Nature is awesome. I say this because it always takes care of itself; case in point, the link above will show you a self-contained ecosystem that relies on nothing more than the sun and a little water.
The back-story of this experiment goes back 54 years to Easter of 1960. According to British newspaper Daily Mail, "Using a ten gallon jug, (David) Latimer poured in some compost, a half-quart of water and carefully lowered in a spiderwort seedling... He then placed the bottle garden by a sun-filled window... and let photosynthesis do its thing." This is where the surprise comes...
He waits TWELVE YEARS to water it again, in 1972, and it hasn't been watered since! How is this possible? Well, the same way we don't have to water the earth; the water cycle (think back to 3rd grade) has water evaporating, condensing, and precipitating. Same thing here where the energy trapped by the glass causes the little bit of water involved to go through this process and, meanwhile, the plant is going through its own breathing process, refreshing the nutrients in the soil.
I may have to try this experiment. It's nothing more than some fertile soil, a plant, a little water and a lot of patience. Earth in a bottle!