Asking if your soil is sweet or sour--and whether your plants prefer that or not--might seem a silly question. But don't go tasting your soil yet--the question refers to soil pH, one of many factors that affect plant health and growth. pH acts indirectly on plant growth by affecting availability of nutrients, the presence of toxins and the growth of soil microorganisms.
Different people refer to soil pH in different ways. pH stands for "potential hydrogen" and measures the acidity of your soil using a scale from 0 to 14. A pH of 7 is neutral. If the pH is lower than 7, people refer to it as acidic, sour or low pH, while a pH over 7 is termed alkaline, basic, sweet or high pH. Soil pH is determined by the type of rock that formed the soil.
pH affects plant growth primarily through its effects on nutrient availability. High or low pH cause deficiencies in essential nutrients that plants need to grow. According to the Clemson Cooperative Extension, acidic soils frequently experience deficiencies in calcium, phosphorus and magnesium. Alkaline soils demonstrate deficiencies in phosphorus and many micronutrients. The availability of aluminum and manganese can also approach toxic levels in acidic soils and impair plant growth. Furthermore, soil pH affects the behavior of soil microbes, encouraging or inhibiting the growth of pathogens and affecting how well helpful microbes are able to break down organic material, freeing the nutrients it contains for plant use.
Determining soil pH requires a soil test, which can usually be performed by your local extension office. If your soil is acidic and you wish to raise it, a lime requirement test will further help you to determine what corrective actions to take. You can also purchase a home soil-testing kit from a garden center. However, as noted by the Ohio State University Extension, these tests do not provide recommendations on pH correction as that would require an analysis of soil type in addition to pH.
Soil pH primarily affects which plants will grow well in your garden. Some plants, such as blueberries and rhododendrons, like acidic soils, while vegetables and most ornamentals thrive in slightly acidic conditions. Plants such as lily and chrysanthemum are able to tolerate slightly alkaline soils. Once you know your soil pH, you will need to choose plants that will grow well in those conditions or attempt to adjust the pH through soil amendment.
The University of Florida IFAS Extension recommends selecting plants that grow well in your soil pH rather than attempting to change it, as pH adjustments can be challenging to implement and short-lived. If a plant you want doesn't grow well in your soil's natural pH, they advise growing it in a container.
Myriad factors influence the pH of soils, and gardeners can use them to adjust the pH to meet their gardening needs. However, soil pH is complicated, and no attempts to correct pH should be made without a soil test and consultation with an extension agent or other professional. Natural processes such as rainfall lower pH by leaching out minerals that make soils alkaline. Fertilizers that contain ammonium, urea or organic matter also tend to lower pH. The most common additive used to raise pH is lime. The effectiveness of all soil additives depends to a significant degree on your soil type.