Soil Chapter 8 Acidic Soils & Salt Affected Soils Pages 229 262 About 2.5 billion acres of mostly cultivated lands are affected by soil salinity Treating ph problems is less difficult than managing and remediating saline soils Acidic soils... Acidic soils mainly occur in areas of high rainfall Areas with rainfall greater than 30 per year 1
Acidic soils... Levels of toxic Aluminum (Al + ) and Manganese (Mn + ) ions increase as ph decreases Microbial activity, including N 2 fixation slows as ph decreases Sources of acidic soils... Acidic rainfall CO 2 dissolved in rainfall releases carbonic acid CO 2 from decomposing organic matter (humus) releases carbonic acid CO 2 from root respiration releases carbonic acid Sources of acidic soils... Release of H + from ammonium (NH 4+ )- containing fertilizers Hydrogen ions (H + ) excreted by plant roots Crop removal removes calcium, magnesium and potassium 2
Causes of acidic soils... Infiltrating water moves acidic cations Al(OH) 2+ and H + into the soil High amounts of exchangeable H + and Al 3+ replace the basic cations leaving soils acidic Causes of acidic soils... The basic cations: Ca 2+, Mg 2+, N + and K + are leached With continued leaching over time soils continue to become more acidic When acidic soils become toxic When ph decreases below 4.7... Al 3+ can become abundant... Existing in the soil solution and on the cation exchange sites 3
When acidic soils become toxic Soil solutions of ph 4.5 and lower... The dominant species in solution can become Al 3+... When the dominant species becomes Al 3+... Soils can become toxic to most plants Plants that do well in acidic soils... Have low requirements for basic cations Ca and K Able to tolerate low nutrient availability Plants that do well in acidic soils... Able to tolerate higher levels of Aluminum (Al) and Manganese (Mn) ex. tea, pineapples, coffee, azaleas, blueberries (no Al for blueberries) and hydrangeas 4
Acidic things to remember... About acidic soils Acidic soils are usually leached soils The more leached the soils are... The more strongly acidic the soils are likely to be Acidic things to remember... Most strongly acidic soils have... few basic cations (Ca, Mg, Na, & K) available for root absorption High quantities of exchangeable Al, H and Mn Low quantities of the more easily leached nutrients (S, B, Na, Mo & Cl) Acidic things to remember... Levels of toxic Al and Mn increase as ph what? Decreases Microbial processes such as N 2 fixation are slowed by falling ph 5
Tropical and subtropical soils In areas of high year-round temperatures and rainfall... Soils can be very weathered and low in basic cations With few nutrients in the soil... Tropical and subtropical soils Plants grow slowly containing low essential nutrient levels in their tissues Most of the nutrients are in the plants... Not in the soils Increasing ph of acidic soils... Lime is added to acidic soils to increase ph Lime improves soil fertility Increases microbial activity and crop production Liming soils reduces toxicity of soluble aluminum 6
Lime Used to increase the ph of soils Most agricultural lime is impure, crushed calcium carbonate (powdered limestone) Next most-used is dolomitic lime containing magnesium (Mg) Several other sources one is wood ash Reaction of lime on acid soils Strongly acidic soils restricts growth of most plants Most crops suffer reduced yields Reasons: Aluminum toxicity Reduced microorganism activity Reaction of lime on acid soils Manganese (Mn) toxicity Iron (Fe) toxicity in a few soils Calcium deficiencies Nitrogen (N), Phosphorus (P) and/or Sulfur (S) deficiencies due to slow organic matter decomposition 7
Benefits of liming soils Raises ph eliminating two major problems Eliminates toxic soluble aluminum (Al 3+ ) Increases soil microbial activity Benefits of liming soils Reduces soluble manganese (Mn) and iron (Fe) as possible toxins Dolomitic limestone adds both calcium (Ca) and magnesium (Mg) Liming makes phosphorus (P) more available Benefits of liming soils Liming makes potassium (K) more available Liming increases the availability of nitrogen (N) Lime reduces the solubility of heavy metals often found in sewage sludges 8
Acidifying soils Lower ph may reduce incidences of certain plant diseases Micronutrient metals become more soluble for better plant adsorption Acidifying soils Materials used to acidify soils Elemental sulfur Iron sulfate Aluminum sulfate Sulfuric acid Acidifying soils 9
Soluble salts and plant growth Salt buildup in soils make it impossible for most plants to grow As salts accumulate in soils only salt tolerant plants will grow Some soluble-salt concentrations can be higher than seawater 3% - 4% total salts Salts... Salts are referred to as soluble salts Readily dissolve in water Solid when dry Can be seen on the soil surface during and after drying Soluble salts... Salts are ionic compounds... have a cation (+) sodium (Na + ), calcium (Ca 2+ ) and magnesium (Mg 2+ ) and an anion (-) chloride (Cl 2- ), sulfate (SO 4 2- ) and bicarbonate (HCO 3- ) 10
Soluble salts... Also occurring in smaller quantities... potassium (K + ) ammonium (NH 4+ ) nitrate (NO 3- ) carbonate (CO 3 2- ) Soluble salts... Typically form as minerals weather Most soluble salts remain in the soil if not leached Soluble salts... Areas of low precipitation, < 15/year Evaporating water brings salts to the soil surface remaining on or within the soil 11
Salinity... Defined as the amount of dissolved minerals or salts in a solution And for our purposes... In a soil solution Saline soils and their causes... Occur in areas of low rainfall Saline parent materials e.g. limestone, sandstone Minimal leaching of parent materials Excess fertilizers Buildup of animal urine Saline soils and their causes... Poor irrigation practices High salinity irrigation water like reclaimed irrigation water Salty groundwater rising too close to the surface from land clearing 12
Saline soils and their causes... Slow to leach heavy clay soils with poor drainage Well water Physical signs of saline soils Saline soils are more challenging to manage than soil acidity Readily identifiable by: White crust on soil surface Leaf margin burn Poor crop stands and yields Measuring salt-affected soils Soil salinity is measured by the electrical conductivity of a soil Electrical conductivity (EC) 13
Measuring salt-affected soils Electroconductivity of a soil measured in decisiemen per meter EC in ds/m ECe in ds/m taken from a soil extract ECw in ds/m taken from irrigation water Salt-affected soils... Saline soils ECe 4, SAR <13 Sodic soils ECe > 4, SAR 13 Saline-sodic soils ECe 4, SAR 13 Normal soils ECe < 4, SAR <13 Saline soils... Saline soils nonsodic soil high in total dissolved salts impairs productivity 14
Saline soil characteristics... High in soluble salts Non-sodic Measured by TDS, EC & OP High saline soils > 2.0 mmhos/cm (or ds/m) Saline soil characteristics... High ECw causes plant failure Higher than normal irrigation amounts are required to leach salts Soil extract = ECe Water extract = ECw Saline soil characteristics... Most plants do not do well in high salinity soils Salts tend to accumulate in soils over time from: fertilization and use of high saline irrigation waters 15
Remediation of saline soils Requires specialized irrigation practices Irrigate heavily with low-salt irrigation water Remediation of saline soils Must have good drainage Salt leachates must be able to evacuate the site Addition of organic matter Sodic soils... Different from saline soils Sodium ions (Na+) cause most problems in soils Sodium (Na+) is the main cause of dispersed (deflocculated) soils Sodic soils are a result of excess sodium in soils 16
Sodic soil characteristics... High in Na + as compared to Ca 2+ & Mg 2+ Measured by SAR & ESP (sodicity) SAR sodium adsorption ratio Higher SAR more Na + as compared to Ca 2+ & Mg 2+ Sodic soil characteristics... [ Na+] SAR = [Ca2+] [Mg2+] Sodic soil characteristics... SAR 13 is sodic with likely poor permeability SAR is also used to test irrigation water 17
Sodic soil characteristics... ESP exchangeable sodium percentage Portion of total exchangeable cations that are Na+ Measured in centimoles of charge per kilogram Sodic soil characteristics... ESP = [Na+] x (100%) [Total salts] Sodic soil characteristics... ESP value = or 15 indicates a sodic soil Sodic clays tend to have a ph of 8.5 or greater 18
Sodic soil characteristics... Sodic soils nonsaline soil high in sodium (Na + ) SAR of 13 or more can be an indicator of poor drainage harmful to productivity Sodic soil characteristics... Sodium (Na + ) deflocculates clay particles Soils high in sodium (Na + ) are referred to as sodic To flocculate or deflocculate... Deflocculation the separation of clay aggregates into individual particles 19
To flocculate or deflocculate... Flocculation aggregation or coming together of individual clay particles formation of soil aggregates Good drainage in clays relies on particle aggregation Black alkali Sodium on clay promotes hydrolysis splits water into H + and OH - H + adsorbs to clay and OH - remains in solution increasing the ph Clay + Na + HOH Clay + H + Na + + OH - Recovery of sodic soils... Treat with gypsum (CaSO 4 ) as deeply as possible Gypsum rates 80 lbs/1,000 sq.ft. Must have good drainage Irrigate with calcium-rich (Ca 2+ ) irrigation water 20
A note about gypsum... Sodium coats positively charged clay particles with negatively charged Na + ions deflocculating clay particles The golf ball analogy dimples on a golf ball are receptor sites Na + ions fill those sites A note about gypsum... Gypsum (CaSO 4 ) can be added to sodic to correct excess sodium levels Calcium in gypsum displaces sodium and neutralizes the negative (-) charge on the clay A note about gypsum... Addition of calcium (Ca2 + ) flocculates the clay particles forming aggregates Gypsum has NO effect on mechanical compaction 21
A note about gypsum... What if the soil is not sodic? It s like chicken soup... It couldn t hurt! Sinks A low spot surrounded by higher ground Rainfall and irrigation runoff and collect in sinks Sinks Sinks characteristically have no way out but... Up through evaporation... or... Down through infiltration 22
Sinks When the water infiltrates or evaporates... It leaves behind salts, sodium and clays Soils in sinks can become unusable Sinks Sinks 23
Sinks Sinks 24