Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Long time growers of cole crops know that the micronutrient boron is critical for production. However, newer growers may be unaware of these requirements. Boron is also subject to leaching with rainfall, particularly in our sandy soils, so available soil boron declines over time.
Cole crops have a moderate to high boron requirement. Symptoms of boron deficiency vary with crop type. Most boron deficient cole crops develop cracked and corky stems, petioles and midribs. The stems of broccoli, cabbage and cauliflower can be hollow and are sometimes discolored. Cauliflower curds become brown and leaves may roll and curl, while cabbage heads may be small and yellow. Of all the cole crops, cauliflower is the most sensitive to boron deficiencies.
It is recommended in broccoli and kale to apply 1.5-3 pounds of boron (B) per acre in mixed fertilizer prior to planting. In Brussels sprouts, cabbage, collards and cauliflower, boron and molybdenum are recommended. Apply 1.5-3 pounds of boron (B) per acre and 0.2 pound molybdenum (Mo) applied as 0.5 pound sodium molybdate per acre with broadcast fertilizer.
Boron may also be applied as a foliar treatment to cole crops if soil applications were not made. The recommended rate is 0.2-0.3 lb/acre of actual boron (1.0 to 1.5 lbs of Solubor 20.5%) in sufficient water (30 or more gallons) for coverage. Apply foliar boron prior to heading of cole crops.
Other fall crops such as beets, radishes, and turnips are also susceptible to boron deficiencies in sandy soils with limited boron fertilizer additions.