Please see the general commercial geothermal page for information on other tax incentives first.
Farms were singled out in particular because they are alible for a program that most organizations are not. More specifically
The Rural Energy for America Program Renewable Energy Systems & Energy Efficiency Improvement Guaranteed Loans & Grants or REAP for short.
This is a program that is administered by the US Department of Agriculture (USDA) and can provide grants or loans to US farmers to add renewable energy technology to their farms including geothermal heat pumps.