Greg,
In answer to the application rate, I think you have to look at it for the whole season. It is also lake-specific.
For my clients, I apply the first fertilizer in early to mid-March. To maintain a bloom into September requires between 5 and 8 gallons of fertilizer per acre. This is between 2220 and 3552 units of phosphate for a season. This rate is sufficient to attain a sub 24" secchi from when the bloom starts into September, barring complications from aquatic vegetation or some such.
Therefore, my cost per acre for raw materials (which incidentally is an extremely small portion of my overall cost) is between $7.50 and $12.
I don't have a handle on costs for the powder or SE for the season yet, but they are going to be significantly higher. For example, I have a one acre pond that required 5 gallons of liquid (2220 P units)last year and achieved a great bloom at a cost of $7.50 for materials. This year I'm on my 4th bucket of SE, using it exclusively, and the bloom is running between 24" and 30". That is 3680 units of P at a cost to me of $72. That difference is what makes me think that not all of the fertilizer is dissolving despite staying in the pond for 8 weeks or more. At any rate, the liquid appears to be more efficiently utilized, at least in this limited example.
I am using SE exclusively on three other lakes, but they are new business so I don't have anything to compare them to. They are also requiring more fertilizer than I expected, though.
It should be pointed out again that because I am using SE in that particular pond, launching a boat is not necessary, so wear and tear on equipment is minimized. Also it requires much less time, and in this business time is money. That is why I'm looking at alternatives to liquid.
The point you brought up about the requirement that liquid be diluted is absolutely true in most cases. However, I've had good success on remote small ponds spraying fine droplets over the surface of the water from a hand-held sprayer. If I can get to it with equipment, I've also used a pump to dilute the liquid in a tank on shore, then applied it to the pond from shore. At any rate, it is always more work and requires equipment.
As for the retail prices of each, they are pretty accurate for SE and powder, at least in my area. The liquid is a little more than 2X cost, because of the need for the consumer to buy the container it's in, etc. So it comes out to about $5/gallon. This is based on what I charge my customers, and I've seen it a lot higher, up to $10/gallon in this area. At $5/gallon, you don't make any money on it really, when you consider the cost of transporting it.
It bears repeating that I don't really care if the raw materials cost 5x as much as liquid, which is probably an appropriate standard. The cost of materials is a small portion of the overall cost when you're talking about fertilization. I'm looking for an effective product that is easy to use and provides a good value to my clients. It's the combination of materials, overhead, and time that they pay for. If a small increase in material cost results in large decreases in one or both of the other areas, I can offer my services for less cost.
