Potassium (K)

• One of the 13 essential elements for plant growth and is considered one of the 6 Major Elements, based on concentration required
• principle form of uptake: ionic form (K⁺)
• potassium primarily moves through the soil via diffusion
• soil factors that affect uptake of potassium by plants:
  • soil moisture movement of ions
  • temperature
  • the level of decomposition of organic matter
• plant factors that affect uptake:
  • hybrid or variety of plant
  • stage of growth
  • interaction with calcium & magnesium, and other elements

Roles of Potassium in Plant Functions:

• involved in the osmotic potential, balancing anions, controlling membrane permeability & electropotential
• water status: controls turgor pressure of the cells and the opening and closing of stomata
• required for accumulation and translocation of the newly formed carbohydrates
• plants concentrate K into structure: stalks, stems, & fiber^IG
• K is needed to make carbohydrates and fiber, but to make proteins, enzymes, and vitamins, plants need all the other elements^IG
• potassium stays locked up tight through plant’s life in the structure^IG
• hay and straw contain lots of potassium^IG
• trees & shrubs concentrate potassium into their woody parts (sawdust & bark are also rich in potassium)^IG
• moves water into the cells of developing fruit (reason for juicy tomatoes)

Target Potassium Levels:

• amount required for healthy plant growth: 10,000 ppm, 1.0%, dry soil matter
• heavy soils deliver potassium far more effectively than light soils do (to plants), therefore heavy soils should be lower in K than light soils (<5% TCEC, down to 2%)^IG
• according to Solomon, target potassium levels depend on TCEC:
  

  TCEC	LBs K
  1-7	255
  8	270
  9	290
  10	310
  11	320
  12	335
  13	350
  14	365
  15	380
  16	390
  17	400
  18	410
  19	420
  20	435
  21	443
  22	451
  23	459
  24	463
  25	475
  >25	see book

Sources of Potassium

• most soil conditioners, mulches, composts you can buy or import are all rich in potassium and comparatively low in everything else^IG
• potassium is the most inexpensive and effective yield-booster, but is a detriment to long term balance^IG

Potassium Deficiency Symptoms:

• yellowing and cell death around the edges of the leaves, or in between veins (may be confused with magnesium deficiency)
• Potassium is very mobile within the plant, so deficiency symptoms will first show in the older leaves, since the plant is moving K from older leaves to newer growth to supply growing points
• Potassium content in leaves naturally declines with maturity of the plant, so some symptoms of deficiency are normal in those older leaves, as the plant ages
• soil conditions related to deficiency in potassium:
  • mineral soils low in organic matter
  • soils with low CEC (Cationic Exchange Capacity)
  • long history of cropping without adequate potassium
  • sandy soils from low-potassium parent material
  • sandy soils subjected to high rainfall or irrigation

Potassium Excess Symptoms:

• too much potassium will bring down brix levels
• soils over-fertilized with potassium and/or calcium (Ca) are commonly deficient in magnesium (Mg)
• leached fields tend to have lots of potassium and little else^IG

Sources:

• Plant Nutrition & Soil Fertility Manual
• IG: The Intelligent Gardener