So anyway, back to the question...
I hope nobody wastes much time at the dirtworks.com site. What's up with that? All advertising all the time! And pop-up windows, too!!
If you take your 15 gallon sprayer, put in 5 gallons of liquid molasses, 5 gallons of liquid seaweed, and fill the rest with water or a good compost tea, that will cover 5 acres of grass, trees, ornamentals, or edibles as a foliar spray. Add a lot more water if you want to drench the soil. This can be sprayed every 2 weeks to improve the general health of the plants. If you use compost tea instead of water, add the compost tea last and use it all up in a day if you can. The microbes in the tea will use up all the oxygen in the sprayer and it will stink - which is something you don't want.
Many liquid organic fertilizers rely on some sort of liquified fish remains. That can smell pretty bad. Some products claim to be free of that smell. Try them at home before you embarass yourself and your client with a smelly yard.
Liquid molasses is best bought by the pound at farm co-ops. Usually you have to bring in your own container or you can buy one from them. It weighs 10.5 pounds per gallon, and at $0.09/pound, a 55-gallon drum should cost about $52.00.
Liquid seaweed can be found, but if you are going to use it in bulk, get dried seaweed and add the water yourself. Here's a link...
I'm writing to them to get pricing and application rate info for the dry stuff. I don't think you could afford to buy the liquid and use it at the rates I've suggested above. I'm guessing the rate will be one pound of seaweed per acre, which is very affordable at $1.00 per pound (retail). But it could be as much as 10 pounds per acre, which is $10/acre, so we'll see what I get back. For something you are going to spray every 2 weeks, I would want to see your cost be in the neighborhood of $2/acre.
Both molasses and seaweed are food for soil microbes.