Cambridge University Press apologises for errors in the above article. The authors’ affiliations should have been given as follows:
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1. Department of Applied Economics, University of Minnesota, St. Paul, MN, USA
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2. Department of Horticultural Science, University of Minnesota, St. Paul, MN, USA
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3. Department of Soil and Crop Sciences, Texas A&M University, College Station, TX, USA
Equation 2 on page 4 should have been printed as:
(2)
The original article has been revised to reflect the correct affiliations.
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