The seawater, the dirt, and the plants, are all composed of numerous substances, thus making them a mixture of substances.
The seawater is mostly composed of water (hydrogen and oxygen) and salt (sodium and chlorine), thus making it a mixture of few substances.
The dirt, it is usually a mixture of multiple types of soil and rocks, the rocks and soils have different compositions, including anything from dead and life matter.
The plants have all sorts of chemicals in them, one of the most dominant being the water, they are also a natural storage of carbon, so they have a lot of it, nitrogen as well, including numerous others, thus making them a mixture of substances.
