I suppose that what you depict, and how symbolic you get, depends a lot on the scale. I was thinking in terms of typical dungeon maps when I suggested drawing individual fields/trees/tracks etc.

I agree with you that as you go to the other extreme farmland might even become indistinguishable from grassland - both being just the flat expanse between symbolic representation of polar ice caps, Himalayan mountain ranges, Amazonian forests and Saharan deserts. Even then, an inference of farmland could be made by representing the largest cities as dots, since the mere presence of a city infers an apron of agriculture, a rich fishing ground, or an important trade route along which supplies may be ferried, whether or not these things are also symbolically represented on the same map.

If showing agriculture was the sole purpose of the map, and the map area was an entire planet, you could draw just a coastline, with a single colour or shade to show the distribution of the agricultural areas.