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Soil pH

Its variation in the landscape, its importance, and how to manage it. Soil pH. What is pH and how do we measure it?. pH is a measure of the hydrogen ion concentration. What is a hydrogen ion???. Periodic Table. Sixteen Essential Elements. Atoms, cations, and anions.

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Soil pH

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  1. Its variation in the landscape, its importance, and how to manage it. Soil pH

  2. What is pH and how do we measure it? pH is a measure of the hydrogen ion concentration.

  3. What is a hydrogen ion???

  4. Periodic Table

  5. Sixteen Essential Elements

  6. Atoms, cations, and anions • Atoms are made of equal numbers of protons (+) and electrons (-), and therefore have zero charge (aluminum metal, Al0) • Cations are single atoms or groups of 2 or more atoms that have lost 1 or more electrons (Al+++) • Anions are single atoms or groups of 2 or more atoms that have gained 1 or more extra electrons (Cl-)

  7. Hydrogen atoms and ions • Hydrogen atoms exist as a gas. It is very explosive and is used as a rocket fuel. It is proposed to provide the fuel for fuel cells for generating electricity used by a future generation of automobiles. Hydrogen atoms react with oxygen to produce energy and water. • Hydrogen ions are highly reactive ions that will degrade most other substances. It is the active part of an acid.

  8. H+ ion 1.6 to 1.7 x10-5 Ångstrom From Wikipedia 1.20 Ångstrom

  9. From Sensorex.com

  10. pH of Common Products and Soils Range found in common products Range found in various soils

  11. pH is a measure of the hydrogen ion concentration, determined with a specially designed voltmeter, reference electrode, and glass electrode that creates a voltage proportional to the hydrogen ion concentration. • The measurement scale varies from 0 to 14, with the small numbers being the most concentrated in hydrogen ions. • In soils, pH values typically range from 3.5 to 8 and are used as an index to indicate the balance between acid and non-acid cations in soil.

  12. pH meter components A pH electrode is an electrode whose output voltage changes as the pH (hydrogen ion concentration) changes. A reference electrode is an electrode whose voltage output stays constant. A pH meter, a millivolt meter with a special high impedance input circuit and circuits to change the electrode's millivolts into pH unit readouts. Optionally, an automatic temperaturecompensator, a device which senses temperature so that the meter can correct for the effects of temperature changes. (Sensorex.com)

  13. Soil pH • Soil pH is a measure of the “active” acidity in soil solution • What is measured with a pH meter • Soil pH alone does not tell us much about how much limestone to apply!!!

  14. Major source of error in measurement of pH • When pH is measured using deionized water, values are often incorrectly high because of low salt levels in the soil solution. We have corrected this problem by now measuring pH in a calcium chloride solution (0.01M), which creates more stable pH readings.

  15. What are the factors that affect the measured value of soil pH? • The amount of acidity in the soil. • The salt content of the soil. • Differences in salt content between samples cause errors in the pH reading. • The new method eliminates this error.

  16. What do we want to measure with a soil pH test? • Ideally, soil pH should be an index ofthe intensity of acidity. But the amount of salt in the soil does affect the pH reading. • Salt in the soil is affected by the amount of manure and fertilizer we apply. • Salt in the soil is affected by the amount of rainfall that leaches salts from the soil.

  17. What are the sources of salts in soil? • Fertilizer • Potash • Ammonium nitrate, UAN Solution, etc. • Manures • Sodium chloride • Potassium chloride

  18. The UGA method for measuring pH avoids the seasonal variation in pH caused only by differences in the soil’s salt content.

  19. Fertilizer and manure application, average Georgia soil pHw wet year average pHw wet year pHw 5.9 dry year ΔpHaverage = 0.6 pHw dry year 5.3 pHCaCl2 January July December

  20. What does the pH reading tell you about the fertility of the soil? • The pH reading will tell you if certain chemical elements might be deficient, or if some might be toxic. • The pH reading can be compared to the desirable pH range for a given crop. Based on this comparison, you can decide if the soil pH should be increased by liming or in rare cases, if the pH should be decreased by adding sulfur or other acid forming materials. • The pH reading does not tell you how much lime to add to an acid soil.

  21. The amount of lime to add depends on the desired pH, the soil pH, and the lime buffer capacity or LBC (more later).

  22. Non acid cations calcium ions magnesium ions potassium ions sodium ions Acid cations hydrogen ions aluminum ions Some of the major exchangeable cations are acid and some are not

  23. Acid/non-acid cations vs soil pH pH - 3.5 H+ Soil Al3+ pH – 5.3 pH - 7.0 Ca2+ H+ Ca2+ Al3+ Mg2+ Mg2+ Ca2+ K+ Soil Ca2+ Soil Al3+ Mg2+ Ca2+ Ca2+ K+ Al3+ Al3+ Ca2+ Mg2+ K+ BS = 0% BS = 50% BS = 100%

  24. What is the most desirable pH for growing plants and why? • The most desirable pH depends on the plant species. Some plants, such as blueberry and azalea, grow best in low pH soil, whereas other plants grow best in slightly acid pH soils. Alfalfa grows best at pH 6.5 to 7.

  25. Blueberry Al & Mn Toxicity Mn & Fe Deficiency 3.0 4.0 5.0 6.0

  26. wheat Al & Mn Toxicity Mn & Fe Deficiency

  27. How Soil pH Affects Availability of Plant Nutrients Very Slightly Acid Very Slightly Alkaline Medium Acid Slightly Acid Slightly Alkaline Medium Alkaline Strongly Alkaline Strongly Acid disk 10 4.O 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 pH

  28. pH 5.5 is a critical point in soils • For most agronomic crops, it is important to maintain surface soil pH above 5.5 because: • Root growth and water extraction by the crop is reduced at pH below 5.5. • If the pH of the plow layer is low, it results in the formation of acid subsoils over time.

  29. Toxic Elements in Soils • Al and Mn Disk 11 Gary

  30. We lime acid soils to restore their ability to produce crops efficiently.

  31. Acid soils are limed because • At low pH, aluminum and manganese are toxic to crops. • At low pH, there is sometimes a deficiency of calcium and magnesium for good crop growth.

  32. Limestone is primarily calcium carbonate, with some magnesium carbonate • Limestone dissolves slowly in soil to release Ca++ and Mg++ cations and carbonate anions. • The carbonate anions react to neutralize H+ in soil, forming carbon dioxide and water. • The Ca++ and Mg++ cations take the place on the soil of the H+ that has been neutralized by the carbonate from the limestone. Soil pH increases as the lime slowly neutralizes the H+ in the soil.

  33. Lime Reactions in Soil • Lime dissolves slowly in soil to produce Ca2+ and CO32-. • CaCO3 + H2O Ca2+ + CO32-+ H2O • Newly dissolved CO32- reacts with H+ in the soil to neutralize it, forming a Ca2+- soil. • H2O + Ca2+ + CO32- + soil-H <==> soil-Ca + CO2 + H2O

  34. Ca2+ Soil Ca2+ Ca2+ Ca2+ CaCO3 CaCO3 Ca2+ + CO3 H2O + CO2 + Al(OH)3 (gas) Al3+ Al3+ H+ + Soil Al3+ Al3+ (Solid) H+ pH - 3.5

  35. How soils become acid

  36. Soils become acidic because • An excess of hydrogen ions are produced in the soil by biological reactions, primarily the transformation of ammonium to nitrate. • Hydrogen ions and the Aluminum ions they produce push calcium, magnesium, and potassium ions off of the exchange sites. • Calcium, magnesium, and potassium in solution can then leach from the soil.

  37. Nitrogen Conversion in the Soil Produces Acidity Organic Matter Manure, etc. 2 NH4 + 4O2 Bacteria2NO3- + 2H2O + 4H+ NH4 - N Fertilizer Sources Note: The H+ is the acidity component

  38. Cation Exchange 2H+ Ca + Clay particle H Ca++ Clay particle + H

  39. The H+ produced from N reactions affects soil in the following ways • The addition of H+ to the soil reduces the soil’s effective cation exchange capacity. • The acid cations (H+ and aluminum ions) are held much more strongly by the soil than the non-acid cations such as calcium and magnesium. • Therefore, the non-acid cations are leached from the soil.

  40. Excess cations (and anions) leach from the soil

  41. Cations (positive) potassium K+ calcium Ca++ Magnesium Mg++ zinc Zn++ Manganese Mn++ Iron Fe++ Copper Cu++ Ammonium NH4+ Anions (negative) nitrate NO3- sulfate SO4-- Chloride Cl- Phosphate HPO4 - Borate H2BO3- Molybdate MoO4-- Some ions in soils used by crops

  42. Acid/non-acid cations vs soil pH pH - 3.5 H+ Soil Al3+ pH – 5.3 pH - 7.0 Ca2+ H+ Ca2+ Al3+ Mg2+ Mg2+ Ca2+ n H+ n H+ K+ Soil Ca2+ Soil Al3+ Mg2+ Ca2+ Ca2+ K+ Al3+ Al3+ Ca2+ Mg2+ K+ BS = 0% BS = 50% BS = 100%

  43. How to sample a field or landscape (Circular 896) • Identify sampling locations or zones. • Determine sampling depth. • Select a sampling time to allow time for lime to react with acid soil, etc. • Use clean sampling tools and buckets. • Handle samples carefully.

  44. Making a lime recommendation

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