Understanding Soil pH

Dirt is that stuff we vacuum from our carpets and wash off our cars. A gardener distinguishes dirt from soil, that wonderful earth where we grow our plants. Soil is essential for plant development. Understanding soil is essential to a gardener's success. Although plants tolerate a wide range of soils, a gardener benefits from learning about the delicate relationship between a plant and the soil in which it develops.

So, what is soil? Soil anchors plant roots and serves as a storehouse for nutrients. It consists of minerals, air, water, dead organic matter, and living organisms. The mineral portion is made up of small fragments of rocks that have disintegrated by weathering. The organic portion is composed of plant and animal remains in various stages of decomposition. How much water and air a soil contains depends on the soil's texture and structure. Soil is composed of three differently sized mineral particles: sand, silt, and clay. The percentage of each determines the soil's texture as well as its physical properties. The ideal soil texture, called loam, consists of equal parts of sand, silt, and clay.

The more intensive the gardening, especially in vegetable plots or flower beds, the more important it is to understand the soil. The information from a soil fertility test is particularly valuable if the ground is being planted for the first time. A soil test is also beneficial if you are developing a permanent landscape. The information it provides allows you to incorporate needed materials before planting. Once a landscape is planted, it is more difficult to make significant changes in soil quality.

Testing soil fertility every three to five years is usually sufficient. Sampling in the early spring or late fall assures that you will have the soil test results and recommendations prior to buying lime and fertilizer. Late fall sampling will also allow you to adjust the pH of the soil in time for the next year's crop. (See Understanding Soil Fertility.)

A basic soil test kit from the Agricultural Analytical Services Laboratory at Penn State University is available through county offices of Penn State Extension for a nominal fee. The kit contains an information sheet for each soil sample, which is necessary for the lab to make lime and fertilizer recommendations. You can go to Penn State's Agricultural Analytical Services Lab website to find out more and download the information sheet.  You will receive the soil test results and recommendations in about 10 days.

Now that you have received the soil fertility report, apply the nutrients recommended on the soil test so your plants will thrive. Applying more soil nutrients than is recommended is detrimental to your plants, can reduce the quality of the fruit, can delay harvest, is bad for the environment, and is a waste of your money.

There are many benefits of applying the correct amount of lime. Liming an acid soil creates a favorable soil environment where plants can thrive. Most plants do well when the soil pH is between 6.2 and 6.8. pH is a measure of the acidity and alkalinity of a material. The pH range is 0 (extremely acid) to 14 (extremely alkaline) with 7 being neutral. When acidic soil is neutralized by liming, soil nutrients are made more available for the plants to absorb through their roots. Usually when we see a micronutrient deficiency in a plant, it is not because there is not enough of the nutrient in the soil, it is because the soil pH has limited the availability of that nutrient.

In addition to raising the soil pH, lime also supplies essential plant nutrients, calcium and magnesium to the soil. Calcium is critical if you want to avoid blossom end rot on tomatoes. Tree fruit is also very sensitive to shortages of calcium. Lime is the most inexpensive way to supply these nutrients. Lime is classified as either high calcium (calcitic) or high magnesium (dolomitic) lime. High calcium lime contains 3% or less magnesium. How do you know which type of lime you need? The soil test results will tell you. 

Whether you should apply pelletized or pulverized lime depends on the type of spreader you have. Pelletized lime is pulverized lime that is moistened and formed into a small pellet so it can be spread with a spinner type spreader. You will need a drop type spreader to apply pulverized lime. Both work equally well. If you spread it by hand, purchase whichever is cheaper.

If you grow plants that prefer an acid soil do not lime the soil around them. Lime can severely hamper the health of plants like blueberry, azalea, and rhododendron. These plants often benefit from a light application of sulfur that pushes the soil pH lower where they like it. How do you know if you need to apply sulfur to help your acid-loving plants? Get a soil test, of course. Yes, if you have an area with specialized needs, you must complete a separate soil test and indicate these plants on the information sheet so that the lab can make the correct recommendations.