Plant nutrients are generally most available to plants in the pH range 5.5 to 6.5. Aluminum may become toxic to plant growth in certain soils with a pH below 5.0. The structure of the soil, especially of clay, is affected by pH. pH determination (soil test) will tell whether your soil will produce good plant growth or whether it will need to be treated to adjust the pH level. For most plants, the optimum pH range is from 5.5 to 7.0, but some plants will grow in more acid soil or may require a more alkaline level.