How to Make Homemade Seaweed Fertilizer

Close-up photograph of seaweed on a sand and pebble beech.

Seaweed is an excellent source of nutrients.

Seaweed is an abundant source of minerals and nutrients that will feed plants and, when mixed with earth, replenish tired soil. As creating a seaweed solution for plants is a relatively simple process that is inexpensive, knowing how to do it is an important tool for any gardener with access to the sea.

You Will Need

  • Seaweed
  • 1 bin or barrel
  • Water
  • 1 long mixing spoon
  • 2 bricks
  • Thin wooden slats
  • Barrel tap

How to Make Seaweed Fertiliser

  1. Collect some seaweed. The precise amount to collect will depend on the size of the bin or barrel you intend to use to make your solution. You should collect enough seaweed to approximately half fill the barrel.
  2. Wash the seaweed thoroughly in fresh water to remove excess salt and debris, as this will pollute your seaweed solution.
  3. Place the bricks in the bottom of your bin or barrel and cover them with the slats to form a raised level in the bottom of the container beneath which liquid can collect
  4.  The slats should not be tightly together, as fluid needs to be able to seep through as the seaweed solution forms.
  5. Place the seaweed into the bin or barrel until it is approximately half full.
  6. Top the barrel off with water.
  7. Allow the container to stand for eight weeks without a lid or covering on it.
  8. Stir the contents thoroughly every other day, making sure that you get right down to the wooden slats, as it is important that the material circulate.
  9. After eight weeks, fit a barrel tap to the bottom of the container and drain out the liquid.
  10. Create a solution of 1 part seaweed liquid, 80 parts water.

Top Tip: Don’t simply throw out the contents of the barrel once you have made your seaweed solution. The decomposing seaweed matter left in the barrel after you tap it can be mixed with soil to make an effective compost. You can then use this to replenish tired soil, or nourish your indoor plants.

Please Note: When seaweed decomposes to create the fluid you then mix with water, it will smell very bad. You should therefore keep this in mind when deciding where to place your barrel for two months, as if it is too near your house it will create an unbearable stench. Also consider your neighbors, so you avoid making a nuisance.


Photograph courtesy of Mike Pennington. Used under a Creative Commons licence.