Fine-tuning the nutrient program for your brassica crops can help reduce the incidence of nutritional deficiencies and improve overall quality and yield. Paying attention to plant symptoms that indicate improper nutrition, conducting regular soil tests, and properly timing your fertilizer applications are three key steps to maximize plant nutrition.
Carl Rosen, head of the Department of Soil, Water, and Climate at the University of Minnesota, provides details on what symptoms to watch for in nutritionally deficient plants, IPM strategies to improve uptake, and application timing.
Common Deficiencies In Brassicas
Nitrogen is the nutrient brassicas require the most, followed by potassium and phosphorus. Boron is another nutrient growers will need to watch for, especially on sandy soils, Rosen says. If the plant has a boron deficiency, that can lead to hollow heart and browning.
He also mentions calcium as a recurring problem for many growers, explaining that the nutrient is typically not missing from the soil, but that localized deficiencies can sometimes occur because the plant is unable to transport the nutrient to young, growing tissue.
“Calcium is not transported as well into new parts of the younger leaves, and so the tips of those leaves will brown. It’s not as much a problem with cauliflower because you can clip those leaves away and the curds are usually OK. Where it can be more of a problem is with cabbage. All of those leaves are tightly packed, and if you get a little bit of browning in those leaves, it really detracts from the quality,” he says.
This can occur from uneven watering or fast growth from excessive nitrogen, Rosen explains, adding that foliar calcium applications are not much help.
“Make sure you have even water; don’t overdo the nitrogen,” he adds.
Molybdenum is another micronutrient sometimes deficient in brassicas that is needed in very small amounts. Cauliflower deficient in this micronutrient can result in whiptail, which is when leaves narrow and curl upwards.
“Molybdenum, unlike other micronutrients, becomes less available at low soil pH levels, but as the pH increases, you’ll often see those problems resolved,” he says.
Manage Your Soil
Maintaining soil health is the first step toward proper nutrient management. With nitrogen, because it is cycled through the soil via organic matter, soils with higher levels of organic matter tend to provide more nitrogen to the plants, while sandy soils tend to be more deficient, Rosen says.
Regarding levels of phosphorus, potassium, and other micronutrients, Rosen says your best bet is to take a soil test.
“There are really good soil tests that can predict the probability of a response to added fertilizer and predict the need for fertilizer before planting,” he says. “I’d also recommend an organic matter test. The more information you know, the better decisions you can make.”
He also suggests testing the soil pH, and says to make sure that you do any pH adjustments well within advance of planting.
“As with most vegetable crops, you want to be in the slightly acidic range. A pH between 6 and 7 is ideal for nutrient availability,” he says.
Application Timing And Methods
Fertilizer timing will depend on whether or not the nutrients are mobile or immobile. Nutrients such as phosphorus and potassium, which are immobile, need to be applied prior to planting or right at planting, Rosen explains.
“With the immobile nutrients, you want to make sure they’re incorporated to a depth of 6 to 8 inches so they are effective. If you use any starter fertilizer, make sure you apply it 2 to 3 inches to the side and 2 to 3 inches below the transplant,” he says.
For nitrogen, which is mobile, Rosen suggests a split application, which is one application at or around planting and then another two or three weeks after transplanting.
In general, Rosen says brassicas can take anywhere between 100 to 180 pounds of nitrogen, up to 150 pounds of phosphorus, and up to 250 pounds of potassium per acre, depending on soil test levels.
The amount of nitrogen or potassium applied to the crop will largely depend on the texture and native fertility of the soil. For example, crops grown on sandy soils should receive lower levels of nitrogen and potassium at more frequent intervals than those grown on finer textured soils.
For organic growers, he suggests using composted manure prior to planting preceded by a cover crop, likely a legume that will contribute nitrogen.
“Make sure you use a good source of compost. Monitor the growth of your plant and maybe do a tissue test just for diagnostic purposes,” he says.
Rosen warns, however, that composts differ in their composition, so that may need to be evaluated as well.
As far as what fertilizers work best, he says it’s less important to identify what the specific fertilizer is and more important to apply the correct amount.
“Go with a reliable source, read the label closely, and make sure you put the right source on at the right rate, at the right time,” he says.
In the end, after you’ve received your soil tests and have made the necessary adjustments, the only thing left to do is monitor the crops and fine-tune your management system from there.
“Because soils vary a lot and growing conditions vary a lot, you have to use your tests to give you a baseline,” Rosen says. “After that, you can just adjust from there.”