Maize yield and quality are both affected by crop nutrition. Maize is responsive to fertiliser, especially nitrogen, phosphorus and potassium. High-yielding maize crops require large quantities of soil nutrients. In a healthy maize crop, all the upper plant leaves and most of the lower leaves remain green until the crop is nearly mature. To ensure optimal use of fertilisers, a soil test should be conducted prior to planting.
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pH
Maize grows best in soils with a pH range of 5.6-7.5. If the pH is be less than 5, lime should be applied three to six months before planting. An alternative is to apply dolomite, which has a similar neutralising value to lime and applies magnesium.
Fertilisers
Nitrogen (N)
A rule of thumb for nitrogen application (kg/ha N) is:
* Maximum rate for growers aiming for a 7 t/ha dryland crop or 10 t/ha irrigated crop from ground that has been cultivated for more than 25 years and has a history of summer crops (such as sorghum, maize, millet, sunflower, cotton) or has been double-cropped with winter cereals
Symptoms of nitrogen deficiency include:
- light green or yellowish leaves, often with a V-shaped yellow area at tips of leaves
- premature drying along the midrib and leaf tips
- stunted and slow-flowering plants
- short and often poorly filled ears.
Phosphorus (P)
A rule of thumb for phosphorus application (kg/ha P) is:
Symptoms of phosphorus deficiency include:
- purplish coloured leaves
- plants with stunted growth
- delayed flowering
- poorly developed root systems
- reduced kernel size and number.
Potassium (K)
Soil testing and strip trials are recommended to help in the early detection of potassium deficiency.
A rule of thumb for potassium application (kg/ha K) is:
Symptoms of potassium deficiency include:
- a scorched appearance on the outer leaf edges of young plants
- yellow to brown discolouration on lower leaves
- a crop with a greater tendency to lodge
- a crop with small ears that fail to fill at the tip.
Zinc (Zn)
Maize is susceptible to zinc deficiency. Zinc may be applied to soil or as foliar fertiliser.
For soil application, apply 30 kg/ha of zinc sulphate monohydrate or zinc oxide into the soil at least three months before planting. This may correct the deficiency for up to eight years.
Fertilised Maize
For foliar application, apply two to four weeks after emergence. A second application may be required two weeks later. Apply 1-1.5 kg of zinc sulphate heptahydrate and an equal amount of urea in 100 L of water. Apply at a rate of 100 L/ha.
If the water is hard (i.e. with high carbonate levels), the water will turn cloudy as insoluble zinc carbonate is produced. Trickle in sulphuric acid (e.g. battery acid) as the tank is filling. The rate is 50 mL/100 L. This will make the zinc available to plants.
Zinc heptahydrate is not compatible with all herbicides. Read zinc products and herbicide labels to ensure compatibility.
Other zinc-based products are available. If these are used, follow the label directions.
Symptoms of zinc deficiency include:
- light streaking followed by a broad whitish band, starting slightly in from the leaf edge and extending to the midrib
- leaf edges, midrib and tip that remain green
- plants stunted with short internodes
- new leaves with sometimes nearly whiteleaf edges and stalks of a purplish colour.
Sulphur (S)
Some soils on the Darling Downs (intensively farmed for more than 20 years) may show responses to sulphur. Usually, 8-10 kg/ha is adequate. Gypsum at a rate of 200-400 kg/ha every three years is the most economical form of sulphur.
Symptoms of sulphur deficiency include:
- pale green or yellow young plants with only limited stunting
- pale green-yellow upper leaves on older plants, with the lower leaves remaining green (the opposite of what occurs with nitrogen deficiency).
Molybdenum (Mo)
Seed normally has sufficient molybdenum, though deficiencies can occasionallly occur in acid soils, especially those that are highly leached. Molybdenum deficiency can be treated with a foliar spray of 300 g/ha sodium molybdate. This is applied when there is sufficient leaf area (e.g. 30 days after planting).
Symptoms of molybdenum deficiency include:
- the tips of the lower leaves turning yellow, browning off and dying shortly after plant emergence
- many plants in a crop dying completely and others being short and stunted.
Deficiencies can be corrected by application of molybdenum mixed with other fertilisers. This may correct the deficiency for up to five years.
Nutrient application
Maize does not tolerate high levels of fertiliser coming into contact with the seed, so care should be taken to avoid this at planting. A starter fertiliser (small percentage of nutrient requirement) may be placed with the seed if low soil temperatures or cool waterlogged soils are likely after planting. This provides the seed with a ready nutrient source.
The starting fertiliser should not exceed 5 kg/ha N and 10 kg/ha P when sown in 90 cm rows. The application of a compound fertiliser (NPK) drilled some 5 cm to the side of the seed is preferable.
