Concentrating Phosphorus in Space and Time (3/4)

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: Here you will find clear descriptions of what you just saw, how they know what they said they knew, why they know it to be trustworthy information. Finally we will ask the question, "So what!" and explore why the information is important.

WHAT do we know?

In this clip from the 2009 public presentation “Agriculture After Norman Borlaug, ” Dr. Crews describes ways of concentrating phosphorus (P) that did not require industrial methods. While some societies practiced agriculture in ways that were solely dependent on the natural (i.e. geological) rate of phosphorus weathering, many traditional agricultural practices manipulated the system to increase the amount of phosphorus available for a particular growing season, which enabled higher crop yields.

Dr. Crews divides these strategies into two categories: concentrating P in space and concentrating P in time. Concentration in space involves gathering phosphorus from a large area, such as watershed, and applying it to a smaller area, such as a farm. Using the natural flood cycles of rivers that collect phosphorus in run-off over an entire river basin, is one method for concentration in space. The river collects phosphorus from the entire watershed through run-off, and the flood deposits this phosphorus on just one portion of the watershed--in this instance, the crops. 

Concentration in time involves “banking” the accumulated phosphorus that results from weathering in a particular area over several years. In other words, crops that use large amounts of phosphorus are not grown in the same location every-year, allowing stores of phosphorus to build up in areas where the crop is absent.

HOW do we know?

Our knowledge of traditional practices stems from studying the remnants of past civilizations, as well as study of traditional practices still utilized in certain areas today. Scientists studying this include anthropologists, ecologists, as well as many other types, and their research often involves on-the-ground fieldwork in places where traditional agriculture was, or still is, being practiced. For example, Tim Crews earned a PhD by studying the traditional agricultural practices in Mexico.

WHY can this be trusted?

Confidence in our basic understanding of the importance of phosphorus comes from centuries of work by biologist, biologists, chemists, and other scientists. For instance, phosphorus was first discovered to be important component of bone in 17711. Subsequently, reproducible observations have found phosphorus as an important element of every living thing. Models of the phosphorus cycle were later developed in the 1970s when ecologists started making estimates of the various pools of phosphorus in the earth system (such as in land, freshwater, and oceans) and then developing estimates of exchanges between these pools.

A basic premise that leads to confidence in estimates of nutrient use by plants and release by decomposition or weather is the principle of conservation of mass, which states that matter is never lost within a system, only shifted from one state and location to another. Scientists like Dr. Crews apply this principle to the phosphorus cycle to qualitatively and quantitatively track the flow of phosphorus in and between systems within the Earth and even some inputs from space dust!

1Pierrou, U. (1976). The Global Phosphorus Cycle. In B. H. Svensson & R. Soderlund (Eds.), Nitrogen, Phosphorus, and Sulfur - Global Cycles (22nd ed., pp. 75–88). Stockholm: Ecological Bulletin.


Research such as Dr. Crews’ helps us to understand traditional agricultural practices, especially strategies for managing limiting nutrients such as phosphorus. This type of knowledge is important for a variety of reasons. For one, an enhanced understanding of existing practices can enable researchers and implementers of policy to identify locally suitable improvements to existing practices. This may lead to greater productivity in a community, sustainability, or both. Another benefit of understanding traditional approaches is the ability to learn from existing practices and identify techniques that may be more suitable or sustainable than current modern practices.

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Further Reading
Further Reading: 

Crews, Timothy E., Kanehiro Kitayama, James H. Fownes, Ralph H. Riley, Darell A. Herbert, Dieter Muller-Dombois, and Peter Vitousek. 1995. “Changes in Soil Phosphorus Fractions and Ecosystem Dynamics across a Long Chronosequence in Hawaii,” Ecology, Vol.76, No. 5 (July, 1995). Ecological Society of America, pp 1407-1424.

Complete PowerPoint "Agriculture After Norman Borlaug" is available on the AGCI website.