The USDA Economic Research Service just released a nice chart showing the total reported taxable farm income and farm losses from 1998 to 2012. Over the 15 year period, the net tax farm income number was never positive, however the trend from 2007 to 2012 increased every year.
Farmers, in total, reported gross profits from farming of about $18 billion in 2012 with offsetting net farm losses of about $24 billion. This resulted in a net taxable farm loss of about $6 billion. This data only reflects the amount income reported on sole proprietor's Schedule Fs. Most larger farm operations report their income on a Form 1065 (partnership return) or Form 1120 or 1120S (Corporation tax returns). Therefore, this does not reflect all reported farm income or loss with the IRS.
The worst year on the chart was 2007 with net negative farm income of almost $20 billion, however, the largest gross loss year was actually 2008 with about $27 billion of gross farm losses.
As long as farmers have the ability to continue to use the cash method of accounting along with Section 179 and bonus depreciation, I would anticipate farmers never showing a net profit.