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See Inside June 2010

Readers Respond on "Fixing the Global Nitrogen Problem"

Letters to the editor from the February 2010 issue of Scientific American

When Less Is More
The global challenge of addressing sustainable use of nitrogen fertilizers is well characterized in “Fixing the Global Nitrogen Problem,” by Alan R. Townsend and Robert W. Howarth.

Where the authors fall short, however, is in considering some of the solutions that agricultural research and innovation offer to address these problems. Conventional breeding and biotechnology are being applied to improve crop utilization of available nitrogen, thus reducing fertilizer demand without sacrificing increased yields. And best practices today, through the use of urease and nitrification inhibitors, minimize off-target movement of nitrogen by reducing evaporation and by maintaining fertilizer nitrogen in the ammonium form, which is less mobile and more efficiently utilized by many crops.  

Technology and innovation, combined with intelligent public policy to promote more timely development and adoption, will be the foundation for sustainable solutions allowing the world to continue reaping the benefits of the Haber–Bosch process.

Michael Shaw and Cliff Gerwick
Dow AgroSciences
Indianapolis

Fill In the Colors
In “Seeing Forbidden Colors,” Vincent A. Billock and Brian H. Tsou review work by Hewitt D. Crane and myself extensively and propose that our results and their replication of the forbidden-colors studies require a new winner-take-all alternative to well-established opponent-color theory. Neither Crane nor I ever posited that our findings of forbidden colors violated or even undermined color-opponent theory. In fact, we felt that our results told us nothing about color opponency in the retino-cortical pathways but much about how the visual mechanisms of the brain processed information once received. Our forbidden colors were neither “parlor tricks” nor hallucinations but the result of a series of experiments that explored the parameters of the “filling-in” mechanism.

Filling in, which functions across both naturally occurring stabilized images and artificially stabilized images, seems to “paint” colors and patterns across areas of the visual scene for which there are apparently no signals emanating from the retina. Our research showed that the color of the filled-in area appears to be determined by chromatic contrast at the perceptual boundary of the stabilized region. When we provided conflicting information at that boundary—for example, red on one side and green on the other—the filling-in mechanism “painted” both colors across the perceptual field. These results and others from our laboratory suggest that the filling-in mechanism functions according to its own rules, independent of color-opponent retino-cortical pathways.

Thus, there seems to be no need to propose a new color-opponent mechanism, because even such a mechanism cannot explain how we see colors where the retina sends no neural signals to higher centers, for example, at the optic nerve head and in the shadow of the many retinal blood vessels.

Thomas P. Piantanida
Greenleaf Medical
Palo Alto, Calif.

Cold and Dark
It seems odd that in the recent article “Cloudy with a Chance of Stars,” in which Erick T. Young discusses formation of massive stars, no mention is made of the potential role of cold dark matter or the alternative theories of modified Newtonian dynamics (MOND). The inadequacy of Newtonian gravity based purely on baryonic mass in describing the physics of galaxies has been well covered in Scientific American. One would think that the additional gravitation provided either by dark matter or by MOND would be significant in the formation of massive stars. Can Young explain why these important effects are ignored in the models he has described?

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