When a farmer a sends a sample of soil energy to the soils laboratory of his state university for routine testing, the sample is often treated with 0.1 normal acid or alkali solution. The quantity of minerals, such as phosphorus, calcium, or potassium, removed by such gentle treatments is considered a crude measure of quantities available to plants (that is, the size of the available pools). A simple soil energy test such as this may provide a useful basis for fertilizer recommendations, but as often as not leaves much to be desired. As with energy, it is evident that the rates of movement or cycling may be more important in determining biological energy productivity than the amount present in any one place at any one time. Turnover time and turnover rate, as convenient measures of flux. It is the flux (= rate of transfer), rather than the concentration, that is of prime importance. Tracers have been a great help in determining rates of movements since tagged atoms can actually be followed as they exchange with organism and environment.

       During the past ten years what has come to be known as mineral cycling has received increasing attention in ecological research. One – at – a – time study of single elements has been superseded by studies of the behavior of groups of linked elements and compounds as related to energy flow and stability. These newer approaches have been made possible by improved techniques of systems modeling. Major breakthroughs have been made in our understanding of the role of energy as the driving force for cycling, and the role played by microorganisms in releases of releases of essential elements from other wise unavailable pools.