Concentrating Phosphorus in Space and Time (3/4)

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Clip Guide

: 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.

SO WHAT

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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Printable Companion Guide:

Guide New--Crews III.pdf

Active Listening Questions

Active Listening Questions:

How have traditional societies overcome the limitation of a slow natural rate of phosphorus weathering in order to increase crop yield?

Why does Tim Crews frame the different strategies in terms of “space” and “time"?

Taking the Reins

taking the reins

Clip Discussion Questions

Reflect on the clip using these questions. Then, record your thoughts in a science journal or discuss them with a friend.

What might be benefits and downsides to each method of concentrating phosphorus that Dr. Crews describes?

Clip Discission Question Printable Companion:

Discussion Questions New--Crews III.pdf

Go Beyond:

“Thinking in Systems” PART 2 - manipulating a system

The idea of this activity is to continue development of a systems model created in Thinking in Systems Part 1 ) with inputs, outputs, stocks (or “pools”), and flows (or “fluxes”). In this part, you will begin to manipulate your system in ways comparable to how traditional farmers discovered techniques to augment the cycle of phosphorus.

Rationale/Overview

Recall the simple system you developed in Part 1.

The example system used in Part 1 was a city bike share system, where stations for specific types of bikes are installed throughout a city, bikes are shuffled around the city by users making one-way trips from place to place, and bicycles leave the system either by getting stolen or decommissioned. In this exercise, you will be asked to consider ways to augment your system to allow for higher productivity or operability within that system while operating within limits to growth that you identify.

INSTRUCTIONS

1. Think about all the dimensions of your system from start to finish. Write down all the pressures for growth on the system as well as all the limits to growth within that system.

Example response from bike share system:

Pressures for growth:

Increasing number of users (people) using the bike share system
Many riders take similar routes, making scarce availability of bikes and/or docking stations
Increasing theft or disrepair of bicycles within system

Limits to growth:

Cost of introducing new bicycles and docking stations within system
Availability of sources to manufacture new bikes and docking stations
Available real estate for docking station expansion

2. Identify strategies that respond to each of the pressures for growth you listed and that are sensitive to the limits to growth you listed. Think about the strategies used in traditional agriculture that Tim Crews described as analogies to strategies that you may choose to implement in your system.

3. Augment your system map to describe how strategies affect the performance of the system. Use additional visuals as needed.

4. When you have finished, graph a potential change within your system based on one of the pressures or limits you described. In the bike system example, you might project growth of bike numbers if the population grows by 10% over the next 15 years. What other factors might play a role and keep your line from being a simple increase or decrease? For our bike share program, an increase in population might also lead to an increase in bike theft.

If you were graphing the current phosphorus cycle, do you think that the pool of phosphorus in the soil would be growing, shrinking, or neither? What are the factors influencing this system?

5. How is your system similar to a natural system like the phosphorus cycle? How is it different? Share your ideas with a friend or record them in your science journal.

Go Beyond Printable Companion:

Go Beyond New--Crews III.pdf

Clip Quiz

Clip Quiz: