In Summary
  • The three primary macronutrients required by the plant are nitrogen for vegetative growth, phosphorus for root development and potassium for flowering and fruiting.
  • The symptoms start from the old leaves, which gradually change from green to pale-yellow. Progressively, the leaves become uniformly yellow, including the veins.
  • The deficiency develops very slowly, and in some cases, the plants have a distinct purpling of the stem and the underside of the leaves.
  • 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.

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.

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.

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