I have done soil tests. It is a good idea to get a baseline and to get recommendations for what to add and how much fertilizer you need. I had good production before I did the tests but I used 3 packages of miracle grow every two weeks. Everything was lush and green but not a lot of flowers and my root crops made leaves but not roots. It was fine for leaf crops and tomatoes were huge and produced a lot of tomatoes, at least more than I could use.
After I did the soil test, I found out I was over fertilizing a lot. I have done 4 tests so far and only one garden has come down enough to add more than nitrogen. The other two gardens are extreme in phos, potassium and high in calcium so I only need to add small amounts of nitrogen. Saves me a lot of money on fertilizer I don't need. I only use complete fertilizers mostly on potted plants but not in ground.
I do add organic matter. I am actually adding twice as much as I did before. It is still raising the phosphorus level but it is improving the tilth of my clay soil.
I get soil tests every 4 years or so just to check the balance and fine tune the fertilizing. A soil test at the University extension service costs $25 and includes pH, major nutrients, tissue analysis if I send a tissue sample for nitrogen. Nitrogen is not included on the test since it is a volatile element. Recommendations are for general garden vegetables and I get fertilizer recommendations per 100 sq ft. It still helps to have a scale since most of the recommendations are in pounds. It is more accurate than a kit test and kits don't give you fertilizer recommendations.
Happy gardening in Hawaii. Gardens are where people grow.