Kale (Brassica oleracea var. acephala) and collards (Brassica oleracea var. acephala) are leafy members of the genus brassica, which also includes such familiar vegetables as broccoli (Brassica oleracea var. botrytis) and cabbage (Brassica oleracea var. capitata). Kale and collards, along with most other brassica crops, make slow early growth and are susceptible to common pests and diseases. Consequently, these crops benefit from health- and growth-enhancing fertilizers such as the various seaweed products.
Fertilizers based on seaweed, often referred to as kelp, come in two common forms: meal and liquid. Liquid kelp is a concentrated seaweed extract that is dissolved in water for easy application as a foliar spray. With a typical nitrogen-phosphorus-potassium analysis of 0-0-1 or even 0-0-0, liquid kelp is not a significant source of major nutrients. Rather, liquid kelp is valued as a source of beneficial micronutrients and growth-enhancing hormones. To apply, mix 1 to 3 tablespoons into 1 gallon of water and spray directly on plant leaves in the early morning.
Kelp meal is made from whole seaweed plants that are dehydrated and ground up. This material is easy to spread by hand; broadcast it at a rate of about 1 pound per 100 square feet and mix it into the top few inches of soil, either before planting or as a side dressing once plants are established. The major nutrient content of kelp meal is low and somewhat variable, with typical N-P-K ratio of about 1-1-5. Kelp meal is beneficial primarily as a source of enzymes, amino acids and readily bio-available trace elements.
Kale and collards are heavy feeders, so you should not rely on seaweed fertilizers to supply the major nutrients for these vegetables. These leafy crops are especially dependent on abundant nitrogen for lush vegetative growth, and seaweed products are not a significant source of nitrogen. Also, brassica crops grow best in calcium-rich soil, and seaweed fertilizers do not supply calcium in meaningful quantities. Although kelp meal can include a significant amount of potassium, the high cost of kelp meal makes it an impractical source for this nutrient, which is abundant in most soils and in many organic soil amendments.
Minor Nutrients and Growth-Enhancing Substances
Despite their low N-P-K analysis, seaweed products contain minor nutrients and growth stimulants that are considered highly beneficial for kale, collards and other brassica crops. Kale and collards are often seeded in early spring when cloudy weather and cool, damp soil leads to slow growth and susceptibility to disease and insects such as flea beetles. These cool-season crops also suffer when summer weather turns hot and dry. The health-enhancing substances in seaweed products help kale and collards to withstand these stresses until weather and soil conditions are more conducive to vigorous growth.
- North Carolina State University: Fertilizer from the Sea -- Fish Emulsion and Seaweed Extract
- Colorado State University Extension: Organic Fertilizers
- How to Grow More Vegetables; John Jeavons
- Four-Season Harvest; Eliot Coleman
- Photo Credit Jupiterimages/Photos.com/Getty Images