How did phosphorus shape early agriculture? (2/4)

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Clip Guide

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 presents a depiction of what agriculture looked like in pre-industrial times— the period of agriculture before fossil fuel-powered equipment was used to manipulate and manage croplands. Dr. Crews describes significant shifts that happened once humans transitioned from hunter-gatherers to becoming farmers of increasing sizes of land, intentionally planting, tending, and harvesting crops for the purposes of food production. 

HOW do we know?

In this clip, Dr. Crews builds upon a simple model of the phosphorus cycle in native ecosystems to illustrate the changes to the phosphorus cycle in soils changed as a result of pre-industrial agriculture. Ecologists often refer to a model like this as a budget because, like a financial budget, it tracks the influx and outflow of resources into a system. Such budgets help us to understand which ecosystems may be receiving different amounts of a nutrient than were previously available or alter ecosystems to grow larger or more productive crops than would have otherwise been possible.

WHY can this be trusted?

Although Dr. Crews and other agricultural ecologists were not alive to study agriculture practices or impacts during pre-industrial periods, they are able to combine their knowledge of physical processes, such as the forces of erosion and leaching, with artifacts and traces left by civilizations practicing pre-industrial agriculture to come to conclusions illustrated in the simple model. Additionally, some societies around the world still practice agriculture in ways very similar to practices carried out centuries ago, providing more support to these conclusions.


Despite agriculture’s essential purpose to provide a stable food supply, it is often taken for granted. Yet scientific study of agriculture enabled advances that allowed agriculture to keep up with soaring population rates in recent decades. Agricultural ecologists like Dr. Crews study agricultural in an ecological context in order to make additional advances in the productivity of crops while considering the impacts such farming might have on surrounding natural ecosystems.

Printable Companion Guide: 
Further Reading
Further Reading: 

The complete PowerPoint for this presentation, "Agriculture After Norman Borlaug," is available on the AGCI website. Click the link or search "Tim Crews" on

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.