As this season is winding down and planning for next year begins, it’s a great time to consider nitrogen applications for your brassica crops as well as what nitrogen credits are made available to the crop following your brassicas.
There’s tremendous variability in the rates and application methods that growers use. Many fail to maximize the efficiency of their fertilizer by applying it at times when the crop doesn’t need it but weeds will hungrily accept it.
Most brassicas have similar seasonal N uptake demands, as depicted in the graph below (fig 1.), which plots nitrogen uptake over a typical cauliflower growth cycle. You can see that N demand dramatically increases around 25 days after transplanting, and 75% of the plant’s nitrogen needs are during the last half of its lifecycle.
Fig 1. Cauliflower seasonal N uptake [1]
Recent research from Cornell shows that when growers apply the same rate of fertilizer but choose to split-apply their nitrogen instead of only applying pre-plant, average yields increased by close to 5% representing an increase from 32.8 ton/A to 34.2 ton/A [2]. Across multiple farms and three years of study, the researchers found a 50:50 ratio of fertilizer application (at-planting : side-dress) produced the greatest yields when the side-dress application was banded at 30 days after planting.
By split-applying your nitrogen, not only are you matching your fertility to the demands of the crop, you are also minimizing the fertility available to your weed bank and reducing losses through leaching. And, by banding your fertilizer, the nutrients in the areas between rows are minimized whilst your brassica crop has maximum availability.
Brassicas, especially cabbage, are excellent N scavengers. This coupled with the fact that much of the plant is left in the field after harvest means that you can expect residual N the following year of up to 10 lb/A. A typical cabbage crop requires 200 lb/A of N over a growing season. After harvest, there is roughly 100 lb/A in the leaves, stump, and roots left on the field. This residue breaks down and is mineralized, ready for plant uptake the following season. Plant available N levels the season following cabbage averaged 8-10 lb/A [3]. This might not seem like much, it’s clearly no legume—but it’s definitely worth considering as fertilizer costs steadily climb.
Tim Morcom
References:
[1] UC Davis, Cauliflower uptake and partitioning, http://geisseler.ucdavis.edu/Guidelines/N_Cauliflower.html#References
[2] Hoepting. C, Understanding Nitrogen Use In Cabbage: New York Study, CCX Cornell Vegetable program, https://rvpadmin.cce.cornell.edu/uploads/doc_857.pdf
[3] Hoepting. C, Understanding Nitrogen Use In Cabbage: New York Study, CCX Cornell Vegetable program, https://rvpadmin.cce.cornell.edu/uploads/doc_857.pdf
