The Potash Development Association (PDA) has issued an update on how best to meet the fertiliser and nutritional needs of forage maize crops.
To produce a good crop, maize plants need to grow very rapidly once they have germinated. To achieve this, a good soil structure, adequate soil moisture and warm soil temperatures (8-12°C) are essential.
Maize can easily produce 50t/ha in a period of four months and therefore has a large demand for nutrients. Any shortage will restrict early growth and final yield.
Although established crops have a well-developed root system, root growth is slow in the early days of the crop, especially if the weather is cold as was the case recently.
Poor root growth means decreased uptake of nutrients and this can be a vicious circle as poor uptake of nitrogen and phosphate can restrict root growth.
Fertiliser on maize
Experiments have shown that maize benefits from fertiliser placed close to the seed, even when the bulk of the soil is adequately supplied with nutrients.
Phosphorus (P) is required particularly by the growing tips of the plant, hence its importance for root growth.
Any shortage, especially in the very early stages, reduces root growth and nutrient uptake and this can adversely affect the growth of the crop for the rest of the season.
Typical phosphate removal is 1.4 kg P/t fresh crop – that is 55 kg/ha P for an average 40t/ha forage crop.
Potash requirement
Potash (K) is the nutrient required in the greatest amount by maize. An average 40t/ha crop takes up around 360kg/ha K by early August.
The demand for potash is particularly large in the period of rapid growth, and the crop needs to take up about 8kg/ha K per day.
The soil must be able to supply both the total demand of 360kg/ha and the daily requirement of 8kg/ha without any hindrance.
This requires an adequate level of readily plant available soil potassium. During maximum periods of growth, the maize plant will contain more potash than nitrogen.
Maize removes potash
Maize crops also remove large amounts of potash, typically 4.4kg/K/t fresh yield which amounts to 175kg/ha for a 40t/ha crop. This quantity must be replaced to maintain soil fertility.
Nutrient offtake is dependent on yield; a 30t/ha removes 130kg K and a 50t/ha crop removes 220kg/ha K.
Potassium has a number of diverse roles in plants. It plays an important role in regulating the water content of the plant.
With an adequate supply of potassium, plants can survive drought stress more easily. It is essential for the transport of sugar from the leaves to the storage organs where the sugar is converted to starch.
Potassium and turgor
Potassium plays a major role in maintaining the turgor (i.e. rigidity) of plant tissue.
Leaves need to be turgid to remain fully extended to maximise the surface exposed to sunlight that provides the energy to convert carbon dioxide (CO2) in the atmosphere, to sugars in the leaves.
Plants well supplied with potssium also seem to be less susceptible to fungal and pest attacks.
Magnesium (Mg) is an essential element in chlorophyll and hence for photosynthesis. Crop removal is 40kg/ha Mg.
The total requirement (to be supplied from manure and fertiliser) should be related to Mg level in the soil and additional Mg is only justified at soil Index 0 when 50-100kg Mg/ha should be applied every 3-4 years.
If both potash and magnesium soil levels are below the optimum then apply the larger amount (100kg Mg/ha).
Sulphur and trace elements
Sulphur is a constituent of protein, together with nitrogen.
Few maize experiments have tested the need for sulphur but deficiencies are possible where soil sulphur levels are below optimum.
Finally, maize is not very sensitive to trace element deficiencies, but boron, copper, zinc, manganese and iron may occasionally be deficient on soils where manure is not applied regularly.