While seaweed fertilizer is often used in the garden, marine kelp (Phylum phaeophyta) is only one form of seaweed, or algae, used as fertilizer. Freshwater algae (Phylum chlorophyta) is being studied by the USDA's Agricultural Research Service, several universities and private companies for use as fertilizer. Because algae thrives in waters rich in nitrogen and phosphorus, it may become a solution to agricultural operations, where nutrient-rich manure leeches into nearby rivers, lakes and bays.

Your Own Green Gold

While you may not have access to marine seaweeds or local, algae-rich waterways, your own garden pond may be a source of algae for homemade fertilizer. Scooped it out with a net and spread it in a thin layer to dry in the hot sun, or placed it in the compost pile and the unsightly green algae becomes a source of nitrogen and phosphorus for your home garden. Because algae is an extremely wet component, Mary L. Seaman of the University of Wisconsin recommends mixing three parts fresh algae with one part wood chips for composting. Composted or dried algae may be mixed directly into the garden soil. You can also blend dried algae with commercial fertilizers and apply it to the garden at a rate of 1 1/2 to 2 1/4 tablespoons per square foot.