Potassium deficiency in tomato plants

The leaves on the right-hand photo (below) show marginal necrosis (tip burn). The leaves on the left-hand photo show more advanced deficiency status, with necrosis in the interveinal spaces between the main veins along with interveinal chlorosis. This group of symptoms is very characteristic of K deficiency symptoms.

The onset of potassium deficiency is generally characterized by a marginal chlorosis, progressing into a dry leathery tan scorch on recently matured leaves. This is followed by increasing interveinal scorching and/or necrosis progressing from the leaf edge to the midrib as the stress increases. As the deficiency progresses, most of the interveinal area becomes necrotic, the veins remain green and the leaves tend to curl and crinkle. In contrast to nitrogen deficiency, chlorosis is irreversible in potassium deficiency. Because potassium is very mobile within the plant, symptoms only develop on young leaves in the case of extreme deficiency.

Role of potassium in tomato plants

High levels of potassium provide high yields in tomato crops. Tomatoes have a relatively high potassium requirement. There is usually 5.2 to 7.2 lb (approx. 2.36 to 3.27 kg) of K taken into the plant for every tonne (1000 kg) of tomato harvested.

Establishment - Promote strong early growth
Vegetative Growth - Maximize concentrations in leaf tissue prior to flowering
Flowering - Fruit Set Maintain plant growth and maximize flower numbers
Fruit Ripening - Maturity Maximize high potassium levels in the fruit and minimize disorders

Tomato plants need potassium because:

• It is essential for protein synthesis.
• It is important for breakdown of carbohydrates, a process which pro- vides energy for plant growth.
• It helps control ionic balance.
• It is important for the movement of heavy metals such as iron.
• It helps the plant to overcome and ward off plant diseases.
• It is important for fruit formation.
• It improves hardiness to overcome adverse weather conditions.
• It is involved in the activation of more than 60 plant enzyme systems that regulate grow rates.
• It assists in controlling guard cells that open and close stomata on the under- sides of leaves that can reduce stress during drought.

Therefore, potassium deficiency can cause poor plant growth and reduce overall yield. It is important to soil test for potassium and adjust levels and pH before planting a tomato crop or any crop. Tissue testing tomato leaves is also a way to monitor potassium levels during the season to ensure proper fertilization through the drip system or as a side-dress application.

Tomatoes have a relatively high potassium requirement compared to nitrogen with over 267 lb/ac of potassium typically being utilized. Potassium is needed throughout the season and is a major component of the fruit at around 250mg K per 100g of fruit. It is essencial to maintain a good balance of potassium with magnesium and calcium. Too much potassium restricts the uptake of these other cations. Use of high levels of potassium is particularly important under saline conditions to maintain plant growth. Excess sodium reduces the uptake and transfer of potassium through the plant and thus potassium levels need to be increased in order to maintain plant growth.

Symptoms

The leaves become dark green and yellowish to white necrotic dots develop near the leaf margins of the older leaves. These merge into brown necrotic areas around the leaf margins. Greenhouse grown tomatoes will often express K deficiency as ‘blotchy’ ripening. The fruit also lacks firmness and has low brix levels.

Deficient plants may be more prone to frost damage and disease, and their symptoms can often be confused with wind scorch or drought.

In tomatoes, the stems are woody and growth is slow. Leaves are blue-green in color, and the intervenal area often fades to a pale gray color. Leaves may also have a bronzed appearance and yellow and orange patches may develop on some of the leaflets. Fruits often ripen unevenly and sometimes have green patches near the stalks.

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.

Prevention and treatment

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 fertilizers. 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.

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.