Fixes for 3 Common Soil Problems

I ran across an article from a Horticultural Digest issued by the University of Hawaii in April of 1981. It got me thinking about common soil problems that we as interiorscapers face.

I thought I’d pass along some information on fixes for common soil problems I picked up from that article and other sources as well.

SOIL PROBLEM: Fluoride TOXICITY

Fluoride toxicity is an especially aggravating problem given the sensitivity to elevated fluoride in so many commonly used plants. Those in the Agavaceae family, such as the Dracaenas, Cordylines, and Yucca, are notorious for their sensitivity to fluoride. In addition, there are Spathiphyllum, Aspidistra, Chlorophytum, and some Marantas and Calatheas that are also touchy about fluoride levels.

Fluoride is a natural element occurring in most rocks and soils. However, concentration beyond the norm can cause discoloration and burning of leaf tissues in susceptible plants. Excess fluorides usually come from one or more of the following– air pollution, fluoridated water, fluoride in the potting medium (sphagnum peat and perlite), and the high levels of fluoride in some fertilizer components (especially those made from the rock mineral apatite, such as 20% super phosphate, as well as calcium nitrates and some limestone products).

The damage and discoloration seen with excess fluoride usually stem from or are aggravated by more than one cause. This includes excessive soil moisture, root injury, high salt level, and bad soil pH. In a perfect world, we could insert a simple soil probe that would spell out mineral imbalances, pH, salt level, and so on. Alas, this is not a perfect world and often the best we can do is trial-and-error fixes.

Fixes include:

  • Controlling the pH
    • This is most readily done by mixing dolomitic lime or other calcium sources into the soil to raise the pH to 6.0 – 7.0.* At this pH, the calcium in the lime (lime is essentially calcium carbonate) ties up the fluoride molecules so they can’t get into the plant tissues. A yearly application of a tablespoon of dolomite for each gallon of soil worked into the soil surface is recommended. Thus, for an 8” pot, 1 T of dolomitic lime should work. **
  • Limiting Fertilizer.
    • Most of these plants will do quite well being fertilized once or twice a year. You can use organic fertilizers or investigate the mineral sources in your fertilizer product to avoid super phosphate base materials.
  • Avoiding Heavily Fluoridated Water
    • It is not usually feasible to use water other than that provided at accounts but using the first two approaches should make controlling for water fluoride unnecessary.
  • Using Growing Mediums with Lower Fluoride Levels

SOIL PROBLEM: SALINITY

I have a theory that elevated salt level in the soil is responsible for more interior plant problems than many people realize. Mostly it happens because of an accumulation of unused mineral salts from fertilizer, though the accumulation is also added by minerals in the water and by the decomposition of the growing media. The result is general loss of vigor and all kinds of leaf discolorations.

Fixes include:

  • Leaching
    • Leaching is the classic treatment for salinity. Different sources suggest using 2 – 5 times the soil volume of water – going with the “more is better” philosophy. I prefer the latter.
  • Leaching with Spreader-Sticker
    • Adding a spoonful of ionic wetting agent to the water has been shown to more effectively remove soluble salts from soil media. A warning has also been issued cautioning that since many commercial growing media already have spreader-sticker in them, adding too much can create more phytotoxic reactions.
  • Adding Sugar
    • When leaching is not practical, adding sugar can promote an explosion of microbial population which will convert and otherwise lock up the soluble salts. [from J.S. Koths, Univ of Connecticut] The recommended mixture is 1 lb. sugar to 5 – 10 gallons of water and 1 cup of solution per 6” pot.**

SOIL PROBLEM: Acidity

The soil in potted plants tends to become more acidic (lower number of pH) over time. The best fix for this is to repot plants into fresh soil every year or so, but this is unrealistic for professional plant services. However, there are things we can do to help keep the soil in our plants “sweet.”

Fixes include:

  • Adding Lime
    • Described above under “Controlling the pH.”
  • Adding a Solution of Sodium Bicarbonate or Potassium Bicarbonate
    • In the Horticultural Digest article, UCR soil scientists Wesley Jarrell, Richard Shephard, and Roy Branson are cited for having solved the problem of acidity in artificial soils after stock has been planted. They found that acidity could be controlled simply by adding sodium bicarbonate (baking soda) or potassium bicarbonate to irrigation water. The recommended mixture is sodium bicarbonate at 1 T per ½ gal water; potassium bicarbonate at 1 lb. per 50 gal water (1 T / 1.5 gal). An application every 2 or 3 months, sprayed on soil or watered in, was found effective.**

*In considering pH, make sure you know the preferred pH of different plants. Aspidistra, for instance, prefers pH in the lower areas, 5.0 and less, so trying to reduce acidity will probably be counterproductive. 

** Measurements and usage advice are anecdotal only, and come from a variety of sources. They’re not to be considered absolute in any way. Use materials at your own discretion.

Marlie Graves, known as The Ficus Wrangler, has been keeping plants beautiful, training techs and relating to clients at half a dozen companies for 30 years. She studied creative writing and psychology in college and went on to start an independent film company with her first husband. She decided to focus on plants full time after completing the NYBG Horticulture School interior landscaping course. Marlie is retired, operates "The Ficus Wrangler" YouTube channel, contributes regularly to several houseplant forums, and is working on a plantcare book based on professional methods.

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