Life is dependent on the availability of about twenty elements.  These are required in the biochemical processes of all organisms, such as nitrogen, phosphorus, zinc and molybdenum, and an additional ten or so required by different species, usually in trace amounts. Those elements needed in relatively large amounts are generally referred to as macronutrients; those needed in trace amounts are called micro- nutrients. In general, macronutrients include the following two groups:

(1) those which constitute more than 1 per cent each of dry organic weight—carbon, oxygen, hydrogen, nitrogen, and phosphorus;

(2) those which constitute 0.2 to 1 per cent of dry organic weight—sulphur, chlorine, potassium, sodium, calcium, magnesium, iron, and copper.

Some of this latter group of macronutrients may be micronutrients for some species and some of the micronutrients about to be listed may be macronutrients for other species. Micro-nutrients, constituting less than 0.2 per cent of dry organic weight, although not present in all species, are aluminum, boron, bromine, chromium, cobalt, fluorine, gallium, iodine, manganese, molybdenum, selenium, silicon, strontium, tin, titanium, vanadium, and zinc.

The availability of nutrients occurs by way of precipitation, dust, and weathering or output by way of runoff and erosion. Nutrients may get bound up in the biomass of an ecosystem for long periods of time, as in the trunks of standing trees in a forest.  From both these perspectives consideration must be given to the availability and source of nutrients in, which implies investigating the budget of nutrients for particular ecosystems.