But an acre of farmland and an acre of suburban front-lawns are wildly different.

Agricultural chemicals are tightly regulated, various environmental groups watch them like a hawk and are quick to push for legislation against new chemicals. The EPA has strict rules on chemical run-off, retention in soil or ability to impact drinking water. On top of this, most farming is relatively low margin and farmers don't want to spray any more than they have to because of cost.

Meanwhile, the average suburban American will gladly dump 10x the appropriate amount of chemicals on their lawn in an attempt to keep up with the Jones' next door. Or think nothing of their fertilizer-infused irrigation spraying half onto the driveway or sidewalk, flowing directly into the storm sewer rather than being absorbed by the soil.

I'd be interested in solid numbers, but my own guess is that they will differ in the opposite direction, with the average farmer dumping many more chemicals per acre than the average lawn owner does. Crop-dusting in some areas happens daily, and when not daily, 1-3x per week. By contrast, very few lawn owners will apply pesticide to their lawn every day. Few will do it even once a week. The norm where I've lived seems to be 1-2x per year, which is hilariously infrequent to a farmer. Even if they over-apply in their very occasional pesticide treatment, they're starting from a baseline of much less frequent application.

Some more data on lawns:

http://earthobservatory.nasa.gov/Features/Lawn/

Which estimates around 31.6 million acres of lawn in the US.