Balanced nutrition

The high energy and protein content of rapeseed puts significant strain on nutrient supply.

To fuel photosynthesis and to exploit nitrogen efficiently, potash, phosphorus, sulfur, magnesium and micronutrients need to be readily available to meet high demand of this crop.

What are the actual needs and how to best fit them?

Nutrient needs

Rapeseed requires high amounts of nutrients (figure 2). Significant amounts of nitrogen, phosphorus, magnesium and sulfur are removed by grain while most of the potassium is found in crop residues and can be used by the following crop. The amount of residual nutrients and a deep tap root, amongst other benefits, makes rapeseed an excellent preceding crop.

Figure 2: Uptake and removal of main nutrients by rapeseed for a yield of 45 dt/ha. Total uptake concerns the entire vegetation phase. Removal concerns grain yield exported from the field. Nutrients in residues are returned to the soil and contribute to a large extent to the nutrition of the following crop [2].

Even though peak uptake of P, K and Mg is between early stem elongation and flowering they have to be available already for early establishment, root development and leaf growth.

Sulfur needs

Oilseed rape is one of the most sulfur demanding crops and application of 20 to 40 kg S/ha (50-100 kg SO3) in spring is required to avoid any risk of S deficiency.

Micro nutrients needs

Boron, manganese and molydenum are most important. To avoid deficiencies limiting yield, these micronutrients can be most efficiently provided by foliar applications in autumn and prior to flowering with, e.g., YaraVita® Brassitrel and Bortac.

Fact

75-80 % of nitrogen is already absorbed before flowering. During the period of peak nutrient demand, from stem elongation to early flowering, the high uptake rate of 4-6 kg N/ha per day can be best assured with nitrate nitrogen. Any nitrogen deficiency at this stage reduces branching, pod set, grain set and rain weight.

Figure 3: Cumulative nitrogen uptake during the growing cycle. Maximum dry matter and N accumulation can only be sustained by readily available nitrate nitrogen [3].

Nitrogen timing, rate and form therefore are critical. After flowering only 20 to 25 % additional nitrogen is taken up by the crop. Most of the nitrogen in the mature rapeseed is translocated from the leaves and stems (figure 3). Excessive nitrogen, however, produces adverse effects such as higher incidence of diseases, lodging, too much vegetative growth and delay of maturity. Furthermore oil content is reduced and glucosinolate content increases.

Autumn nitrogen application

Oilseed rape is very efficient to capture residual nitrogen from the soil in autumn, preventing winter leaching. But additional nitrogen application in autumn is required if residues from the previous crop are too low for sufficient nitrogen supply. 

Winter rapeseed needs to accumulate significant amounts of biomass between sowing and the winter dormancy period in order to initiate sufficient leaf, branch and flower meristems developed to ensure a high yield potential. Nitrogen uptake during this period needs to be about 50 to 80 kg/ha with 15-25 kg stored in the roots. Part of the nitrogen accumulated in autumn is lost during winter with falling leaves, but remineralized and taken up again in spring.

Reasons for autumn application can be:

  • Low residual nitrogen in the soil
  • N fixation by decomposition of large amounts of straw
  • Compensation for late sowing
  • Achieve a growth stage that ensures good winter hardness and yield potential, i.e. 8-10 leaves and root collar diameter of 8 mm.

Figure 4: Rapeseed yield increases with autumn fertilization. Results from 4 years on 2479 plots in Germany [4].

Several field trials in Germany have demonstrated consistently superior yield and oil content with autumn N application if N uptake before winter dormancy has been less than 50 kg N/ha (figure 4). Readily available nitrate nitrogen is most suitable to promote dry matter production and to improve winter hardness by boosting potassium uptake.