Thinking Medieval with a Twist: magic boosts crop yields

On Earth, for 99% of the history of the agriculture before genetic engineering and GMOs became a thing, farmers used genetic engineering the old fashioned way, by selective breeding.   Scarterran farmers use the same basic principles with their crop selection, but with judicious use of Plant theurgy this process is much faster on Scarterra than on Earth.   The norm is that farmers pick the best samples from their harvest and use those as seeds for the next year and they eat the mediocre seeds. At least in theory, this mean each new crop will be incrementally better than the last year's   Even without modern farming technology, the food crops we use today are more advanced than the crops of yesterday. Below is a painting made roughly 350 years ago compared against a modern watermelon. This is what three centuries of selective plant breeding can do.     In Scarterra, Plant theurgy is a thing. Plant theurgy is better at making temporary changes to plants than making permanent changes, but it can make a small difference. It can accelerate the natural crop evolution caused by selective breeding, effectively getting five years progress in one year.   Also, due to plant theurgy preserving seeds and various factors that result in Scarterra having more proverbial Marco Polo types of explorers than Earth did, new crops can. Potatoes are a staple of European cousine now but they originated in South America and weren't grown in Europe till the mid 1500s. In Scarterra, potatoes have been grown for so long that Scarterrans forgot where potatoes came from originally. The same goes for wheat, rye, and all the other major staple crops. They are so widely available that their origins have been mostly forgotten.