text: Personally, I don't worry about all that stuff and just try to make sure I have good rich soil with all the macro, micro, trace nutrients and minerals the plant needs (which is a lot more than just NPK). Then the plant can just take what it needs, when it needs it.
  It's like if I were to try to give you all the nutrients you need, one at a time. Today you need oranges for vitamin C and spinach for folate and iron, etc. Or I can just put a big buffet in front of you and let you pick what you want/need and eat a diversity of stuff all the time.
  So would you recommend I ammend the soil so that it has high levels of all three macros and just let the plant take what it needs? Or should I just leave the soil as it is?
  In the long run, best thing you can do for your garden is start a compost pile (if you aren't familiar with composting, see our Compost Forum). But that won't help you right now. Since you know you are low in potassium, you probably do want to add some. Wood ash is a good source, but is alkaline and will raise your soil pH. People often add lime to their soil, deliberately to do that. If your soil needs or can handle raising the pH, that would be a good source. Otherwise kelp meal or greensand are good sources, that will also add other minerals and micro-nutrients.
  We all have different ways to approach the garden, so I'll throw out my 2 bags full. I use a fair amount of bagged stuff, being in town and all. I am also working on the soil as a living mass. To add to your mix you put in, I would add bagged poo (black kow from Lowes my preference), and a bag of compost -soil helper. I like cotton burr, but also use Natures Helper from HDep. I am trying to use more of the Epsoma -tone product to build the soil. It has low numbers, is organic, has extra stuff in it. It's found at the garden center and box stores. Then plant!
  When I went through the Master Gardener program part of the curriculum was an individual project. My project was to compare the results of home test kit results to lab test results.
  Finding: IF the directions are followed the test are reliable and the results are very close to lab test results. If the directions are NOT followed the results are garbage.
  Conclusion: Have your soil lab tested. $10/$20 dollars. Well worth the money and much less expensive than randomly amending your soil.
  Question: How confident would you be with a Physician who prescribed medication without an evaluation or diagnosis?
  I would do as rainbow said and just add more organic matter and compost to the garden. Different plants will use nutrients in different amounts at different times in their life cycle.
  Nitrogen is a volatile nutrient. Even on a lab soil test, usually nitrogen is recommended but not tested. That is because nitrogen levels are changing all the time. Too much nitrogen at the start and your seeds may fail to germinate. While plants are actively growing they use the most nitrogen, but if you want more fruit, you want to avoid putting in too much or your will have lots of leaves and very little fruit or bulbs if you are growing roots. Usually the safest way to apply nitrogen and the way it will be recommended on a soil test would be to divide the total nitrogen requirement into two or three applications.
  You can always supplement with fish emulsion or AACT or add some wood ash (do not add too much, it will alter the pH), greensand, kelp meal, or throw in more banana peels, potatoes and other high nitrogen materials into your compost pile.