November garden tasks: soil testing

November, with the time change here in Georgia, brings fewer daylight hours and a welcome drop in temperature. There are still plenty of things to do in the yard and garden this month.

November is a good time to complete a soil test in your garden and flower beds. For just a few dollars, you can know exactly what to do to your soil to give your plants the best opportunity to thrive.

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. That is, 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 (pH 7.0) soil! Simply put, soil pH is a measurement of how many hydrogen (H+) ions are in 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 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 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 leaf veins.

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 others.

Its effect on nutrient availability is a key reason that 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 (oxidize) sulfur to sulfuric acid. The acid reacts to produce sulfate (SO4) and hydrogen ions. It is these hydrogen ions which lower the soil pH.

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

Best of success with your November soil testing! More on this later.

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