For optimum growth, soil should be tested periodically throughout the growing season. It is especially important to test before planting in the spring and when preparing beds in the fall. It is also a good diagnostic tool to employ if you feel your plants are not growing well. The Rapitest Soil Test Kit provides gardeners with both a simple to use and very quick method of measuring and tracking these important factors – soil pH and three macro-nutrients – Nitrogen (N), Phosphorus (P) and Potassium (K). . Soil Test Kit – $23.99. The kit contains 4 colour comparators, 40 test capsules (10 each for soil pH and three macro-nutrients N, P, K), instructions for adjusting soil pH, fertilization guidelines and a pH preference list for over 450 plants. . Knowing the correct soil pH level is essential for optimum plant growth. The availability of nutrients depends on the soil having the appropriate pH needed by a crop or ornamental plant. Micro-nutrient availability is especially vulnerable to incorrect soil pH.