Soil testing will help you determine what nutrients your soil has so that you will know if you need to amend the soil (add fertilizer or nutrients).
When I first started gardening I had no idea what I was doing and could not make sense of all the information coming at me about fertilizing. I purchased some garden soil, added some fertilizer, and planted some things. The next year, things didn't grow so good. It wasn't until I got a soil test that I learned I had been adding unnecessary nutrients (ones that accumulate in soil) that could harm my plants and contribute to environmental pollution.
Soil testing is the only way to be sure you are only adding nutrients that your soil and plants need and will use. I suggest soil testing for new gardeners as a way to educate yourself about your soil and what it needs to grow the type of plants you want to grow.
Tip #1 - Contact your local conservation district
Many conservation districts offer FREE soil testing to residents. Most at least have soil testing resources and referrals to soil testing labs.
In King County: King Conservation District
In Snohomish County: Snohomish Conservation District
Tip #2 - Send in your soil samples
The soil lab will give you recommendations on your soil but you will have to do some research to see what applies to you. Most soil sample testing labs have detailed and complete instructions on how to take soil samples and send them in. They will then email you a copy of your soil analysis. Here are a couple that I have used and like.
In Puget Sound: Simply Soil Testing USA
"A soil test is important for several reasons: to optimize crop production, to protect the environment from contamination by runoff and leaching of excess fertilizers, to aid in the diagnosis of plant culture problems, to improve the nutritional balance of the growing media and to save money and conserve energy by applying only the amount of fertilizer needed. " – UMASS Center for Agriculture, Food, and the Environment.
Tip #3 - How to read a soil test report
So, now you've received your soil test report back and you're not sure what it all means? Here's a great handbook from Snohomish Conservation District that will take you through interpreting your results and figuring out what nutrients and how much to add, if needed.
I mainly look at the N-P-K and the pH. Nitrogen is generally the nutrient that will show as lacking. That is because nitrogen is highly water soluble, meaning it leaches out of your soil easily. We got a bit of rain here in the PNW so generally we need to supplement with nitrogen sources each spring.
Tip #4 - Feed the soil
Compost is a nutrient source (average N-P-K of compost is 2-2-1) and it's importance is not to be underestimated. However, it requires active soil organisms to break it down into the nutrients your plants need. Soil organisms are less active in cold soil and become active again as soil warms in the spring. Get your garden beds top dressed (1/2" or so) with compost in the late winter to get those soil buddies working on it and ready for spring planting!
Besides compost applications, I also use a balanced (low N-P-K, like 4-4-4), organic fertilizer when planting things like tomatoes, peppers, squash, cucumbers and all brassicas (cabbage, cauliflower...). Putting a granular fertilizer in the planting hole or row gives the plants access to some fertilizer during the first month of growth. I supplement that with a liquid fish fertilizer (Alaska Fish Fertilizer or other that is mostly nitrogen, 5-1-1) that I dilute and use to water-in transplants and then apply weekly until the plants are established. Once established and growing I side dress the individual plants with more of the granular fertilizer (sprinkle fertilizer around the base of the plant and mix into the soil surface with your fingers).
Tip #5 - When to test for soil contaminates
If you are planting vegetable crops in soil that you are not sure the history of, or anytime you plan to grow an edible garden near an older home (think lead paint), it is important to test the soil for contaminates. Some contaminates can make their way into leafy vegetables and could cause issues if the contamination levels are high or for individuals who might be more susceptible.
This is another reason to contact your local agriculture extension or conservation district to find out if there are any known soil contaminate issues in your area. Some areas with a history of agriculture or industrial use have known issues with certain "persistent" chemicals. These chemicals are inorganic compounds that remain in the soil indefinitely. Arsenic is one issue in the Tacoma area due to a copper smelter. Read more.
It's ok to make mistakes!
Don't be too hard on yourself and try not to get overwhelmed with the details! Start small and simple and learn as you go. And then test your soil again.... And don't forget to have fun!