Forage Maize Fertiliser Requirements

Nutrition of the maize crop

To produce a good crop, maize plants need to grow very rapidly once they have germinated. They will do this if the soil moisture and structure are good, the temperature is warm and nutrition is adequate. Maize can easily produce 50 t/ha in a period of four months. To support this rapid growth it has a large demand for nutrients and 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. Poor root growth means decreased uptake of nutrients and this can be a vicious circle as poor uptake of N and P can restrict root growth. For this reason experiments have shown that maize, like many vegetable crops, benefits from some N and P fertiliser placed close to the seed, even when the bulk of the soil is adequately supplied with nutrients.

Nutrient uptake

Nitrogen (N) is the key nutrient for obtaining maximum yield and quality, but also the one most difficult to get the amount available to the crop just right to achieve optimum yields. This is because the amount of fertiliser N to apply should supplement the amount that becomes available during the growing season from the mineralisation of soil organic matter. Nitrogen encourages leaf growth and an inadequate supply means smaller leaves, reduced photosynthesis producing less starch for storage in the grain, and less total yield and protein. If the nitrogen supply is too large, then there will be excessive leaf growth increasing the proportion of leaf and stem to grain at harvest. Thus the starch content of the silage will be reduced and yields may also suffer. Too much nitrogen may also delay maturity and result in lodged crops. The average maize crop yielding 40 t/ha removes 160 kg N/ha although peak uptake is 210 kg N/ha.

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 P₂O₅/t fresh crop - that is 55 kg/ha P₂O₅ for an average 40 t/ha forage crop.

Typical symptoms of P deficiency (illustrated) are reddening or purpling of older leaves. These are often seen when conditions in the early part of the growing season are cold and wet and growth is inhibited and by soil structure problems that limit the availability of soil P for root uptake.

Potash (Potassium, K) is the nutrient required in the greatest amount by maize. An average 40 t/ha crop takes up around 360 kg/ha K₂O by early August. The demand for potash is particularly large in the period of rapid growth and the crop needs to take up about 8 kg/ha K₂O per day. The soil must be able to supply both the total demand of 360 kg/ha and the daily requirement of 8 kg/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 K than N.

Maize crops also remove large amounts of potash, typically 4.4 kg K₂O/t fresh yield which amounts to 175 kg/ha for a 40 t/ha crop. This quantity must be replaced to maintain soil fertility. Nutrient offtake is dependent on yield; a 30 t/ha removes 130 kg K₂O and a 50 t/ha crop removes 220 kg/ha K₂O.

Potassium has a number of diverse roles in plants. It plays an important role in regulating the water content of the plant and with an adequate supply of K 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. It 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 in the atmosphere to sugars in the leaves. Plants well supplied with K also seem to be less susceptible to fungal and pest attacks. Poor cob fill resulting from K deficiency is illustrated.

Magnesium (Mg) is an essential element in chlorophyll and hence for photosynthesis. Crop removal is 40 kg/ha MgO. 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 50-100 kg MgO/ha should be applied every 3-4 years. If both potash and magnesium soil levels are below the optimum then apply the larger amount (100 kg MgO/ha). The picture illustrates typical magnesium deficiency symptoms of browning/reddening of the leaf tips and margins and pronounced yellow striping of the leaf veins.

Sulphur (S) is a constituent of protein together with nitrogen. Few maize experiments have tested the need for sulphur but deficiencies are possible in areas where the soil sulphur level is below optimum. Sulphur is applied in animal manures, but much of this is unavailable.

Trace Elements. 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.