Phosphorus Cycle

The measure of how quickly and to what extent sunlight is converted into organic material by plants during photosynthesis is called primary productivity. Some ecosystems have high primary productivity, while others have very low productivity. For example, the ocean is the world's most productive ecosystem because of its huge area. Oceanic algae create new plant biomass, or weight of living material, on a vast scale. However, plant primary productivity is not simply dependent on the availability of sunlight alone. In addition to water, other vital inorganic nutrients are required for growth and optimum primary productivity. Phosphorus is one such nutrient.

In ecosystems, rarely will all required nutrients be used up at the same rate. When one nutrient is used before other nutrients, it is called a limiting nutrient. Limiting nutrients prevent growth with their absence. When returned to the lacking environment, limiting nutrients jump-start productivity, which continues until the limiting nutrient again is depleted. Phosphorus is a limiting nutrient in many terrestrial and aquatic ecosystems. The productivity of the primary producers in these areas is limited, held in check, by the amount of available phosphorus that is so vital for life. This fact is why phosphorus is a main component of agricultural and household plant foods and fertilizers. The addition of phosphorus that is normally in limited supply allows for maximal plant growth.

Normally, because phosphorus availability is limited in the phosphorus cycle, plant growth in lakes is also limited. A major problem with the use of phosphorus in fertilizers is the process of artificial eutrophication. Eutrophication is a large increase in the primary productivity of a lake. Eutrophication can be harmful to the natural balance of a lake and result in massive death of fish and other animals as dissolved oxygen levels are depleted from the water. As the growth of algae and aquatic plants goes unchecked, the lake slowly stagnates, becoming fouled. Artificial eutrophication can occur when run-off rain water from agricultural fertilizers that are used in excess reaches lakes. Another human cause of artificial eutrophication is run-off from mines. Mining in areas where rock is rich in phosphorus minerals can create dust that is blown by wind into nearby water systems. Similarly, rain-water can wash from mining areas to nearby lakes. A third cause of artificial eutrophication is the introduction of phosphorus into phosphorus-limited lakes by man-made laundry detergents. Many detergents in the past contained phosphorus. Effluent from households eventually made its way to lakes where massive plant overgrowth occurred, spoiling the natural balance present. Today, considerable progress has been made in producing phosphate-free detergents, which do not cause artificial eutrophication and preserve the normal cycling of phosphorus. The phosphorus cycle. Geological uplift accounts for the presence of the phosphate rocks (upper left). Illustration by Hans & Cassidy. Courtesy of Gale Group.