Go to Home Depot or Walmart garden centers. Ask them about "Jobe's fruit tree spikes". They look like "wood spikes" that you drive in the ground for securing your tent. But they're made of some organic material that is driven into the ground a certain distance from the plant, where the moisture in the ground soaks into it releasing the nutrient from it.
Here are links that may be helpful:
http://www.walgreens.com/store/produ...&id=prod386469
http://www.acehardware.com/product/i...ductId=1335050
I would not recommend a blend where the Phosphorus is higher than the Nitrogen and Potassium components, e.g. the Tomato blend(8-24-8), etc. Request one with something like a 16-4-4 or 16-8-8 blend. I don't know if there a spike with a 12-8-16 blend, but that would be ideal.