NRES 201 Lecture 39 Fall 2014 Potassium NRES 201 Potassium 1 Potassium Today s lecture topics Terminology Potassium in plants Potassium in soils The potassium cycle Losses of soil potassium Factors affecting potassium availability Potassium fertilizer source 2 1 NRES 201 Lecture 39 Fall 2014 Potassium Terminology The elemental symbol K stands for kalium The Latin word for potassium Potash as a trade term Refers to fertilizers that contain K Source http www growveg com Derived from pot ashes Plant residues burned to make soap The ash contained K2CO3 and other salts Potash production was an early industry in the colonial U S First patent in 1790 3 Plant nutrient concentrations Source Havlin et al 1999 4 2 NRES 201 Lecture 39 Fall 2014 Potassium Functions of potassium in plants Enzyme activation Single most important function of K 80 enzymes especially in meristem Plant water relations Osmotic regulation K is the major plant cation Transpiration K supply controls stomatal guard cells Source http evolution berkeley edu 5 Plant energy relations Required for ATP production Essential for any process that requires ATP such as Photosynthesis Starch translocation Lodged corn N assimilation Physiological stability Control of excess N effects Neutralization of organic acids Source http extension udel edu 6 3 NRES 201 Lecture 39 Fall 2014 Potassium Potassium content of soils Key points Soils contain plenty of K But the fertilizer industry doesn t want you to know it 7 The potassium cycle Source Havlin et al 2005 8 4 NRES 201 Lecture 39 Fall 2014 Potassium 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 organic exchange sites Microbial uptake But no transformations Loss of K occurs by Leaching Erosion Runoff But no volatilization 9 Forms of soil potassium Water soluble K The least abundant form of soil K 8 15 lb A 6 in Illinois soils More abundant in saline soils Exchangeable K Far exceeds water soluble K Less abundant than exchangeable Ca the major exchangeable cation in most soils Measured by soil testing 200 600 lb A 6 in Illinois soils 10 5 NRES 201 Lecture 39 Fall 2014 Potassium Nonexchangeable K K fixed in interlayer spaces of certain 2 1 clay minerals especially Illite Vermiculite Source Tisdale et al 1993 p planar not specific for K e edge specific for K or NH4 i inner 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 the clay 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 profile 12 6 NRES 201 Lecture 39 Fall 2014 Potassium Mineral forms of soil potassium Quantitative 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 K 13 Sand and silt minerals All are silicate minerals Micas Muscovite Biotite Contain interlayer K Potash feldspars Orthoclase Microcline Sanidine Empirical formula KAlSi3O8 Muscovite Source http skywalker cochise edu Orthoclase 14 7 NRES 201 Lecture 39 Fall 2014 Potassium Crop removal of potassium Generalizations K Ca Mg K P K N Uptake in stover versus grain Nutrient removal by corn 180 bu A 15 Much greater crop removal When the vegetative matter is harvested As with a forage legume Or when corn is grown for silage 16 8 NRES 201 Lecture 39 Fall 2014 Potassium Leaching losses of potassium Depends 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 layer 17 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 Ca and or Mg 18 9 NRES 201 Lecture 39 Fall 2014 Potassium Decreased leaching losses by liming highly acid soils Due to less competition with exchangeable H and Al3 And an increase in pHdependent CEC Source Brady and Weil 2008 19 Erosion 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 profile 20 10 NRES 201 Lecture 39 Fall 2014 Potassium Plant availability of soil potassium Readily available Water soluble Exchangeable Mineral K Moderately available Nonexchangeable clay fixed Slowly available Mineral forms Most of the soil supply All sand and silt minerals Nonexchangeable K Measured by soil testing Exchangeable K 21 Factors affecting potassium availability Soil clay and organic matter content Affects CEC Deficiencies may develop in Sandy soils Organic soils Type of clay affects CEC K fixation Source Tisdale et al 1993 22 11 NRES 201 Lecture 39 Fall 2014 Potassium Climatic 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 conditions 23 Wetting and drying Increases K availability in soils with low exchangeable K Causes clay to crack Fixed K is released from edge positions Decreases K availability in soils with high exchangeable K Due to clay fixation Dry soil test usually higher than wet soil test 24 12 NRES 201 Lecture 39 Fall 2014 Potassium Lower values Soil K testing measures exchangeable K Higher values Source Khan 1991 25 Freezing and thawing Increases K availability in soils with low exchangeable K No effect for soils with high exchangeable K Soil test values higher in spring than in fall Not only because freezing and thawing promote the release of fixed K But also because K salts leach out of Source http www aganytime com plant residues 26 13 NRES 201 Lecture 39 Fall 2014 Potassium Source Khan 1991 27 Soil pH Deficiencies more likely on acid soils than on neutral or calcareous soils Acidity Decreases base saturation of CEC Decreases pH dependent CEC Increases Al toxicity Acidic soils less problematic for K than for Ca or Mg due to huge reserves of nonexchangeable K Source http corn agronomy wisc edu 28 14 NRES