Essential Nutrients

Potassium (K)

Form used by plants:

K⁺

Important functions:

• Regulates water status and activation of many enzymes

• Involved in photosynthesis and respiration

• Involved in the formation and translocation of sugars, proteins, starch, and plant growth hormones

• Required in stomata opening and closing

• Promotes root growth

• Increases plants resistance to disease and improves winter hardiness

• Fruit size (fruits are strong sinks for K), color, and acidity are related positively to potassium concentrations

Ideal foliage range for apple leaves:

• 1.3-1.75%

• < 1.2% may mean K is limiting

• Young trees and nonbearing trees have higher leaf K then mature cropping trees.  Levels strongly influenced by crop load, tending to be higher with light crops and decreasing as crop load increases.

• Levels of leaf K for optimal crop quality is also dependent upon the nitrogen status of leaves
  ex. McIntosh 1.0-1.25 parts N to 1.0 part K are adequate
  Delicious 1.25-1.5 N/K are adequate
  Therefore, a Delicious sample that has 2.4% N should contain between 1.6-2.0% K.

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Crop requirement:

125-175 lbs/Ac/year which is equivalent to 150-210 lbs K₂O. 

60-90 lbs K are removed from orchard each year by crop.

 

This value will also depend upon N status and variety. For example, for varieties such as Delicious, ratios of 1.25 to 1.50 (N:K) appears to be adequate.  This translates to the following, a Delicious sample containing 2.4% N should contain between 1.6-2.0% K.

Ideal soil range:

150-250 ppm or 0.4-0.6 meq/100 g soil (values based on the ammonium acetate extraction method. If sodium bicarbonate method is used, values may be slightly lower).

 

low: < 150 ppm (< 0.4 meq/100 g soil)

high: 250-800 ppm (< 0.6-2.0 meq/100 g soil)

excessive: > 800 ppm (> 2.0 meq/100 g soil)

 

Orchard soils low in K are rare.  The relationship between soil tests, tissue tests and response of fruit trees to K fertilizer has not been adequately determined.

 

K is strongly sorbed by soil components and thus is not readily mobile in soils.

Best indicators: 

 “Leaf analysis in conjunction with soil testing of samples collected from both the topsoil and subsoil provides the best estimate of the amounts of potassium that are available for uptake by fruit trees.” (From Tree Fruit Nutrition book)

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Mobility in plant:

Remobilized within plant

Deficiency symptoms:

• Symptoms are visible when the concentration is below 0.75% of leaf dry weight.  Leaves containing 1% K do not show visible signs but fruit will be smaller or poor color development occurs.

• Older leaves are affected first. Leaf scorch will show up first. Undersides of leaf margins may become darkened or browned (etching). Slow growth, tip and marginal chlorosis/necrosis advancing toward the midrib may also occur. Some species show a recurving of petioles along with upward rolling of leaf blades. Necrotic speckling and scorching of leaves is also common. 

• Shoots are slender and spurs are weak.

• Fruit will be smaller, poorly colored and low in acidity (lacks flavor). Fruit may hang on tree after leaf fall.  Trees may be more susceptible to winter injury and blossoms to frost injury (also if high N/K ratio exists).

• Time of fruit maturation is delayed.

Increased risk of K deficiencies on:

• Heavy crops of fruit

• Orchards fertilized with high N levels

• Inadequate watering

Excess problems/Interactions with other elements:

• Magnesium, manganese, and calcium deficiencies become more pronounced with excess K.

• High levels of Ca can limit K uptake in some soils.

• An adequate soil supply of boron must also be maintained in order to optimize the utilization of K (to support active root development for K uptake).

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