In module four, and in your education previous to this course, you've learned about the water cycle, in which water evaporates from bodies of water, condenses into clouds, and then is returned as rain to drain again into groundwater, lakes, and oceans. Each of the major crop nutrients, and most chemical elements on the earth's surface, has a similar cycle in which the nutrient is transported and transformed from one place to another, spending time in different 'pools', analogous to the division of water into lakes, rivers, clouds, rain, and the ocean. Just as rainwater and groundwater may be of more immediate use to crop plants than the ocean, different pools of the same nutrient differ in availability to plants. For example, most soils hold a tremendous amount of nitrogen in large organic molecules, but only the smaller soluble pool, and some smaller molecular forms of N, are directly available to plants. The way that soil nutrients move through the earth system, including within food production systems, is called nutrient cycling. The objective of this module is for you to understand the main features of nitrogen (N) and phosphorus (P) cycling in human-managed soils. Earth scientists sometimes use the term "biogeochemical cycling" to emphasize that each nutrient’s cycle represents the geological and atmospheric sources of the nutrients, the biology of organisms that often transform nutrients from one form to another, and the chemical nature and interactions of each element.
As an example of biogeochemical cycling, think of the important element carbon (C). Carbon has a chemical nature that allows it to be a fundamental molecular building block for all living things. In addition, there is an impressive atmospheric pool (a sort of geologic pool) of non-organic carbon dioxide. Interacting with this atmospheric pool, green plants and algae play a fundamental role in turning atmospheric CO2 into biological organic carbon in living things and the remains of living things, such as plants, that fall back into the soil. Scientists refer to this large set of interacting parts with geological, biological, and chemical attributes, earth's system that "processes" and recycles carbon in a certain sense, as the biogeochemical C cycle. Another example is phosphorus (P), which will be described in more detail on the following pages: The earth’s crust is the primary source of all P, which is then weathered by geological and biological processes and also in human fertilizer factories, held or retained strongly by soil clay minerals after application by farmers, and eventually occupies a key role in every living thing as one of the elements within the DNA molecules encoding our genes. It’s essential to realize that humanity and human systems are now major players within these nutrient cycles including C, P, and nitrogen. We can see this in activities such as mining (and eventually threatened depletion) of phosphorus sources for fertilizers or fixing of large amounts of nitrogen for fertilizers with a massive expenditure of energy and emission of carbon dioxide through the use of oil and gas.