Farmer's Weekly (South Africa)

Why plants need nitrogen


When a plant has a high level of nitrogen, its leaves turn dark blue-green, a colour that best promotes photosynth­esis. This is the process whereby light energy is collected by the plant and converted to starches and sugars, which are then used for plant growth.

When the level of nitrogen in the plant is low, the leaves have a palegreen colour and could even be yellow in extreme cases. Photosynth­esis will be insufficie­nt and the plant will therefore not grow properly.

In the case of maize and grain sorghum, an inverted V-shaped pattern on the leaf tips is a sign of a nitrogen deficiency. The leaf edges die off, inhibiting plant growth.

Nitrogen occurs naturally in the soil and in the atmosphere. Legumes work with bacteria to fix (trap) nitrogen and make it available to the plant.


Nitrogen (N) can also be provided to the plant via fertiliser. Fertiliser­s that contain only nitrogen are known as single nitrogen carriers. They include:

• LAN (28% N): limestone ammonium nitrate;

• ASN (27% N): ammonium sulphate nitrate;

• Urea (46% N);

• Ammonium sulphate (21% N);

• UAN (32% N): urea ammonium nitrate;

• ANS (19% N) and (21% N): ammonium nitrate solution.

Nitrogen is also mixed chemically or blended with phosphate (P) and potassium (K) in various ratios to meet the nutrient requiremen­ts of plants. Examples include:

• 4.3.4 (33% active Zinc [Zn], N, P and K) + 0,5 % Zn: 12% N, 9% P, 12% K + 0,5% Zn;

• 3.2.1 (25% active Zn, N, P and K) + 0,5% Zn: 12,5% N, 8,3% P, 4,2% K + 0,5% Zn;

• 2.3.2 (22% active Zn, N, P and K) + 0,5% Zn: 6,3% N, 9,4% P, 6,3 %K + 0,5% Zn;

• 2.3.4 (30% active Zn, N, P and K) + 0,5% Zn: 6,7% N, 10% P, 13,3% K + 0,5% Zn.


Nitrogen should be added to summer crops in two applicatio­ns:

• At planting, band-place it in the row mixed with other nutrients such as phosphorus and potassium. Place it 5cm below, and 5cm away from, the seed;

• Depending on the type of plant, apply the nitrogen as a top-dressing after the plant has emerged.


Nitrogen fertiliser­s that contain large quantities of ammonium and amine nitrogen have a greater acidifying effect on soil than nitrate-containing fertiliser­s. LAN (28% N), for example, has the least acidifying effect because of its nitrate content and the 20% lime included.

Ammonium sulphate contains only ammonium nitrogen and sulphur, which accelerate­s soil acidificat­ion. It is generally used in irrigation areas where the pH is high and the acidifying aspect therefore has a neutralisi­ng effect.

The table below shows the classifica­tion of nitrogenou­s fertiliser­s according to the degree to which they lower the pH value of the soil (increase its acidity).

• Source: ‘Fertilisat­ion – nitrogen’. 2001. Department of Agricultur­e. Retrieved from pdf?timestamp=1618741794­180.


 ?? FLICKR ?? The yellowish-green leaves of this young squash plant are a sign of nitrogen deficiency.
FLICKR The yellowish-green leaves of this young squash plant are a sign of nitrogen deficiency.
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