NRES 201 Lecture 37 Outline of Last Lecture I. Phosphorus in plantsII. Phosphorus in soilsIII. The phosphorus cycleIV. Factors Affecting Phosphorus AvailabilityV. Phosphorus fertilizer sourcesOutline of Current Lecture VI. TerminologyVII. Potassium in plantsVIII. Potassium in soilsIX. The Potassium cycleX. Losses of soil PotassiumXI. Factors affecting Potassium availability XII. Potassium fertilizer sourceCurrent LecturePotash as trade term - refers to fertilizers that contain KEnzyme Activation = Single most important function of KPlant water relations - - Osmotic regulation: K is the major plant cation- Transpiration: K supply controls stomatal guard cellsPlant 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 Ko More abundant in saline soils- Exchangeable Ko Far exceeds water soluble Ko Less abundant than Cao Measured by soil testingThese 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 Ko 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 intensityLeaching 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 rainfallo Rainfall - amount, intensityo Composition of exchangeable cations Competition for exchange sites Retention decreaseso Fertilizer practices Leaching promoted by K fertilization and by excessive N fertilizationo Liming Decreased leaching losses by liming highly acid soils- Due to less competition with exchangeable H and Al- Increase in pH dependent CECFactors affecting K - - Soil clay and organic matter content - o Affects CECo Deficiencies may develop in sandy and organic soilso Type of clay affects CEC and K fixation- Wetting and drying - o Increase in K availability in soils with low exchangeable Ko Decrease in K availability in soils with high exchangeable Ko Dry soil test usually higher than wet soil
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