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I tested my soil using the burpees test packet. Seems my soil is deficient. K is just about at medium. The pH is a solid 6, but N and P or nonexistent.

I know I can increase N with some compost from the store, but not sure about phosphorus. I imagine my N P K fertilizer should be enough to fix things but not sure if I should supplement that.

You put this under seed starting. Are you particularly thinking about for planting seeds or just in general?

I think the best amendment for deficient soils is compost, which has NPK + a lot of trace minerals and micro nutrients + a lot of soil organisms and micro-organisms to help plants use the nutrients.

If you don't have your own compost, you can buy it, but the quality of commercial compost varies widely, so be careful and ask about where it comes from, what the ingredients are. Start your own compost pile, so that later in the season you will be able to add good homemade compost.