Maybe I am biased but I wouldn't use chemical fertilisers like you mention as they destroy soil life. Healthy soil teeming with soil life is the answer. Build your soil up with old manure, compost and any organic matter and 'numbers' won't be necessary.Use crop rotation starting with a leaf crop after you have added manure, etc to the soil. When the leaf crop is finished plant a fruit crop (beans, capsicum, tomatoes zucchini, etc), then finally a root veg crop. Re-fertilise the soil ready to start the cycle again. plants need more than N-P-K and organic matter will achieve this, building up the soil life, increasing the capacity of the soil to hold water, increasing disease resistance and making more micro-nutrients available to your plants. adding some lime in late autumn or winter will also help. Trust this helps.

- John