Kelp is a seaweed that can be used as fertilizer. Kelp can be used as a natural fertilizer because it contains high levels of nitrogen, phosphorus, and potassium. Kelp also contains significant amounts of other nutrients, including calcium and magnesium. Kelp can be applied as a foliar spray or ground into a pulp and spread on the soil as a fertilizer. Kelp is also effective at controlling weeds.