AND THEIR ACTIVITIES Nitrifying bacteria are also inhibited when the pH is less than 5.5 Slide 24 Significance of Soil Reaction(pH) Mycorrhizal Fungi will thrive in the pH most suitable to their host plant For many that is a more acidic pH Slide 25 Pools of Soil Acidity Active Acidity H+ activity in the soil SOLUTION exchangeable on the colloids (clay & organic matter) have immediate effect on soil pH H+ H+ H+ H+ H+ H+ H+ Shazral Slide 26 Pools of Soil Acidity Reserve Acidity H+ is BOUND by organic matter and clays like these barnacles to rock Slide 27 Buffering Capacity Ability to resist a change in pH ensures stability in the soil pH prevents drastic fluctuations in pH It takes less material to change the pH in a sandy soil Slide 28 Alkaline Soils usually contain calcium and magnesium carbonates above pH of 8.4, usually sodium carbonates and bicarbonates The carbonates must be dissolved if alkaline soil is to be made neutral Slide 29 Acidifying the Soil Many soils have few exchangeable hydrogen and much exchangeable Calcium, Magnesium and sometimes Sodium Slide 30 Acidifying Materials Sulfur is the cheapest and most commonly used material Slide 31 2S + 3O2 + 2H2O 2H2So4 4H++ 2SO42- Step 1: microorganisms convert S into Sulfuric Acid Step 2: this then combines with the carbonate materials H2SO4 + CaCO3 CaSO4 + H2O +CO2 Step 3: when all the carbonate materials have been dissolved the Sulfuric acid can thenreact with cations on the clay and OM surfaces colloid = Ca+2+H2SO4 colloid =H+ CaSo4 It takes about 6 to 8 weeks for sulfur to act during the summer Slide 32 Altering pH To lower pH apply: Sulfur Organic Matter Slide 33 Acidifying Materials Peat - 1 m3 has the same acidifying power as 300-650g of Sulfur Fertilizers - use acidifying fertilizers when you need to add nutrients (not to just alter the pH -if you add too much you will have salinity problems and possible toxicities) Slide 34 Acidifying Materials Iron (ferrous) Sulfate don’t use it regularly though as the IRON will reduce the availability of Phosphorus Slide 35 Ammonium fertilizers will acidify soil through the following reaction: NH4+ + 3O2 2N2O- + 2H2O +4H+ 2N2O- + O2 2NO3 Ammonium based fertilizers will not change pH of alkaline soil for many years Slide 36 Altering pH To raise pH add Lime Lime contains Calcium and Magnesium carbonates the rate of application depends on a soil’s CEC, the texture and the initial pH Slide 37 Raising the pH: Liming can use calcium carbonate the more finely ground, the more rapid the pH changes Slide 38 Origin of Carbonates In most cases the parent material constitutes the initial source of carbonates,either because they already existed there, or because although they were not originally present, the carbonates have formed in the soil as a result of the weathering of original minerals rich in calcium Slide 39 Test for Carbonates Soils with Calcium Carbonates (calcareous material) will effervesce (fizz) when a few drops of 10%HCl is added to the soil. Slide 40 We often plant in this calcareous layer Slide 41 We often plant in this calcareous layer