NRES 201 Lecture 37 Outline of Last Lecture I Phosphorus in plants II Phosphorus in soils III The phosphorus cycle IV Factors Affecting Phosphorus Availability V Phosphorus fertilizer sources Outline of Current Lecture VI Terminology VII Potassium in plants VIII Potassium in soils IX The Potassium cycle X Losses of soil Potassium XI Factors affecting Potassium availability XII Potassium fertilizer source Current Lecture Potash as trade term refers to fertilizers that contain K Enzyme Activation Single most important function of K Plant water relations Osmotic regulation K is the major plant cation Transpiration K supply controls stomatal guard cells Plant energy relations are required for ATP production It is essential for any process that needs ATP such as photosynthesis starch translocation and N assimilation Physiological Stability Control of excess N effects and neutralization of organic acids Forms of Soil Potassium Solution K Least abundant form of soil K o More abundant in saline soils Exchangeable K o Far exceeds water soluble K o Less abundant than Ca o Measured by soil testing These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Non exchangeable K o K is fixed in interlayer spaces of certain 2 1 clay minerals especially illite and vermiculite Fixation is the result of size K is smaller than Ca or Mg The amount of fixed K depends on clay content and the type of clay Mineral forms of soil K Major form of soil K Amount present depends on soil parent material and weathering intensity Leaching losses of K Depends on o 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 o Rainfall amount intensity o Composition of exchangeable cations Competition for exchange sites Retention decreases o Fertilizer practices Leaching promoted by K fertilization and by excessive N fertilization o Liming Decreased leaching losses by liming highly acid soils Due to less competition with exchangeable H and Al Increase in pH dependent CEC Factors affecting K Soil clay and organic matter content o Affects CEC o Deficiencies may develop in sandy and organic soils o Type of clay affects CEC and K fixation Wetting and drying o Increase in K availability in soils with low exchangeable K o Decrease in K availability in soils with high exchangeable K o Dry soil test usually higher than wet soil test
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