While the main period of nutrient uptake in corn is during stem extension or shooting (V6 to tasseling) unless centre pivot fertigation is practiced, fertilizer needs to be applied before this because crop height and density means it is difficult to enter the field with conventional machinery after this time.
Uptake and removal figures vary according to the crop being grown. The difference in nutrient offtake between grain and silage corn needs to be considered when planning the nutrient program of the following crop. There may also be significant differences in nutrient utilization between varieties.
Nitrogen is required in large quantities and helps to maximize DM growth and yield. Over 200kg/ha is needed to sustain a 7t/ha grain corn crop. Higher levels of nitrogen than any other nutrient – around 16kg/t - are removed in the grain. Corn grown for silage or corn cob mix will also remove similar quantities of nitrogen. Split applications work best, particularly on lighter soils to maintain good nitrogen availability. Too much late-applied nitrogen can lead to lodging and excessive late growth.
Phosphorus – while only needed in small quantities - is required at early stages of crop development to ensure good root growth and to boost shoot and leaf growth. Crops take up around 85kg P2O5/ha.
Potassium is needed in large quantities – at levels similar to those for nitrogen. Total crop uptake is around 200kg/ha. Most of this potassium is used in the leaf and stem and peak potassium-demand is during stem extension when uptake is faster than for any other nutrient. As a result, significant quantities are found in the stover and removed when the whole crop is ensiled. While lower amounts of secondary macronutrients are required – ranging between 25-50kg/ha for a 7t/ha grain corn crop - supplies of calcium, magnesium and sulfur are important to maintain corn yield.
The key micronutrients taken up in greatest quantities are iron and manganese. These two micronutrients are also those which are lost in greatest quantities when the whole crop or straw is removed from the field. However, boron and zinc – which are the two most important micronutrients needed in relatively large quantities – are removed in greater quantities in the grain. It is increasingly common to apply zinc with the seed at planting to ensure good rooting and shoot development.