Agriculture (Comprehensive Sites) - Botany - Soil Science
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ASA American Society of Agronomy
This site is from the main Agronomy association in the US and has extensive links and news as well as job opportunities.

Agronomic Links Across the Globe
Purdue University organizes the sites, for crop, soil, and environmental issues, by US National, US States, and International.

CSSA Crop Science Society of America
And this site is from the main Crop Science association in the US.

Greenbook
Free data from the Crop Protection Reference (CPR). You can read agrochemical product labels and Material Safety data Sheets (MSDS). This site also includes a fee-based service.