One of the meaningful bits of engineering one can do on Earth for furthering Martian agriculture, is the recycling of water. It's going to cost energy and worker-time to extract water out of the soil. Even machinery that condenses water out of the Martian air will require energy and maintenance. (Possibly passive structures that condense water?)

For this reason, we may want to research habitats that can almost completely retain moisture in the form of water vapor and extract it out of the air. Such requirements are quite uncommon for structures on Earth, doubly so if you also add in the requirements of enclosing useful agricultural acreage and pressurization. Submarines are probably good at doing this, but the price per square meter of floor would be prohibitive. We'd want to be able to build such structures on Mars from in-situ resources.

Building such structures on Earth and growing crops like potatoes within them would constitute worthwhile research in preparation of colonizing Mars.

(One side effect of such research might be the ability to settle and develop land in places like West Texas, much of which gets very little rain and has land that sells at single-digit dollar prices.)