I agree about the small diesel. I do small residential jobs. But I do a lot of them. That 15 hp motor on my 300 gallon Finn just doesn't have enough horsepower to pump as thick a slurry as I'd like through a small diameter hose.
They tell me that a 600 gallon Finn has the same clutch and pump assembly as my smaller machine. I haven't really checked it out yet, but the thought has occurred to me that it might be possible to install a 25 hp motor on it like the bigger machine.
For years I pulled around an 800 gallon bowie even though I didn't need it's capacity. But it's 33 hp motor gave it the ability to pump anything I asked it to. I miss that, but I sure don't miss the towing weight of the bigger machine. Of all the manufacturers I've talked to, the only one willing to put a big motor on a small machine was Bob Jones of Bowie.
Regarding plastic tanks. I don't see why someone couldn't build one with mechanical agitation. It's not about plastic vs steel, it's about manufacturing profit margin vs the ability to do the job. I wouldn't be surprised if Easy Lawn hasn't played around with the idea. They're now making a pretty innovative high-end mechanical agitation machine. (I think it has a stainless steel tank?)
By the way I stopped putting corrosive chemical fertilizer in my machine a long time ago. I'm convinced that most of the nutrients leach into the soil faster than the seeds can germinate and grow into the soil. I do ocassionally use an organic bio stimulant when the situation warrants it. But for the most part, I ask my customers to fertilize 2 weeks after they start watering. It works with my homeowner customer base. It you probably couldn't get away with it on most commercial jobs...
And I've heard that about Alaska. From time to time I've used charcoal to raise the temperature a degree or two. A Finn rep once told me one of their bio stimulant additives will get the seed to germinate in colder soil than it otherwise would. Do you guys do something similar?