|Blotchy ripening in tomatoes caused by a potassium deficiency|
Potassium deficiency influences plant water regulation by affecting cell turgor and the opening and closing of stomata. It is most common on light, sandy soils, as well as chalky or peaty soils with a low clay content. It is also found on heavy clays with a poor structure.
Plants require potassium ions for protein synthesis and for the opening and closing of stomata, which is regulated by proton pumps to make surrounding guard cells either turgid or flaccid. A deficiency of potassium ions can impair a plant's ability to maintain these processes.
As potassium is mobile in the plant, it moves to the younger leaves when supplies are short. Although the growth of deficient plants may not be seriously impaired, the yield and quality of fruit are often greatly reduced. Deficient crops are prone to wilting.
Potassium deficiency causes yellowing and scorching of older leaves. These symptoms begin at the margins of the leaf and spread between the veins towards its centre. Large areas of tissue around the major veins remain green until the disorder is well advanced. A brown scorch develops in the yellow areas and spreads until the leaf is dry and papery. As each leaf dies, others further up the plant develop the same symptoms. These symptoms can develop rapidly in hot weather. Fruit may not expand fully at the stem end, although they look swollen at the tip end, a symptom that is also caused by water stress.
Potassium from a fertiliser side-dressing will move from the soil surface to the roots only if the soil is very sandy. A soil test can be used to determine the rate needed. Fertigation or drip feeding can also be used to treat a deficient crop. Foliar sprays are less effective and can burn leaves.
Prevention and cure can be achieved in the shorter term by feeding with home-made comfrey liquid, adding seaweed meal, composted bracken or other organic potassium-rich fertilisers. In the longer term the soil structure should be improved by adding plenty of well rotted compost or manure. Wood ash has high potassium content, but should be composted first as it is in a highly soluble form. Common forms of inorganic fertilizers include: Potassium Nitrate, Potassium Sulfate, and Monopotassium Phosphate.
- (2004). Nutrient disorders of greenhouse Lebanese cucumbers. New South Wales - Primary Industries - Agriculture. Agfact H8.3.3. First edition. Retrieved: 2010-07-31.
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|Adverse conditions||Bruising · Frost · Greening · Magnesium deficiency · Nitrogen deficiency · Oedema · Potassium deficiency · Soft tubers · Spindly sprouts · Split roots · Sun scald|
|Diseases||Alfalfa mosaic virus · Andean potato latent virus · Bacterial soft rot · Black dot · Blackleg · Black potato blight · Alternaria brown spot · Common scab · Dry rot · Early blight · Fusarium wilt · Gangrene · Late blight · Potato deforming rust · Potato leaf roll virus · Potato spindle tuber viroid · Potato virus Y · Potato yellow dwarf virus · Potato yellow vein virus · Powdery scab · Rhizoctonia solani · Ring rot · Septoria leaf spot · Silver scurf · Skin spot · Southern blight · Southern bacterial wilt · Tobacco ringspot virus · Tomato spotted wilt virus · Verticillium wilt · Wart disease|
|Pests||Aphid · Capsid bug · Keeled slug · Leaf miner (Pea leafminer) · Potato cyst nematode · Potato psyllid · Potato rot nematode · Root-knot nematode · Rosy rustic moth · Wireworm|
|Adverse conditions||Blossom drop · Blossom end rot · Catfacing · Dry set · Fasciation · Frost · Greenback · Hollow fruit · Leaf roll · Leggy plants · Magnesium deficiency · Nitrogen deficiency · Oedema · Phosphorus deficiency · Potassium deficiency · Split fruit · Sun scald · Tomato stem primordia|
|Pests||Aphid · Cabbage whitefly · Flea beetle · Greenhouse whitefly · Leaf miner (Tomato leafminer, Vegetable leafminer) · Potato cyst nematode · Potato psyllid · Root-knot nematode · Slug · Thrips (Eastern flower thrips)|