Vegetable brassicas have many and varied growth habits, as a result, there are big differences in the nutrient requirement between brassica types to produce a unit of yield; the ratio of the nutrients which are taken up by the plants; and the amount which is removed from the field by harvest.
When broccoli is harvested, for example, only around 20% of the total fresh weight of the crop is removed. Similarly, the buttons on brussels sprouts constitute less than 30% of the harvested material.
However, with cabbages and kohlrabi at least 50% and up to 70% of the plant is removed at harvest. Brassicas can take up high rates of nutrients. For example, broccoli utilizes more than 3kg/ha/day of N, K and Ca with uptake peaking at the start of curd development.
Nitrogen is needed in large quantities and helps to maximize growth and yield. Key nitrogen-need is during intensive leaf production. Total nitrogen-requirements vary according to brassica type and range between 1.6kg/t to 4.7kg/t.
Phosphorus is required early on in the plant’s development to ensure good root growth and to boost establishment as well as throughout vegetative growth. Brassicas utilize around 0.33-0.67kg P/t of total fresh matter.
Potassium is needed in large quantities – often at levels above those for nitrogen. Total crop requirement, depending upon brassica type is between 2.7-5.1kg/t, with significant proportions being removed in crops such as cabbage at harvest.
Calcium is the fourth most important macronutrient. It is also needed in relatively large quantities at around 2.2kg/t in broccoli and up to 2.4kg/t in cabbage. Peak calcium requirement is at the beginning of head formation. While most of the calcium is utilized for leaf growth, the relatively small amounts that are found in the harvested heads, buttons or leaves help boost crop quality and storage characteristics.
Magnesium is required at lower levels than in other crops and removals are between than 0.1 - 0.3kg/t depending on crop.
Soil and leaf tissue analyses should be used to assess nutrient imbalances. By getting the nutrient balance right, yield and quality will be maximized. This is particularly important for cations. Where potassium, magnesium or calcium is available in excess, then uptake of the other cations is reduced.