You need to learn PWM circuits. The simplest way is to use a 555 timer chip. Check the AAC ebooks. There are lot of tuts. Learn the theory and try to make one. As for switching the 19A load. It is not that of a problem based on the power of the recent Mosfet's available today.

Your load handling is the easy part. The PWM drive circuit is the complicated one. Get that right and ur circuit is done.

So go thru the books or google PWM using 555. make a circuit with a Led as output and post the results. Then you'll get ways to use the Mosfet.

 

If you don't want to roll your own here's one already built, capable of 30 amps @12v.
http://www.canakit.com/30a-motor-speed-controller-pwm.html

 

My experience with MOSFET's is that what worked at low current may not work at high current very well. Lot's of new problems crop up when the power increases. You should plan to dissipate some heat from your MOSFET, a large heatsink will be necessary. Since you're driving an inductive load (your fan motor) you'll need a diode rated at half the power your fan uses (10Amps or so). Check out the attached picture for the basic circuit for a motor controller. IMHO driving the PWM on the MOSFET gate would be easier with an Arduino than a 555 timer but it all depends on what you already know I suppose.

You have the issue of wanting all the electronics to be protected from the weather, but also able to dissipate lots of heat. I've been sucessful with weatherproofing but you also create an oven when you seal everything up.

 

If you don't want to roll your own here's one already built, capable of 30 amps @12v.
http://www.canakit.com/30a-motor-speed-controller-pwm.html

I think gerty has provided you the link to exactly what you need. If you buy a cheap project enclosure to house the motor controller, all that will be left for you to do is incorporate it into your tractor.

 

If you don't want to roll your own here's one already built, capable of 30 amps @12v.
http://www.canakit.com/30a-motor-speed-controller-pwm.html

Good job finding this for him. Not everyone wants to build electronics projects, especially farmers. They've got more hard work to do than most of us.

scobar, if you didn't see it, at the bottom of the page, there's a part number for a recommended case for it.

 

If you don't want to roll your own here's one already built, capable of 30 amps @12v.
http://www.canakit.com/30a-motor-speed-controller-pwm.html

Nice find..if I do say so myself