Below are some sources I have been using since the start of this term:

http://www.commodities-now.com/ Commodities Now

http://www.commodityonline.com/ Commodity Online

These provide selected information about a variety of commodities, and they also gives insights of how differentiate news are related to  commodities that we are interested in. For example, in http://corncommentary.com/2012/09/24/no-farm-bill-now/  it briefly explains how farm bill works and how past farm bills affected the commodity market, not necessary to search “farm bills” then.

In library I found a book “Methods to analyse agricultural commodity price volatility” by Isabelle Piot-Lepetit; Robert M’Barek. It was published in 2011, which is quite up-to-date. I will probably read it in the coming long weekend.