Where potassium, magnesium or calcium is available in excess, then uptake of the other cations is reduced. Common macronutrient ratios for nitrogen to potassium, phosphorus and magnesium for different brassica crops are shown here. Tissue analysis will confirm visible deficiency symptoms and reveal hidden deficiencies where visual plant symptoms are not yet present. The type, position and timing of the tissue selected for tissue analysis varies according to local practice.
For young plants, whole leaves of the same physiological age are sampled. For plants in mid-growth, the upper leaves are usually taken. Once the head is formed, then wrap leaves or curds can be analyzed. It is particularly important to take into account the different parts of the plant that are commonly sampled.
Around 50 % of the nitrogen in the leaf in Brussels sprouts is recycled to the growing buds. This can be aided by nitrogen-applications at the onset of bud growth which will also help green the sprout. Brassica crops are moderately tolerant to salinity. As a result, high rates of nitrogen can be applied relatively early to the crop. However, well timed, split applications of nitrogen will ensure growth is not limited and maximize yields.
Brassicas are highly responsive to applied nitrogen and in field studies, nitrogen supply is often too low to properly confirm peak demands.
The total nitrogen uptake of brassica crops is often between 200 - 400kg N/ha. However rates of up to 600kg/ha produce yield responses at no loss in curd quality in cauliflowers and broccoli. In these crops, nitrogen determines the balance between vegetative and reproductive growth.
Trials confirm that the improvement in growth provided by nitrate-nitrogen sources produces the highest yields.