I am often curious where the idea that quilting will detract from other elements of the quilt. That quilt definitely needs additional quilting both in the embroidered tractor blocks and in the plain blocks. If you use a white thread that blends with the fabric it will enhance the quilt as well as reinforce it and make it more durable. I agree with juliasb that the embroidered tractors are lost in that massive sea of white with nothing to break up all that negative space.
I think quilting in both the tractor blocks and the plain blocks would not only bring the embroidery to the forefront, it would look much better. Here is a visual example of an embroidered quilt I did several years ago showing before and after pics of each block so you can see what I am talking about.
Birds of a feather sneak peek of what I am working on
Birds of a feather finished
Obviously what I did would not fit that quilt but you definitely need something. I do like the idea of doing straight line quilting to resemble the tractor furrows. Maybe do them going horizontal in the tractor blocks and vertical in the plain blocks. Here is a link to a google search of clip art tractor tracks to get your creative juices flowing.
https://www.google.com/search?tbm=is...&bih=753&dpr=1
Another option is to do crops in the plain blocks. Ears of corn, sheafs of wheat, that idea. Quilts with this much negative space need quilting. If done densely enough you will be able to take up the extra fabric that has resulted in the ditch work stablizing. Use a walking foot if you are not comfortable doing FMQ