900 MHz radios are supposedly limited to 1 Watt. You are also allowed to use a 6dB gain base station antenna, so if you don't factor antenna line loss, that gets you to 4 Watts Effective Radiated Power (erp). If you keep the base station feed-line less than 100 feet and use good quality coax, you should be able to come out around 3 Watts erp.

Remember that you should factor receiver antenna height into the line of sight equation also, as I'll assume you nice map considered base station antenna to a point on the ground. 15 feet of elevation at the tractor or sprayer will add considerably to your 'view' of your tower.

Using a 'gain' type of antenna on the rover can double or even quadruple the amount of signal that your rover radio hears, so you could be looking at an effective signal of over 10 Watts if you use gain antennas at both ends.

Trees and other green foliages that interfere with line of sight to the tower really eat 900 MHz signal because the short wavelength acts just like a microwave in trying to heat the water in the foliage and all that energy gets soaked up before it can get to the receiver antenna.

One of the good things about 900 MHz is that it bounces, and bounces, and bounces, so just because you can't see the base with your eye, doesn't necessarily mean that you rover won't have a nice strong signal as a result of a double or triple carom on its way to your rover. My rover radio 'sees'  the base tower even when I'm in the pole shed with the south facing doors open. The base antenna is west of the shed so I know the signal is bouncing off one or more surfaces to get inside the shed.

If you are within 3 miles of your base, my gut feeling is you'd have no trouble with reception. Such is free advice though and you know what they say about that.