I'm a little late to this party but I'll pitch in my $0.02 as an appraiser in a rural area and lurker of this sub. The ASFMRA offers qualifying General Sales and Cost approach classes that are specific to appraising farm ground. The "standards" and techniques they teach are commonly used and applied in the "Uniform Agricultural Appraisal Report" or UAAR. This form is standard for most lenders and Farm Credit associations in rural areas. Like any other assignment, appraising farm land is as simple or complex as the subject property in question.
In a typical vacant land scenario, the unit of comparison is usually a particular land use (pasture, cropland, waterways, etc.) then, more specifically, soil type (class, productivity, slope). Subsurface mineral rights are noted for their presence or separation from the subject - income derived from a producing well is commonly treated as personal property and not part of the valuation assignment. Wind leases can have a significant impact on value. Most wind farm developments are specific to an area and comparable sales with similar leases can usually (hopefully) be found. Once a vacant land value is established, a value attributed to a lease (or presence of mineral rights) can be extracted.
Some wrinkles to the equation can be property that is bought and sold for recreational use - which I am starting to see more of. In Kansas, most ground is used for farm production, so the recreational properties typically stand out from a sales perspective.
Hope that helps!