ORIGINAL: kdsberman
I have 2 plots, one being about 1/3 acre or so and the other about 1/4 acre.
Are you willing to take a soil sample? If so, you can probably find an extension office near you to take it to. It took me a week to get my results emailed to me once I dropped off the sample.
If you don't want to go through that, my estimate would be to use about 25 pounds of P and 25 pounds of K on your 1/4 acre plot and about 30-40 pounds of each on your 1/3 acre plot. You can usually find straight phosphorus and potassium fertilizer at farm stores or fertilizer suppliers. Or you could look for something like a 0-45-45. Just FYI, the potassium is commonly referred to as "potash". If you would rather just use the 5-20-20 because it's readily available, just use 2 bags on each plot and call it a day. Just make sure you apply your fertilizer right before it rains. Sit back and watch your plot explode!!!