I see this as a common sense issue but since no one has responded to your question, I'll take a stab at it.
A farm/ranch appraisal is just that. An appraisal of a farm/ranch. If you have to ask what a farm/ranch is, you probably shouldn't be appraising one but for purposes of discussion, I believe that a property that derives reasonable income from agricultural endeavors would qualify as a farm/ranch.
Furthermore, the definition of farm/ranch varies from person to person and from location to location. I know several city folks that consider themselves proud owners of 5 acre farms or 10 acre ranches. Although it is possible to generate substantial agricultural related income from 10 acres or less, it is not the norm in my market area. Ten acres is often not enough to even be a hobby or "gentleman" farmer.
To answer your question as to the qualifications to appraise a farm/ranch, check with USPAP. To obtain competency, I belive you must have proper education/training and experience working with a seasoned veteran. I often see reports of farm appraisals where residential appraisers list a house and 5 acres on a URAR and on the "other" comment line, list 100+ acres and adjust for it. They don't realize that the 100+ acres is worth substantially more than the house and 5 acres. They don't know the soil type, productivity, condition, or utility of those acres. They don't comment on noxious weeds (or know what they are), they don't mention water sources for pasture or cropland, they don't mention the ag related buildings and if they are functional or not. They don't understand the use or capacity of grain storage facilities.
In short, I have seen many farm appraisal reports performed by residential (and commercial) appraisers who don't know what they don't know. If you're interested, check with ASFMRA or contact a local ag appraiser.