The foam markers are a common and standard way to prevent overspray for tractor spray rigs (as well as most truck mounted systems).
The other method farmers use is GPS but not a standard uncorrected unit. The professional units use corrections from a variety of external sources such as WAAS, Omnistar, Starfire, etc.
For greater accuracy (<2cm) you can use RTK GPS which uses corrections from a second GPS unit. This second unit may be purchased by the farmer and hosted on his property (costs an extra >$15,000). The other way is to subscribe to companies that provide a subscription service where the corrections are sent to your GPS receiver by mobile phone or radio network.
To do it on the cheap, try using a 'normal' GPS module (without corrections) and put the output through a Kalman filter and see how much deviation you get over a few days.