ABSTRACT
Energy may flow through to us from the Sun, but the Earth is finite and has material limits. All life on Earth requires water and mineral elements to survive. For example: the water that makes up 70 per cent of our bodies; the phosphorus that is incorporated in our bones and in the molecule adenosine tri-phosphate (ATP) that powers our cells; the nitrogen that is combined in the amino acids that makes up the proteins in our muscles; the potassium that we need for cellular reactions and osmotic control; the sulphur that makes up some of the amino acids in our proteins. Even the very carbon we eat in our food and breathe out as carbon dioxide. If we only used these once then they would have run out long ago, and life would have faded away. Instead, they are part of the great cycles, where each is taken up and used by plants and animals (including humans) and then returned once again to the Earth. Both the hydrologic and sedimentary cycles are intertwined with the distribution of six important elements: hydrogen, carbon, oxygen, nitrogen, phosphorus and sulphur. These make up 95 per cent of living things (Daily 1997).
