Seaweed fertilizer is a type of fertilizer made from seaweed. Seaweeds are like really big, slimy underwater plants. When seaweed is dried out, it still contains lots of things that plants need to grow like minerals, vitamins, and nitrogen. Fertilizers help give plants the nutrients they need to grow strong and healthy. Seaweed fertilizer is a good way to make sure your plants get the nutrients they need.