 Commercial fertilizers can be toxic and pollute local waterways. Some tips for collecting seaweed. Collect when it's wet to avoid collecting other waste with it. Only collect seaweed that is loose or floating and not still growing on rocks or on the seafloor. And you can use seaweed as mulch. You can use it in compost. You can dry it and till it into your soil in your garden or even use it as a spray for your plants.