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 dalrrd.gov.za/Portals/0/InfoPaks/nitrogen. pdf?timestamp=1618741794180.
ADD NITROGEN IN TWO APPLICATIONS: IN THE SOIL AT PLANTING AND AS A TOP-DRESSING AFTER EMERGENCE