For healthy and fast-growing plants, a gardener must pay attention to their soil’s pH, as well as the levels of Nitrogen (N), Phosphorous (P) and Potash (K) in the mixture. The Luster Leaf Rapitest Soil Test Kit makes soil testing easy. After dissolving a capsule into watered-down soil, users determine the results with a “color comparator” chart. An included guide instructs readers on adjusting their soil to achieve ideal growing conditions for more than 400 different plants. Available on Amazon; $12.59 for 40 kits (10 each for pH, N, P, and K).