Soil testing — Part one

In October, the daylight hours get a little shorter each day here in Georgia, while we welcome the drop in temperatures. Leaves begin to fall and we consider what to do with them. We nurture our fall flower beds and cole crops. There are things to do in the yard and garden this month.

One of these is to complete a soil test in our garden and flower beds and lawn. For just a few dollars, we can know exactly what to do to our soil to give our plants the best opportunity to thrive. This article serves as an annual reminder of how to go about completing this all important checkup.

First, a soil chemistry lesson is in order. A pH scale of 0 to 14 is used to show how acidic or alkaline (basic) a soil is. A neutral soil has a pH of 7. Values under 7.0 indicate increasing soil acidity, while values above 7.0 indicate increasing soil alkalinity. 

Soil pH utilizes a logarithmic scale, so a single unit of difference indicates a tenfold change. Soil at pH of 5.0 has 10 times the degree of acidity as a soil with a pH of 6.0 and 100 times the acidity of a neutral soil! 

Simply put, soil pH is a measurement of how many hydrogen (H+) ions are in the solution. Acidic soils contain more hydrogen ions while alkaline soils contain fewer hydrogen ions. 

To help understand pH, consider some common substances. Orange juice has a pH of 3.5, meaning it is strongly acidic. Baking soda has a pH of 8.4, meaning it is moderately alkaline.

Soil pH strongly influences nutrient availability to plants, and therefore affects plant growth and health. Even if your soil contains the proper amount of nutrients, plants will not be able to utilize them if the soil pH is too high or low.

If you’re noticing the signs of nutrient deficiencies in your lawn or garden, such as yellowing of leaves or stunted growth, it’s possible that your soil pH is too high or too low. It is always a good practice to have the soil pH tested before undertaking significant garden or landscaping projects.

Most plants we deal with perform best in soil with a pH of 6.0 to 7.0. At this pH range, there is good availability of nutrients for plant uptake. If your soil is overly acidic or too alkaline, certain nutrients will be less available to plants. 

One of the most commonly seen nutrient deficiencies is lack of iron caused by high soil pH. Iron becomes less available to plant roots at a pH above 6.5. This deficiency shows up as yellow leaf tissue with leaf veins remaining green. 

This is often called “interveinal chlorosis.” “Chlorosis” is a term for yellowing of plant tissue. “Interveinal” refers to the areas in between the veins of leaves.

A pH below 5.5 can also cause nutrient deficiencies. Calcium, magnesium, potassium, nitrogen and phosphorus all become less available to plants at low pH levels. Exceptions are blueberries, azaleas, gardenias, camellias, and a few others, which actually prefer a much lower pH. 

Its effect on nutrient availability is a key reason why pH is important for healthy plant growth. Fortunately, there are ways to correct a soil’s pH.

If soil is too acidic, you can increase its pH with lime. Lime is a natural material derived from limestone rock. There are two types of limestone, calcitic limestone and dolomitic limestone. Both contain calcium carbonate (CaCO3), but dolomitic limestone also contains magnesium carbonate (MgCO3), which supplies magnesium while raising pH.

One factor which influences how quickly lime reacts to raise pH is the application method. Lime reacts faster when it is tilled into the soil than when it is broadcast on the soil surface. Always till or rake lime into the soil if possible. 

Expect to wait a few months before lime begins to raise soil pH. If you need to raise your pH by more than one pH point, you may need to apply lime on multiple occasions. Lime takes months to break down and do its work in the soil, so applying it now, in the fall, is considered a best management practice. 

Elemental sulfur can be used to lower the pH of an alkaline soil. Before applying elemental sulfur, it is helpful to understand how it works. When applied to the soil, microbes convert sulfur, by oxidation, to sulfuric acid (H2SO4).  It is these positive hydrogen (H2) ions which lower the soil pH. 

Just as with lime, sulfur works faster when incorporated into the soil rather than broadly cast on the surface. If a significant soil pH change is desired (over one point of change on the pH scale), it is best to apply sulfur months before planting and incorporate it into the soil well.

More on soil testing next time.

Tim Lewis is a Georgia Green Industry Association Certified Plant Professional, gardening writer, and former Perry High School horticulture instructor. He can be reached at (478) 954-1507 or timlewis1@windstream.net