Presentation on theme: "1. Leach bases from soil Acid Soil Formation چگونه خاک اسيدي تشکيل مي شود."— Presentation transcript:
1. Leach bases from soil Acid Soil Formation چگونه خاک اسيدي تشکيل مي شود
As H+ goes on clay = Al +3 is solubilized and hydrolyzed 2- Replace exchangeable bases with acid Sources of acidityWater H 2 O ------------ H + + OH - CO 2 from soil respiration CO 2 + H 2 O ----------- H 2 CO 3 ---------H + + HCO 3 - Organic acids or O.M. decomposition RH ------------- R- + H+ Oxidation of S and N S -------- H 2 SO 4 -------- 2 H+ + SO 4 -2 NH 3 -------------- HNO 3 ---------- H+ + NO 3 -
3- Human-Induced Changes 1- Chemical fertilizers, ammonium-based N materials NH 4 + ¾(O2) ---------HNO 3 2- Acid rain N and S gases emitted from combustion processes
4- Waste disposal sewage sludge - organic acids from decomposition of organics - inorganic acids from oxidation of inorganics mining wastes - oxidation of sulfide (S -2 ) minerals S -2 ----- (O 2, H 2 O)------------ H 2 SO 4 5. Wetland drainage coastal acid sulfate soils iron sulfide (FeS) and elemental S (S o ) FeS and S o ¾(O 2, H 2 O)---------------- H 2 SO 4
pH خاک روي جذب عناصر غذايي توسط گياه تاثير دارد؟ There is no simple relationship of pH to nutrient availability because of the complex combination of soil types, soil moisture regimes and meteorological factors.
C. Jones and J. Jacobsen, 2001, Plant Nutrition and Soil Fertility, Montana State University Extension Service. Document can be downloaded from: http://www.montana.e du/wwwpb/pubs/mt44 49.html.
Elaine Ingham of Oregon State University suggested that in a spoonful of healthy agricultural soil you will find up to 60 million bacteria, 150-500 feet of fungal hyphae, 10,000 protozoa, 20-30 beneficial nematodes (not root feeders) and several thousand mites, springtails and other micro arthropods. She points out that in many heavily cropped soils many of these life forms have disappeared. What affects the activity of soil microorganisms? Decomposition of OM added to the soil depends on these things: Temperature (ideal is 25-30C, poor is below 10C pH below 5 decreases OM Soil texture OM increases as the clay content in mineral soils increases. It combines with clay to form more stable structures protected from more microorganism attack.
Base saturation is one of the most important properties of soil. Understanding base saturation can help us better understand and manage our soil. Soil nutrients that are positively charged (cations) either spend time on the soil exchange sites, clay and organic matter, or in the soil solution. The ratio between cations on the exchange sites and cations in soil solution is directly affected by the base saturation percentage of each cation. This is why we need to take base saturation into account when calculating fertilizer and amendment applications and the amount of fertilizer and amendments required to correct soil problems can vary greatly depending upon the soil's base saturation. Base Saturation Percentage (BSP)
The relationship between nutrients attached to the soil clay and organic matter and nutrients in the soil solution.
How Do You Calculate Base Saturation? Base saturation is defined as the percentage of the soil exchange sites (CEC) occupied by basic cations, such as potassium (K), magnesium (Mg), calcium (Ca), and sodium (Na). The base saturation percentages are calculated for each cation then added up to determine base saturation.
CEC = K ppm/390 + Mg ppm/120 + Ca ppm/200 + Na ppm/230 + H (buffer pH) CEC = Acid (meq/100g) + Base (meq/100g) Base Saturation = Base (meq/100g)/CEC X 100
The base saturation can be overestimated if soils contain appreciable amounts of free limestone, gypsum, or excess salts. It is assumed that the base cations are occupying soil exchanges sites (CEC). If the soil cations are not attached to the exchange sites, the base saturation is over estimated. Base saturation overestimation is mostly commonly associated with free calcium from limestone and gypsum, but can possibly occur with the other three cations.
بين pH و درصد اشباع بازي و درصد اشباع هيدروژن هر خاک رابطه اي وجود دارد. HSP pH 5.577.63.5 HSP BSP 50 100 0 BSP 50 100 0
BSP در pH=5.5 چقدر است؟ pH = [ 187-0.3 (CEC)-HSP] / 24 CEC = 13 meq/100g شکل قبل با استفاده از فرمول زير ترسيم شدهاست که مختص يک خاک مخصوص است. 5.5 = [ 187-0.3 (13) -HSP] / 24 HSP= 50% HSP +BSP = 100% BSP = 50% HX چقدر است؟ HSP= (HX/CEC )100 50= (HX/13 )100 = 6.5 meq/100
The distinguishing characteristic of saline soils from the agricultural standpoint, is that they contain sufficient neutral soluble salts to adversely affect the growth of most crop plants. For purposes of definition, saline soils are those which have an electrical conductivity of the saturation soil extract of more than 4 dS/m at 25°C (Richards 1954). This value is generally used the world over although the terminology committee of the Soil Science Society of America has lowered the boundary between saline and non-saline soils to 2 dS/m in the saturation extract.
Soluble salts most commonly present are the chlorides and sulphates of sodium, calcium and magnesium. Sodium and chloride are by far the most dominant ions, particularly in highly saline soils, Many saline soils contain appreciable quantities of gypsum (CaSO4, 2H2O) in the profile. The pH value of the saturated soil paste is always less than 8.2 and more often near neutrality (Abrol et al., 1980). In field conditions, saline soils can be recognized by the spotty growth of crops and often by the presence of white salt crusts on the surface.
EC measurements are quick and sufficiently accurate for most purposes. To determine EC the solution is placed between two electrodes of constant geometry and constant distance of separation. When an electrical potential is imposed, the amount of current varies directly with the total concentration of dissolved salts.
How Do You Measure Soil Salinity Soil salinity is mostly measured in terms of EC of the soil solution using EC meters Units of decisiemens per meter (dS/m) Millimhos per centimeter (mmhos/cm) both are equivalent units
- water is added to 200–400 g air-dry soil - stirring until the soil is fully saturated. - Allow the sample to stand for at least 4 hours. - Use a suction filter (pressure plate) or a centrifuge to extract sufficient water. - Measure the ECe of the extract. 1- Saturation Extract
2- Soil water extract - Prepare a mixture of soil:water (1:5 by weight) - Shake for 1 hour - Allow the soil to settle and filter the solution. - Measure the EC of the solution above the settled soil. 3- Piezometers: This method is quick
Sodium adsorption ratio (SAR) SAR = (Na + ) /(Ca +2 + Mg +2 ) 1/2 where (Na + ), (Ca +2 ), (Mg +2 ) = concentration of ions in the solution (mmol/L) SAR = (Na + ) /[(Ca +2 + Mg +2 ) / 2] 1/2 where (Na + ), (Ca +2 ), (Mg +2 ) = concentration of ions in the solution (meq/L)
SOME USEFUL CONVERSION FACTORS mmol (+)/1 = 10 × EC (EC in dS/m or mmho/cm) OP = 0.36 × EC (EC in dS/m or mmho/cm) mg/l = 0.64 × EC (EC in dS/m or mmho/cm) mg/l = 640 × EC
Management of salt-affected soils Leaching remove soluble salts leach soil only if it is nonsodic requires good drainage and drainage area source of "good" water, low EC and low SAR
Na + removal (for sodic soils( replace Na + with Ca +2 and Mg +2 commonly use gypsum = CaSO 4 · 2H 2 O 2 Na-X + CaSO 4 ------- Ca-X + Na 2 SO 4 S o and H 2 SO 4 in calcareous soils S o ------------ H 2 SO 4 by microorganisms CaCO 3 + H 2 SO 4 --------- CaSO 4 + CO 2 + H 2 O After Ca +2 (Mg +2 ) replace Na + - leach out the Na + salts