I think these kind of things are great fun to consider and discuss!
The first thing in my mind is the commonality of magic. kigmatzomat makes a great point about how a first level magician can improve farming. But, it is irrelevant if there is only one guy in the county that can do it. How common do you all perceive formalized magic (i.e. PC level magic) in your campaigns?
As for the commonality of this, it is at most 2 cantrips (mold earth, shape water) and a couple of 1st level ritual spells (unseen servant, floating disk, alarm). Anyone who makes it 4th level in any class can have either the cantrips or the rituals via Magic Initiate:Wizard or Ritual Caster(wizard) feats. To be honest, just having Mold Earth is sufficient. (See below)
I will debate the impact of one person who can do this per county. I spent a decade as a construction engineer and have done my own drainage work with a 50hp tractor. The amount of work machinery can do compared to a person with shovels is just staggering. But don't take my word for it: How Long Does It Take To Dig a 6 Foot Hole? Production Rates For Manual and Mechanical Excavation in Light, Medium and Heavy Soil - Estimation QS
The fastest a person can manually move dirt is under 1 CY/hr. One 5ft cube is 125 cubic feet, or about 4.5CY. Our cantrip-weilder can excavate 2700CY/hour. That means every hour of the farm mage is worth 225 effort-days. One 12-hour day of the farmmage doing heavy labor is equivalent to 2700 effort-days. That is 90 adults working for an entire month. For one day's effort.
Any project that big would not use that many people and would bring in horses, oxen, teamsters, drovers plows, wagons and a small crew of workers. All of which is now unnecessary. This person would replace more than a dozen horses or oxen plus all their handlers. Keep in mind, each animal needs 5+ acres of pasture so displacing 12 animals is like adding an extra farm to the county. And unlike horses or oxen, they won't get hurt, don't get tired, can't kick or fall on a handler and have no significant equipment to break or maintain. They also don't need as much food as even that reduced work crew would require, effectively adding more sellable produce for the region.
This one mage would radically improve soil conditions for hundreds or even thousands of acres by installing drainage, creating ponds for water retention, building levees to control flooding, all of which results in more productive fields. Having the fields tilled 5 feet deep every couple of seasons eliminates soil compaction and also significantly improves plant growth. I would seriously expect this one person to increase crop production by 10-15% for the entire county. If they have free time, they can improve the roads which will also effectively increase food production by reducing spoilage in transit and let each wagon and their livestock haul more goods.
Assuming these farm mages are paid like modern heavy equipment rental and operator, they are well into the top 10% of wage earners.
If magic is rare, the pay would be even higher (think brain surgeon) and they would likely have a bodyguard, a servant, a special carriage to work from, a lavish pavilion tent and the best food.
Which may explain why adventurers are rare; why bother when you can be a pampered farm mage?