Taking the Mystery out of Soil pH

Most gardeners know that soil pH is an important soil characteristic, but they may not be aware of what pH really is and what it does. pH is measured by a soil test. How it affects productivity, plant growth and yields is important to all gardeners.

One of the biggest reasons pH is important is because it affects nutrient availability. Most nutrients are available in good quantities around 6.5 as you can see in the scale below.  The wider the bar the more available the nutrient, however, the skinny portions don’t mean the element is not there; it means it is not available to the plant. Some organic soils have pH of 7.5 or higher, which means plants potentially will show copper, zinc, iron, boron and manganese deficiencies. It is too difficult to change pH of most organic soils, so the alternative is to apply these nutrients as foliar sprays during the growing season. Phosphorous is limiting in low pH soils, especially early in the season when soil is still wet and cool. That is why many blueberry plantings have purple phosphorous deficient leaves that will eventually turn green when temperature warms.

The main elements affecting soil pH are calcium, magnesium and potassium and on their own these elements keep pH on the alkaline side. Soil pH is related to how well the soil retains these elements. In nature, sand typically has a lower pH than clay since water (rain) moves through sand faster than clay, sand can be quite high in pH. Most non-amended, well-drained sand soils will generally stabilize at a pH around 5.0 to 5.5, a value too low for most crops except blueberries and potatoes. Most commercial fertilizer applications also lower pH. When pH drops low enough, lime will be recommended to bring it back up.  The amount of lime needed depends on soil type.

Nutrient availibility at various pH levels

Occasionally, pH needs to be lowered. If it only needs to be lowered slightly, it can be done through standard fertilizing practices using an acidic fertilizer such as ammonium sulfate. If more significant changes are needed, you will have to add sulfur. Sulfur rate depends on soil type – don’t guess, get a soil test done. Lowering pH with sulfur is a biological process. Soil borne bacteria takes sulfur, combines it with oxygen and water to make sulfuric acid. The rate this occurs depends on the amount of water and oxygen in the soil and temperature. If sulfur is applied in the fall, little change occurs until the next year when the soil warms. Aluminum sulfate is often recommended to lower pH, but be careful using it since the aluminum portion can become toxic and becomes more available at a lower pH.

Soil micro-organisms

Bacteria are generally more prevalent at higher pH and fungi at lower pH. Not surprisingly a soil pH between 6 and 7 will be the best environment to host both. When it comes to soil microbes, the more, the better. By making soil friendlier to good microbes it can help work against the bad ones. Microbes are also responsible for making soil nitrogen available. There is a lot going on underneath your feet!

Knowing that pH affects microbial activity can help combat some diseases like potato scab and club root. Both diseases are fungi whose severity is related to soil pH.

I hope after reading this you have a better understanding of the importance of soil pH in the performance of your lawns and gardens. Do yourself a favor and conduct periodic soil tests to determine nutrient levels as well as pH. You should have a soil test done at least every three years, more often, on sandy soils since they can change more quickly, and more often if you have a problem site you are trying to correct. Contact your county extension to obtain a soil test. Recently I had the opportunity to tour the soil test lab at Michigan State University. It was a very enlighting experience.

Soil samples to be tested	Testing for organic materials