NRES 201 Lecture 39 (Fall 2014): Potassium1NRES 201Potassium12Today’s lecture topics- Terminology- Potassium in plants- Potassium in soils- The potassium cycle- Losses of soil potassium- Factors affecting potassium availability- Potassium fertilizer sourcePotassiumNRES 201 Lecture 39 (Fall 2014): Potassium2TerminologyThe elemental symbol- K stands for kalium- The Latin word forpotassiumPotash as a trade term- Refers to fertilizersthat contain K- Derived from pot ashes- Plant residues burned to make soap- The ash contained K2CO3and other salts- Potash production was an early industry in thecolonial U.S.- First patent in 17903Source: http://www.growveg.comSource: Havlin et al. (1999)Plant nutrient concentrations4NRES 201 Lecture 39 (Fall 2014): Potassium3Functions of potassium in plantsEnzyme activation- Single most important function of K- > 80 enzymes, especially in meristemPlant water relations- Osmotic regulation- K is the major plant cation- Transpiration- K supply controls stomatalguard cells 5Source: http://evolution.berkeley.eduPlant energy relations- Required for ATP production- Essential for any process that requires ATP,such as:- Photosynthesis- Starch translocation- N assimilationPhysiological stability- Control of excess Neffects- Neutralization oforganic acids6Lodged cornSource: http://extension.udel.eduNRES 201 Lecture 39 (Fall 2014): Potassium4Potassium content of soilsKey points- Soils contain plenty of K- But the fertilizer industry doesn’t want you toknow it7The potassium cycleSource: Havlin et al. (2005)8NRES 201 Lecture 39 (Fall 2014): Potassium5- K is taken up from the soil solution-As K+- Soluble K+is in equilibrium with- Exchangeable K+- And nonexchangeable (clay-fixed) K+- No occurrence in organic forms- Except as exchangeable K+on organicexchange sites- Microbial uptake- But no transformations- Loss of K occurs by- Leaching-Erosion- Runoff- But no volatilization9Forms of soil potassiumWater-soluble K- The least abundant form of soil K- 8-15 lb/A-6” in Illinois soils- More abundant in saline soilsExchangeable K- Far exceeds water-soluble K- Less abundant than exchangeable Ca, themajor exchangeable cation in most soils- Measured by soil testing- 200-600 lb/A-6” in Illinois soils10NRES 201 Lecture 39 (Fall 2014): Potassium6Nonexchangeable K- K fixed in interlayer spaces of certain 2:1 clayminerals, especially:- Illite- Vermiculitep = planar (not specific for K+)e = edgei = innerspecific for K+(or NH4+)Source: Tisdale et al. (1993)11- Fixation is the result of size- In solution, K+is smaller than Ca2+or Mg2+- And is small enough to enter the holes in theclay lattice- Amount of fixed K depends on:- Clay content- Type of clay- 200-1500 lb/A-6” in Illinois soils- And up to 18,000 lb/A for the profile12NRES 201 Lecture 39 (Fall 2014): Potassium7Mineral forms of soil potassiumQuantitative occurrence- The major form of soil K- Amount present depends on:- Soil parent material- Weathering intensity- 20,000-45,000 lb/A-6” in Illinois soils- And up to 500,000 lb/A for the profile!Clay minerals- Illite- Vermiculite- Contain interlayer (fixed) K13Sand and silt minerals- All are silicate minerals- Micas- Muscovite- Biotite- Contain interlayer K- Potash feldspars- Orthoclase- Microcline- Sanidine- Empirical formula:KAlSi3O8Source: http://skywalker.cochise.eduMuscoviteOrthoclase14NRES 201 Lecture 39 (Fall 2014): Potassium8Crop removal of potassiumGeneralizations- K >> Ca  Mg- K >> P- K  NUptake in stover versus grain- Nutrient removal by corn (180 bu/A))15Much greater crop removal- When the vegetative matter is harvested- As with a forage legume- Or when corn is grown for silage16NRES 201 Lecture 39 (Fall 2014): Potassium9Leaching losses of potassiumDepends on- Soil type- Highest with• Sandy soils Low CEC Low water-holding capacity• Organic soils Low clay content limits K fixation• Tropical soils Low CEC High rainfall- Reduced by a dense or compact layer17- Rainfall- Amount- Intensity- Composition of exchangeable cations - Competition for exchange sites- Retention decreases in the order:H+(Al3+) > Ca2+> Mg2+> K+= NH4+> Na+- Fertilizer practices- Leaching promoted by K fertilization- And by excessive N fertilization - Liming- Exchangeable K displaced by addition of Caand/or Mg18NRES 201 Lecture 39 (Fall 2014): Potassium10- Decreased leachinglosses by liming highlyacid soils• Due to less competitionwith exchangeable H+and Al3+• And an increase in pH-dependent CECSource: Brady and Weil (2008)19Erosion losses of potassium- The bad news:- Losses can be considerable• Because finer particles are selectively removed• And these particles have the highest CEC- The good news:- Mineral reserves occur throughout the soil profile20NRES 201 Lecture 39 (Fall 2014): Potassium11Plant availability of soil potassiumReadily available- Water-soluble- ExchangeableModerately available- Nonexchangeable (clay-fixed)Slowly available- Mineral forms- Most of the soil supply- All sand and silt mineralsMineral KNonexchangeable KExchangeable K21Measured by soil testingFactors affecting potassium availabilitySoil clay and organic matter content- Affects CEC- Deficiencies maydevelop in:- Sandy soils- Organic soils- Type of clay affects:-CEC- K fixationSource: Tisdale et al. (1993)22NRES 201 Lecture 39 (Fall 2014): Potassium12Climatic conditions- Root growth- Controls K uptake- Root growth restricted by:• Poor drainage• Cool, wet weather• Droughtiness- Rate of weathering- Determines release from mineral forms- Promoted by:• Warm temperatures• Moist but not waterlogged conditions23- Wetting and drying- Increases K availability in soils with lowexchangeable K• Causes clay to crack• Fixed K is released from edge positions- Decreases K availability in soils with highexchangeable K• Due to clay fixation- Dry soil test usually higher than wet soil test24NRES 201 Lecture 39 (Fall 2014): Potassium13Lower valuesHigher valuesSource: Khan (1991)Soil K testing measures exchangeable K25- Freezing and thawing- Increases K availability in soils with lowexchangeable K- No effect for soils with high exchangeable K- Soil test values higher in spring than in fall• Not only becausefreezing and thawingpromote the releaseof fixed K• But also because Ksalts leach out ofplant residues26Source: http://www.aganytime.comNRES 201 Lecture 39 (Fall 2014): Potassium1427Source: Khan (1991)Soil pH- Deficiencies more likely on acid soils than onneutral or