7/24/2014 - By Harrell's

Somehow we have managed to run out of nutrients to cover in our monthly newsletter, so for the next several months I will be covering different topics in plant science and chemistry and how they affect ornamental plants. You know, the 60's slogan, "better living through chemistry". In organic chemistry, I quickly found out that all plant and animal physiology was a chemical reaction, and that all chemical reactions were a mathematical equation. And not an easy one at that!

One of the most significant principles relative to growing ornamental plants is pH or "potential Hydrogen" . Some of the basics: pH is the measurement of hydrogen ions, H+. pH goes from 1-14, with 7 being neutral. Anything lower than 7 is considered acidic, anything higher is considered basic or alkaline. Going one full digit , say 6.0 to 5.0 increases the acidity by 10 times. As it is a logarithmic scale, going from 6.0 to 4.0 increases acidity by 100x. Now how the heck does that affect plants?

Source: hubcap.clemson.edu

Soil pH mainly affects plant nutrient availability. Most soil nutrients are best available to the plant when the soil pH is between 5.5 and 6.5. When soil pH is below 5.0, iron, manganese and aluminum are more soluble and available. When soil pH is above 7.0, manganese and aluminum get tied up and are unavailable to the plant, but iron is 50% available. With a small decrease in pH to 6.5, 90% of the soil iron is available. Minor adjustments make a big difference. Low pH levels will make magnesium, potassium and phosphorous less available.

Another reason soil pH is important is that beneficial microorganisms are "happiest" at a pH of 6.5, while "mean" fungal organisms like Fusarium are more active at lower soil pH's.  Most of our Central Florida and SE woody growers are growing plants that like more acid soils around 5.0-5.5, with a few exceptions like pines and azaleas preferring the 4.5 range. Most interior and flowering plant growers prefer the 6.0 range.

A lot of this is because of their nutrient availability requirements. We alter our amounts of dolomite in soil mixes based on what types of plants are being grown. In Homestead our Hibiscus, Mandevilla and foliage growers will often add 10-12 lbs. per cubic yard dolomite to get in that 6.0-6.5 range, while our central Florida woody grower will add 5-6 lbs. dolomite to be in the acidic range that they prefer.

Dolomite is a limestone with good levels of calcium as well as magnesium. It is preferred over straight limestone due to its magnesium content. With calcium being a critical secondary nutrient, how do we provide it for plants that prefer acid soils? By adding gypsum or calcium sulfate, which are relatively pH neutral.

Soil pH needs can also influence what types of fertilizer to use. Some woodies like sulfur based nutrient sources such as sulfate of potash or SCU, because sulfur acidifies soil, while plants in "sour" soil, or very acidic soil, may need it "sweetened" up by top dressing with dolomite. A tablespoon per gallon of soil is a good rule of thumb. Where topdressing dolomite is not feasible, such as in bedding plants, consider drenching Limestone F, a flowable dolomite. Fluoride sensitive plants such as Dracaena 'Warneckii' or 'Janet Craig' and Easter Lilies like a bimonthly application of dolomite to prevent higher pH levels tying up Fluoride.

Water quality, next month’s topic, definitely affects soil pH through time. In Homestead, where well water pH levels are in the high 7's, soil pH will creep up over time. Pure water is neutral at 7  but rain water is usually acidic and around 5.0. Hmmm. Well rain water reacts with the carbon dioxide in our atmosphere to form carbonic acid. While this is a natural occurrence, with industrial pollution, things like sulfur compounds further react with water making sulfuric acid or acid rain. But water quality is our topic for next month, so I will leave you on the edge of your seats until then.