Agronomist's notebook: Yellowing of plant leaves; causes, remedies

Many farmers believe yellowing of crop leaves, which is known as chlorosis, is caused by nitrogen deficiency.

Therefore, to solve the problem, they add nitrogenous fertilisers to the soil or the growing media.

But this certainly does not solve the problem for some, because yellowing in plants can be caused by nutritional or environmental problems such as water stress due to under-watering or overwatering, which results in waterlogging.

Compacted soils and change in soil pH also cause the problem.

In some cases, it’s easy to fix the problem immediately but sometimes this requires one to try several solutions until there is an improvement; for example, adjusting the soil pH to allow nutrients uptake.

Yellowing of leaves is normally an indication of plant stress. It leads to stunted growth for the leaves or the entire crop since photosynthesis does not take place effectively due to lack of chlorophyll.

The chlorotic leaves are more prone to scorching and leaf diseases, which makes the plant unhealthy. Sometimes the yellowed leaves fall off prematurely but if the conditions are corrected, the plant can regain its healthy state and develop new leaves.

Under-watering normally results in yellowing and eventually leaf fall as excess water leads to poor aeration and poor root development. This can be corrected by providing adequate water to the plant and encouraging proper drainage, for example by raising beds in poorly drained soils.

The leaves can also turn yellow due to root injuries or compacted soils that prevent proper root development. This makes the roots unable to take up nutrients from the soil, eventually leading to yellowing of leaves.

To prevent this, care should be taken while carrying out management practices like weeding to avoid root injuries. Adding manure to the soil during land preparation helps improve the soil structure.

Iron deficiency symptoms usually start on young terminal leaves and spread out to the older ones. It is rarely caused by lack of iron in the soil but is due to the high soil pH (alkalinity) which inhibits uptake. Soil pH refers to the acidity or the alkalinity of the soil, which affects different nutrients’ uptake.

LOWER OR OLDER LEAVES

Iron uptake takes place when the pH is slightly acidic, hence the need to lower it in case it is high using sulphur – such as gypsum to 5.5-6.5. It’s also a common problem when using cocopeat as a growing media and use of water with a high pH.

The pH of water can then be lowered using an acid such as nitric.

Iron is an important element that is required by the plant in small quantities and plays a role of enzyme function in plants. Chelated iron can be used to supply the mineral to the plants if all other factors are constant. Foliar fertiliser with iron can also be effective.

Nitrogen deficiency starts on the lower or older leaves. It’s easily diagnosed especially when the older leaves are turning yellow and the younger leaves are dark-green. This is because nitrogen moves from the older tissues to younger ones.

In most cases, the whole leaf turns yellow or pale-green. This lowers the leaves photosynthetic surface area, which helps the plant to make its own food, thus reducing yields, especially for leafy vegetables such as sukuma wiki, spinach, and lettuce.

Nitrogen is highly soluble in water, making it susceptible to leaching beyond the root zone. This is a common problem with areas that are prone to flooding and have a poor drainage.

Nitrogenous fertilisers should thus be provided to the plant, with the quantity depending on the soil test analysis. They should be top-dressed to encourage the plants’ vegetative growth.

Unlike nitrogen, magnesium deficiency also results in yellowing of leaves while the leaf veins remain green. This also begins from the older leaves and progresses to the younger ones as the deficiency progresses.

Pests such as aphids that feed on the plants by sucking the sap also lead to yellowing of the leaves especially if they are in large populations. Aphids are highly prolific, hence the need to control them before they multiply.

Plants that receive less light often start yellowing on the side that is away from the source of light.

Adequate light should, therefore, be provided to the plants. An ageing plant starts yellowing on the lower leaves, which eventually drop off, leading to low yields. The farmer should consider clearing the crop, especially for leafy vegetables.

It is, therefore, vital for the farmer to identify the major cause of yellowing in plants and correct them accordingly.