Nitric acid contributes to acid rain. Figure 11-15 illustrates the phosphorus cycle. Note the absence of an atmospheric compartment. Very little phosphorus is readily available to organisms, most is tied up in rocks, sediments, and soils. Hence, it can be a limiting nutrient in ecosystems.
Soluble phosphate is made available to organisms by the weathering of the mineral apatite.
However, if the soil has a low pH, the phosphate binds to clay particles and becomes insoluble. Phosphate may also become unavailable by combining with iron and aluminum in soil. In freshwater lakes, most phosphorus is found in the plankton. As they die, they sink and take this phosphorus with them to the sediments. This is especially true when a lake is stratified in the summer and phosphorus can be a significant limiting nutrient.