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Agronomist notebook: Know signs of various nutrient deficits in crops

Saturday July 20 2019

Stanley Ngeno checks on vegetables in his farm in Bomet.

Stanley Ngeno checks on vegetables in his farm in Bomet. The three primary macronutrients required by the plant are nitrogen for vegetative growth, phosphorus for root development and potassium for flowering and fruiting. PHOTO | STANLEY KIMUGE | NMG 

ANN MACHARIA
By ANN MACHARIA
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The other day I met Samuel, a teacher and farmer who grows crops in the field. He had invited me for a tour of his farm as he sought to get things right.

As we moved around his two-acre farm, I was impressed by how he had mastered the art of practising sustainable agricultural practices that include agroforestry, mixed cropping, and pomology.

Such diversity is essential since a farmer is assured of some income from one crop in case things go wrong with another.

What made me earn the invite is that a test had showed his soil was lacking some major macro-nutrients.

The three primary macronutrients required by the plant are nitrogen for vegetative growth, phosphorus for root development and potassium for flowering and fruiting.

More often, farmers confuse the deficiency symptoms in crops. The purpling in cabbages is due to lack of nitrogen in the soil and not phosphorous as Samuel thought.

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Yellowing of crops is more often confused with nitrogen deficiency but the plants yellow due to various reasons. For example, on the farm, the entire collard greens (sukuma wiki) leaves were yellow, an indication of nitrogen deficiency.

The symptoms start from the old leaves, which gradually change from green to pale-yellow. Progressively, the leaves become uniformly yellow, including the veins.

This means that the farmers should top-dress with nitrogenous fertilisers such as urea to curb the situation.

On his tomato farm, the crop’s leaves were yellowing, but the veins were still green, an indication of magnesium deficiency.

Iron deficiency results in yellowing at the tip of the plants. This is mostly in young seedlings, especially those grown in a soilless media.

In most crops, phosphorus deficiency develops on the older leaves, showing necrotic spots and plants are dwarfed and have stunted growth.

The deficiency develops very slowly, and in some cases, the plants have a distinct purpling of the stem and the underside of the leaves.

UNAVAILABILITY OF THE MINERALS

In cases where the leaves’ edges appear burnt, and the plants seem to be lodging, this is potassium deficiency. More often than not, the stem of the plant is usually also sensitive to disease infestations hence the need to top dress with potassium nitrate.

Sometimes failure of the crop to take up the nutrients is not only due to unavailability of them in the media or soil.

However, it may be due to injured roots, waterlogging, too dry, too acidic or too alkaline soils.

The macronutrients are usually absorbed when the pH is slightly alkaline while the micronutrients are absorbed when the pH is slightly acidic hence the need for farmers to check on the pH of their soil and water.

Proper indication of the deficiency symptoms helps in the correct control.

While identifying the deficiency symptoms, it’s critical to determine if they are due to mobile or immobile nutrients based on where the symptoms first appear in the plants.

Mobile nutrients are always in a position to move from the older leaves to younger ones where supply is inadequate. They include nitrogen, phosphorus, potassium and magnesium.

Immobile nutrients do not move readily within the plants hence deficiency appearing in the young leaves can also be localised. This includes micronutrients such as iron and zinc.

To prevent such nutrient deficiencies, farmers should conduct soil analysis to know the one’s available in the soil. This helps in controlling the deficiencies’ symptoms before the crop is affected.

Also, a farmer should seek advice from experts to determine the type and quantities of fertilisers to use during the production period to avoid excess or inadequate application.

The method of fertiliser application, form of the fertiliser, that is, liquid or solid, and what stage of the crop was it applied is also critical.

For example, application of nitrogenous fertilisers during planting is not ideal since they are soluble, which makes them be leached before the plant absorbs it.

Also, nitrogenous fertilisers help the plant in vegetative growth hence the need to top dress it.