Supersoil Organic Fertilizer?

Is it ok to use different types of organic fertilizer in the same quantaties to equal the 15-21-0 NPK ratio? Or does Subcool use those specific fertilizer because they breakdown how he likes? Also why is the K at 0 and is it ok to add some fertilizer with some K into the soil or will that cause a problem?

Aurora mixes the Roots soil with a significant amount of coco coir. As coco breaks down, it naturally releases K. Also, roots soil has kelp meal listed as an ingredient on the bag. So while super soil isn't amended with a high K fertilizer, there seems to be a sufficient amount to finish my plants off beautifully. Looks weird on paper I suppose, but you can't really argue with the results.