GPS on its own especially with systems like WAAS is quite accurate these days, generally within a few meters. If you need more (which planting most crops does) there's also RTK [1], which permits centimeter-level accuracy from relatively inexpensive / simple setups. Here's an example of a local RTK base station with radio link in the industry [2], and additionally it's possible to get these corrections off cell network data connections [3]. Radio offers you more hyperlocal corrections which could improve accuracy a touch, but comes with all the downsides you'd expect of a local radio system so both are viable options.
Hmm, I always thought of RTK as including carrier-phase tracking, which would make it overkill for farming if the base-station is fixed (on the order of 2mm relative-position accuracy; absolute positional accuracy is not much better than DGPS without carrier phase-tracking). Wikipedia is ambiguous as to whether or not RTK necessarily includes carrier-phase tracking.