Nitrogen side dressing for forage crops
Nitrogen (N) is an essential nutrient for plants and is required in the largest quantities by brassica crops. Nitrogen is also a component of plant proteins, which include compounds responsible for plant structure and nutrient storage.
A healthy growing brassica crop typically contains a minimum of two to two and a half percent of its weight as N. This is cropping demand. The amount of N needed for a crop is dependent on crop yield. The higher the crop yield, the higher the N requirement for that crop. To estimate the amount of N required for a crop, the expected yield is multiplied by the concentration of N in the crop. In other words, 1,000 kg of Dry Matter (DM) will contain 20 to 25 kg of N.
Plants can obtain some of the N from the soil and the balance needs to be supplied from fertiliser. This is called a side dressing. How much N the soil can supply to a growing crop depends on factors including soil temperature, soil moisture, carbon to nitrogen ratio of the soil, pH, and paddock history. Soil tests such as Potentially Available Nitrogen (PAN) taken to 15 cm can help estimate N that will be supplied by the soil, so N fertiliser recommendations can be adjusted to meet the crop total uptake. As a rule, if a tested paddock has had a continuous cropping history, the PAN test will be lower than what you would typically find in a paddock with a history of long-term pasture.
Applying N fertiliser at the right time and in the right amount means the nitrogen is used most efficiently and most profitably. Consider a N side dressing once the crop is established, just before canopy closure. The amount of N to apply as a side dressing will depend upon the crop demand, estimated N supplied by the soil, and the amount of N that has already been applied up to that point.
If weather conditions become dry, the potential yield of the crop will be influenced by lack of soil moisture. The amount of N to be applied at a side dressing will need to be adjusted accordingly to suit the conditions and the projected yield. If dry conditions are a common occurrence, plan to apply your nitrogen early to encourage the crop to establish before weather conditions create stress on the plant.
Typically, N side dressings are in the form of urea. However, if rainfall is not guaranteed to fall within eight hours of applying N side dressing, use a urease-inhibited urea product such as SustaiN to reduce potential N loss to the atmosphere.
To organise a PAN test and estimate your N side dressing requirements, contact your local PGG Wrightson Technical Field Representative.
