Potassium: Properties, Functions, and Availability in Plants and Soils, Study notes of Earth Sciences

Download Potassium: Properties, Functions, and Availability in Plants and Soils and more Study notes Earth Sciences in PDF only on Docsity! NRES 201 Lecture 39 (Fall 2008): Potassium 1 NRES 201 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 • 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 NRES 201 Lecture 39 (Fall 2008): Potassium 2 Typical plant Macronutrient concentration† C 45.0% O 45.0% H 6.0% N 1.5% K 1.0% Ca 0.5% Mg 0.2% P 0.2% S 0.1% †Expressed as a percentage by weight of dry matter. Functions of K 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 NRES 201 Lecture 39 (Fall 2008): Potassium 5 Forms of soil K Water-soluble K • The least abundant form of soil K • 8-15 lb/A-6” in Illinois soils • 0.0004-0.0008% of surface soil Exchangeable K • Far exceeds water-soluble K • Usually less abundant than exchangeable Ca, the major exchangeable cation in most soils • Measured by soil testing • 200-600 lb/A-6” in Illinois soils • 0.01-0.03% of surface soil Nonexchangeable K • K fixed in interlayer spaces of certain 2:1 clay minerals, especially: - Illite - Vermiculite p = planar (not specific for K+) e = edge i = inner specific for K + (or NH4+) Source: Tisdale et al. (1993) NRES 201 Lecture 39 (Fall 2008): Potassium 6 • Fixation is the result of size Ionic radius Ion (nm) Mg2+ 0.078 Ca2+ 0.106 K+ 0.133 NH4+ 0.143 • Amount of fixed K depends on: - Clay content - Type of clay • 200-1500 lb/A-6” in Illinois soils • 0.01-0.08% of surface soil Mineral forms of soil K 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 • 1-2.2% of surface soil Clay minerals • Illite • Vermiculite • Contain interlayer (fixed) K NRES 201 Lecture 39 (Fall 2008): Potassium 7 Sand and silt minerals • All are silicate minerals • Micas - Muscovite - Biotite - Contain interlayer K • Potash feldspars - Orthoclase - Microcline - Sanidine - Empirical formula: KAlSi3O8 Crop removal of soil K Generalizations • K >> Ca ≥ Mg • K >> P • K ≤ N Uptake in stover versus grain • Nutrient removal by corn (180 bu/A)) lb/A Grain 36 Cornstalks 104 Total 140 % in stover 74 NRES 201 Lecture 39 (Fall 2008): Potassium 10 Erosion losses of soil K • Losses can be considerable - Finer particles are selectively removed - These particles have the highest CEC • Partially compensated by: - Mineral reserves throughout the soil profile Plant availability of soil K Readily available • Water-soluble • Exchangeable Slowly available • Nonexchangeable (clay-fixed) Unavailable • Mineral forms • Most of the soil supply • All sand and silt minerals NRES 201 Lecture 39 (Fall 2008): Potassium 11 Factors affecting soil K 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) Climatic conditions • Rate of weathering - Determines release from mineral forms - Promoted by: • Warm temperatures • Moist but not waterlogged conditions • 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 NRES 201 Lecture 39 (Fall 2008): Potassium 12 Lower values Higher values Source: Khan (1991) • 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 promotes the release of fixed K • But also because K salts leach out of plant residue