Phosphorus deficiency in tomato plants

A phosphorus deficiency in your plants will cause stunted growth and prevent them from producing fruit as expected, yet is is often overlooked or misdiagnosed as a nitrogen deficiency or simply as poor soil. Although it is a bit difficult to determine for the untrained eye, with a little effort you can lean to recognize signs that your plants are suffering from a phosphorus deficiency. Correcting the problem early will resolve the issue and your plants will thrive.

Left - Normal plant
Right - Phosphorus deficiency

The necrotic spots on the leaves on the figure are a typical symptom of phosphorus (P) deficiency. As a rule, P deficiency symptoms are not very distinct and thus difficult to identify. A major visual symptom is that the plants are dwarfed or stunted. Phosphorus deficient plants develop very slowly in relation to other plants growing under similar environmental conditions but with ample phosphorus supply.

Phosphorus deficient plants are often mistaken for unstressed but much younger plants.

Developing a distinct purpling of the stem, petiole and the lower sides of the leaves. Under severe deficiency conditions there is also a tendency for leaves to develop a blue-gray luster. In older leaves under very severe deficiency conditions a brown netted veining of the leaves may develop.

Need of phosphorus

Phosphorus is not needed in large quantities. Around 44.6lb/ac is all that is required during the season for field grown crops. Phosphorus is used during early growth ensuring a good start for the crop and again at flowering and fruit set.

Establishment - Maximize root development
Vegetative Growth - Ensure continued growth
Flowering - Fruit Set - Fruit development
Fruit Ripening - Maturity - Boost tomato nutritional quality

Symptoms of phosphorus deficiency

When deficient in phosphorus, tomatoes have rigidly erect leaves which are dark green to bluish green in color. The stems are thin and fibrous with a dull purple discoloration. Flowering and fruiting is poor, fruit are small and firm and yellow prematurely. In some cultivars, phosphorus deficiency can also appear as small brown areas which develop interveinally on the lower leaves.

These are the five symptoms of phosphorus deficiency:

1. Small thin stalks. Plants that do not receive enough phosphorus develop weak thin stalks that appear spindly. Healthy plants need strong stems to support the weight of fruit. Thin stocks not only are not able to support the fruit of the plant, they break easily in the wind and may not survive the season. If your plants fail to develop stocky stems, phosphorus may be the culprit.
2. Stunted growth. Plants suffering from a phosphorus deficiency may be stunted and short in stature. In initial stages they may appear healthy and look like much younger plants. If your plants are smaller than expected or smaller than other plants of the same age, they may need addition phosphorus.
3. Purple veins may appear on the leaves or the leaves may take on a purplish color. Check both the top and bottom of the leaf for purple veins. This is evident on new growth as well as older leaves. In severe cases the entire leaf may take on a purple hue.
4. Older leaves may appear bluish-green instead of the characteristic rich green of new growth. Unless the plant normally has a bluish tint, blue-green leaves signal a need for phosphorus.
5. Reduced blooms and/or onset of fruit. Phosphorus promotes blooming and fruit production. A deficiency may cause the plant to stop producing blooms or the blooms and fruits to be small and lack their usual color. For good fruit production plants require phosphorus.

Treatment

A phosphorus deficiency can be caused by inadequate amounts of phosphorus in the soil, but due to the nature of phosphorus absorption it can also be caused by low soil temperatures. This is particularly tree of tomatoes. Adding plastic mulch to increase soil temperatures may solve the problem. To increase the level of phosphorus in the soil apply high phosphorus fertilizer usually labeled as fertilizer for blooming plants. Water soluble fertilizer works the fastest and may bring results in a short period of time.