css unit 3 part 3 soil pH and fertility

Significance of Soil pH​

- Nutrient Availability and pH.​ As raise pH (acid to neutral)​ increase availability of N, P, K, S, Ca, Mg.​ decrease availability of Fe, Mn, B, Cu, Zn.​ ​ - Optimum pH near​ 6.5 for nutrient ​Availability.​ - Soil Organisms.​ - Greater Fungi and less Bacteria in highly acid soils.​ - pH requirement of some disease organisms is used as a management practice to control disease.​ - Best known is Potato Scab controlled in acid soils. ​ - Damping-off disease in nurseries controlled by maintaining soil pH at 5.5 or less.​ - Earthworms are inhibited by high soil acidity.​ - Toxicities in Acid Soils.​ - Fe and Mn are most available in highly acid soils. Mn toxicity may occur when pH is 4.5 or less.​ - Al solubility high at low pH and can restrict root growth.​ - Preference of Plants.​ - Most field and vegetable crops prefer pH of 6.0 or greater.​ - Plants that prefer pH of 5 or less.​ Blueberries, azaleas, orchids, sphagnum moss, jack pine, black spruce, and cranberry.​ - Species have evolved for all natural soils.​ pH from 4 to about 8.5 in MI.​

Management of Soil pH​

- Two approaches to assure plants will grow without serious inhibition from unfavorable soil pH.​ - Select plants that will grow well at the existing pH​ - pH of the soil can be altered to suit the needs of the plants.​ - Most soil pH changes are done to reduce acidity and increase the pH by liming.​

Management of Soil pH​

- Benefits and Adverse effects of raising pH.​ Benefits​ Increase macronutrient availability ​ Supply Ca and/or Mg through liming.​ Increase Base Saturation and reduce Acid Saturation​ Adverse​ Decrease micronutrient availability​ Particularly Fe, Zn, and Mn​ Many times micronutrients routinely added because of lower availability. ​ pH alteration​ Reduce by adding sulfur ​ Increase by adding liming material - OH-​

example

2 Soils each at pH = 5.5 and increase to 7.0​ Soil A - CEC = 2 cmol(+)/Kg soil - 50% BS​ 1 cmol(+)/Kg soil bases​ 1 cmol(+)/Kg soil acid ​ Need to increase BS from 50% to 85%, or add .7 cmol(+)/Kg soil of lime to equal 1.7 cmol(+) bases.​ MW of Calcite = 100 g/M, 1 g/cmol, 0.5g/cmol(+)​ So, 0.7 cmol(+)/Kg soil x 0.5 g/cmol(+) = .35g Calcite/Kg​ That equates to 700 lbs. lime per acre to increase pH from 5.5 to 7.0.​ ​ ​

factors controlling soil pH

A couple of factors controlling soil pH:​ Parent Material​ Climate and Leaching​ N fertilizers​ Management of soil​

Management of Soil pH​

Lime Requirement​ Agricultural lime is a soil amendment containing calcium carbonate (CaCO3), magnesium carbonate, and other materials, which are used to neutralize soil acidity and furnish Ca and Mg for plant growth.​ Calcitic lime - CaCO3​ Dolomitic lime - CaMg(CO3)2​ The amount of liming material required is dependent on initial pH and the amount of reserve acidity in the soil. Soils with the largest CEC's offer the greatest resistance to change in pH and are the most highly buffered.​ ​

Liming Materials​

Most commonly used are the calcitic limestone's, but others exist.​ Neutralizing Value: ability of other materials to neutralize acidity on the basis that CaCO3 = 100%. Also relates to the purity of the natural material.​

Considerations in using lime​

Purity and neutralizing value.​ Particle-Size of the material.​ Too large and very slow to react, too fine and dusty.​

Reserve Acidity​

Soil testing is done to determine the amount of reserve acidity and amount of liming material is needed to increase pH to the desired level.​

pH versus H and Base Saturation

equation shows the relationship between soil pH and H and base saturation for Michigan conditions. pH =(187 - 0.3 (CEC) - %H saturation)/24 example: •Assuming that we have a soil with a CEC of 13 cmol(+)/kg, if the CEC is 100% saturated with H, the lowest pH possible is 3.46. pH = (187 - 0.3 (13) - 100)/24 = 3.46 or 3.5 •We can also use this equation to determine the percent H and base saturation if we know what the pH and CEC values are. that finds percent hydrogen saturated, and since we know CEC must be 100% saturated with either H and/or bases, you take the H saturation and subtract that from 100% to find the percent base saturated

AT QUIZ QUESTIONS

pH is a measurement of a soil acidity. TRUE The least leached horizon in an Alfisol like the Miami loam is the C horizon. TRUE A pH of 7.0 indicates that the soil reaction is: NEUTRAL Using the pH equation, determine the pH of a soil with the following properties:%H saturation: 38%CEC: 16 cmol(+)/kg EQUALS 6.01 As soil pH increases from 5.0 to 7.0, the availability of micro nutrients decreases. TRUE Decreasing the soil pH from 7.0 to 6.0 results in an increase in OH- ions by a factor of 10. FALSE What is the color of the indicator dye when the pH is equal to 9.0? PURPLE As soil pH increases from 5.0 to 7.0, the availability of macro nutrients increases. TRUE In pure water at a pH of 7.0, the concentration of OH- ions is greater than H+. FALSE Hydroxyl (OH-) is a monovalent cation. FALSE For a quick analysis of pH, an indicator dye is used. TRUE As soil pH decreases from 7.0 to 5.0, the availability of micro nutrients decreases. FALSE

pH meter

•A more accurate method for determining soil pH is the pH meter. •A small amount of soil is weighed, distilled water is added, and the sample is stirred. •A pH electrode is then inserted and the pH of the soil-water suspension is determined using a pH meter. •To determine soil pH with a pH meter, one part of soil is mixed with two parts of distilled water or a neutral salt solution. •The soil-water suspension is mixed periodically over a 30 minute period to allow the soil and the water to reach an equilibrium condition and then the pH of the suspension is measured with a pH meter.

Neutral Soils

•A soil is considered to be neutral if the pH is 7.0 or in the range of 6.6 to 7.3. •Soils with a neutral pH range include some saline soils and some leached soils containing little if any carbonates or soluble salts. •The cation exchange sites in soils are essentially 100% saturated with base cations; Ca+2, Mg+2, K+, and Na+.

common reactions

•A typical reaction that takes place in the soil involves ammonia (NH3) and water. In the soil, ammonia reacts with water to form ammonium (NH4+) with hydroxyl as a byproduct. •This reaction increases the OH- concentration of the soil solution, thus increasing the soil pH. NH3 + H2O = NH4+ + OH-

alkalinity

•Any process or reaction that contributes OH- or that consumes or removes H+ from the soil solution contributes to soil alkalinity. •The hydrolysis of sodium carbonate is a common reaction in sodic soils. This reaction contributes OH- to the soil solution. Na2CO3 + H20 = 2 Na+ + HCO3 + OH- •The hydrolysis of calcium carbonate is a common reaction in calcareous soils. This reaction contributes OH- to the soil solution. CaCO3 + H20 = Ca+2 + HCO3- + OH- •The weathering by hydrolysis of soil minerals such as feldspars, micas, and calcite contribute OH- to the soil solution. This reaction is important in minimally and moderately weathered soils. 3 CaAl2Si2O8 + 6 H20 = 2 HAl4Si6O10(OH)2 + 3 Ca+2 + OH- •The application of anhydrous ammonia (NH+) to soils results in an immediate and temporary increase in soil alkalinity by contributing OH- to the soil solution. NH3 + H20 = NH4+ + OH-

acidity

•Any process or reaction that contributes protons or H+ or removes OH- from the soil solution contributes to soil acidity. •Root and microbial respiration produce CO2 and are common reactions in the soil that contribute H+ to the soil solution. CO2 + H20 = HCO3- + H+ •Also, the dissolution of atmospheric carbon dioxide into precipitation causes normal rain to have a pH of 5.65 and results in adding H+ to the soil. •A common reaction in aerated soils is nitrification which results in the contribution of H+ to the soil solution. NH4+ + 1.5 O2 = NO2- + H2O + 2 H+ NO2 + 0.5 O2 = NO3- •The hydrolysis of aluminum (exchangeable Al+3 and hydroxy-Al) is a common reaction in acid soils that results in the contribution of H+ to the soil solution. Al3+ + H2O = Al(OH)2 + H+ Al(OH)2 + 2 H2O = Al(OH)3 + 2 H+ •The oxidation of sulfur by microorganisms produces sulfuric acid and contributes H+ to the soil solution. S2- + 1.5 O2 + H2O = 2 H+ + SO4-2 •Soil pH is the net result of the reactions which contribute H+ and OH- to soils.

neutrality

•Any reaction that contributes equal amounts of H+ and OH- tends to produce soils with a neutral reaction. This is a common reaction that occurs with the application of potassium fertilizer. KCl + H20 = K+ + OH- + H+ + Cl-

pH Profile

•As the carbonates are leached out of the soil by the downward movement of water, the soil becomes less alkaline. •With increased leaching, the surface of the soil takes on an acidic nature and the rate of mineral weathering and the translocation of colloids increases. •With an acid leaching and weathering environment, the depth of soil leaching increases.

Acid Soils

•Leaching by water plays an important role in the development of acid soils by removing components that contribute to alkalinity (such as carbonates, exchangeable Ca+2, Mg+2, K+, and Na+), while soil components contributing to acidity (such as aluminum compounds) accumulate. •Soils in the pH range of 6.5 to 5.5 are considered to be slightly or moderately acid. •Leaching removes some of the basic cations, however, the CEC has declined, but the effective cation exchange sites are still largely saturated with basic cations. •The pH in these soils is controlled by the hydroxy-Al hydrolysis. •Soils in the pH range of 5.0 to 5.4 are considered to be strongly or or very strongly acid soils. •There is a progressive removal of exchangeable basic cations by leaching which results in a progressive saturation of the CEC with Al+3. •At a pH of 5.0, soils are about 100% Al+3 saturated and the pH in these soils is controlled by exchangeable Al hydrolysis. •Soils in the pH range of 4.0 to 2.0 are considered excessively acid. •There is a presence of strong acids like sulfuric acid which comes from the oxidation of S.

Alkaline Soils

•Soils with a pH in the range of 8.5 to 10 and contain 15% or more Na+ are considered sodic soils. •The pH of a soil in this range is controlled by the hydrolysis of sodium carbonate (Na2CO3). •Soils with a pH in the range of 7 to 8.3 and contain less than 15% Na+ are considered calcareous soils. •The pH of a calcareous soil is controlled by the hydrolysis of calcium carbonate (CaCO3).

pH Continuum

•The actual pH of any soil sample or soil horizon is the net result of the effects of the soil forming factors and soil management practices. •Soil pH commonly ranges from a high of pH 10 to a low of pH 4 or less. •In general, soils in the arid regions tend to be neutral or alkaline and soils in the humid regions tend to be acid.

pH significance

•The maximum availability of nutrients in mineral soils occurs near a pH of 6.5. •At this value, both macro and micronutrients tend to be in ample supply for most plants. •In general, the availability of macronutrients increases as pH increases and decreases in availability as the pH decreases. •The availability of micronutrients (except molybdenum) increases as the pH decreases and decreases as the pH increases. •The maximum availability of nutrients in organic soils occurs near a pH of 5.5, due to the influence of the organic acids on the availability of nutrients.

determining pH and indicator dye

•There are 2 methods commonly used for determining the pH of a soil, with an indicator dye and with a soil pH meter. •For a quick analysis of pH, an indicator dye is used. •The indicator dye solution reacts with the H+ ions in the soil solution and changes color. The color of the solution is then compared to a color chart of know pH values. •To determine soil pH with an indicator solution, a small amount of soil is placed on a piece of wax paper that has been folded length-wise. •A few drops of the solution are then added to the soil until 2 drops more than the soil can absorb have been added. •The wax paper is then tilted back and forth to mix the soil and the indicator solution. •After waiting about 30 seconds, a drop of the excess indicator solution is then "teased" away from the soil with a toothpick and the color of the indicator solution is compared to a color chart of known pH values.

water molecule

•There are equal quantities of H+ and OH- ions in distilled water and it has a pH of 7.0, or has a neutral pH •As the concentration of H+ increases, pH decreases or becomes more acidic. •As the concentration of OH- increases, pH increases or becomes more alkaline. •This relationship between pH and the concentrations of H+ and OH- can be represented graphically. •There is a logarithmic relationship between the concentration (moles per liter) or H+ and OH- and pH. •As the concentration of H+ decreases (increased OH-), the pH increases. As the concentration of OH- decreases (increased H+), the pH decreases. •At a pH of 7.0, the concentration of H+ and OH- are equal. •Changing the pH by 1 unit results in a change in the concentration of H+ and OH- by a factor of 10.

summary

•pH is a measurement of a soil's acidity or alkalinity. •The greater the H+ ion concentration, the lower the pH or the more acidic the soil is. •The greater the OH- ion concentration, the higher the pH or the more alkaline the soil is. •At a pH of 7.0, there are equal concentrations of H+ and OH- ions. •For each pH unit increase, the H+ ion concentration decreases by a factor of 10 and the OH- ion concentration increases by a factor of 10. •For each pH unit decrease, the H+ ion concentration increases by a factor of 10 and the OH- ion concentration decreases by a factor of 10. •Any process or reaction that contributes OH- or that consumes or removes H+ from the soil solution contributes to soil alkalinity. •Any process or reaction that contributes H+ or that consumes or removes OH- from the soil solution contributes to soil acidity. •In general, as the pH of a soil increases, the availability of macronutrients increase and the availability of micronutrients decrease. •As the pH of a soil decreases, the availability of macronutrients decrease and the availability of micronutrients increase.

soil pH

•pH is a measurement of acidity or alkalinity and is defined as the negative logarithm of the hydrogen ion activity, where the hydrogen ion activity is measured as moles or grams per liter. pH = log 1 / [H+]