Makes sense. Another thought I just had is to make sure you are applying the same amount of total N per K with each of the three medias. This will make your results more comparable. It would take a different test than the standard soil test.. out here the total N test runs $11.50. So for $40 you can standardize your N, and have a more reputable comparison. If you decide to apply at the same N rate and need help calculating it... just pm me. Either way good luck and have fun with your experiment.