As we’re into spring now, we tend to look at our plants closely in anticipation of the year’s anticipated growth of colorful flowers and fragrance. After an enduring winter, we occasionally see things that concern us and we become alarmed, such as when our plants have yellow leaves. Before running frantically to the local garden center to buy every product on the shelf, it’s a good recommendation to research and analyze the situation so that an accurate diagnose can be made.
So, if there is an issue, when diagnosing the problem, it’s recommended to look at all the possibilities that may have contributed to this condition. First of all, try to recall the recent history of the plant, including the over-all health condition the plant has been in the past and try to remember any instances or past care or situation including the weather to determine what is more likely the issue. Second, research any deviation that occurred, such as a new construction process of your home involving chemicals or physical occurrences such as tractor use. Occasionally, in the process of construction, workers could have spilled chemicals in washing equipment or workers may have used heavy equipment near plants possibly damaging roots. Also, notice if the condition involves one plant or if all plants are showing stress conditions. Third, notice if any insects are crawling of flying around the plant and if the yellowing is occurring on the lower portion of the plant or the upper portion. Don’t be alarmed if the lower portion is showing signs of yellowing on a few leaves. Sometimes this is the case when sunlight is limited such as when plants are crowded or the plants are against the house. Finally, notice any physical injuries such as a broken or cracked limb or mechanical injuries from weed eaters are present. All of these factors and more can and will contribute to yellowing of leaves.
One plant that has historically been subject to yellowing leaves issues is the gardenia. There are a couple of issues involving gardenias and that is leaf yellowing and leaf drop.
Watering: In general over-watering is not good for all plants. Gardenias are especially sensitive plants and are responsive to soil conditions as to over-watering. If your irrigation/sprinkler system control schedule is set too long and you have been watering these plants regularly, you may be having issues. In addition, over-watering promotes issues such as leaf diseases and root diseases such as root rot. If you water less frequent the probability of root rot decreases.
Leaf Drop: Another issue gardenias are subject to is leaf drop. First of all, the ideal condition setting for gardenias is in full sun. Gardenias need plenty of sun, and if leaves start to drop, it may be related to too much shade. In shade you’ll probably notice the plant will grow less dense and spindly as well. If gardenias are placed in full sun you have better results with a healthier and denser plant.
Sometimes when a plant doesn’t look good, the home owner may think the plant has a disease or needs water. Another factor that is related to this condition includes nutritional/fertilizer needs. The first thing to do before spending any money on “snake-oil” type treatments, that may not work, is to obtain a soil test kit from your local LSU AgCenter extension Office and conduct a soil test as instructed from the kit. After you receiving the results, if needed, you can consult with the local Extension County Agent for further assistance. It’s recommended to perform this test in the fall of the year to prepare for the spring if needed.
In terms of humans, not everyone eats and enjoys onions and some people may be even allergic to them and other foods and as a result, problems may occur if they are eaten. You can think of the plant in a similar way, not all plants are alike in nutritional requirements. For instance, azaleas grow well with a soil pH range of 5.0 – 5.5 while tomatoes grow well with a soil pH of 6.0 – 6.8. If plants are not in their ideal soil environment, problems may occur, such as exhibiting yellowing and chlorotic leaves. It may sound contradictory, but by simply adding fertilizer may not be the cure. Without the proper or suitable soil pH, the plant will NOT absorb nutrients no matter how much you add and the plant will still look mal-nourished. The pH has to be at the specific level required for the specific plant and therefore, the plant will be able to absorb the nutrients. So, soil chemistry is very important.
pH and nutrient availability: Some nutrients become insoluble if the soil pH is too high or too low, limiting the availability of these nutrients to the plant root system. Nitrogen (N) is readily available in a soil pH mid-range of 6.0 - 8.0, then below 6.0 and above 8.0, nitrogen becomes less available. Simply stated, nitrogen (N) will “green-up” plants but unlike iron (Fe) it will promote growth and inhibit and possibly delay and reduce fruiting. Potassium (K) is most available from slight acidic to alkaline soils that are above 6.0 and tapers off in its availability below pH 6.0.Iron (Fe) is readily available in acidic soil (i.e. below pH 7), where its optimum range is pH 4.0 - 6.5. Iron (Fe) is involved in the manufacturing process of chlorophyll, and it is required for certain enzyme functions. Unlike nitrogen (N), Iron (Fe) will “green-up” a plant without promoting excessive growth. Finally, molybdenum (Mo) is readily available in higher pH (i.e. above pH 7) and limited in acidic (i.e. below pH 7).
pH and the availability of specific nutrients: Other nutrients are subject to pH fluctuation. Soil chemistry and pH is quite challenging. Landscape plants may exhibit nutrient deficiency or toxicity symptoms, as a result of highly acidic or alkaline soil pH.
In acidic soils, the availability of plant nutrients such as potassium (K), calcium (Ca), and magnesium (Mg) is reduced, while availability of potentially toxic elements such as aluminum (Al), iron (Fe), and zinc (Zn) are increased.
In alkaline soils, iron, manganese (Mn), zinc, and boron (B) are commonly deficient.
Plants and their optimum pH and problems associated with pH: Each plant has an ideal pH range it best performs at. Some plants can tolerate a wide range. Optimal ornamental ranges can vary from 4.5 - 8.0. For instance, azaleas require a specific pH range of 5.0 - 5.5, citrus needs a range of 5.5 - 6.5, in contrast, verbena prefer a range of 6.0 - 8.0. Leatherleaf fern is one plant that is tolerant of a wide range of soil pH. It can grow and look great in soil having a pH range of 4.0 - 6.5. If the pH range does not match the plant requirements, although there may be ample nutrients in the soil, the plant simply cannot and will not be able to absorb them. There are varying degrees of pH and growth responses.
Altering pH: To alter the soil pH takes time. In short, you can typically raise or increase the soil pH with the use of agricultural limestone. If you wish to lower the soil pH, sulfur or a fertilizer with sulfur such as ammonium sulfate, is normally used. This process may take up to four to six months or even longer depending on the soil or media type. In general and as a rule, the heavier the soil (i.e. more clay or organic matter) the longer the process of change.
Alkaline soils: The common materials used to lower the soil pH is with the use of elemental sulfur (S), microbes oxidized the elemental sulfur into a sulfate (SO4) and H+ resulting in the lowering pH. Hydrogen (H+) ions also are produced with the use of ammonium (NH4) based fertilizer. The addition of soil organic matter (SOM) will also aid in lowering the pH of your soil.
Acidic soils: Depending upon where you live and soil type (i.e. sandy vs clay), over time, with rain in our humid climate, soils become acidic due to the leaching. In addition, with the addition of fertilizers and organic matter, the acidulation process is enhanced.
Monitoring of soil pH is important, for the fact if the pH becomes very acidic, (below pH 5.4), some minerals such as aluminum and manganese become very soluble and often are toxic to plants. Leaf yellowing, puckering, and burning symptoms may appear. Plant nutrients such as calcium and magnesium often are deficient in acidic soils. For legumes, beneficial soil bacteria that fix nitrogen on the roots of legumes such as beans and peas will not survive in acidic soils, and then those crops will suffer.
The common material used in raising soil pH is with the use of lime. Lime is a source of calcium (Ca) that comes in many forms such as calcium carbonate - CaCO3, dolomite - CaMg(CO3)2 , Calcium oxide - CaO, or Calcium hydroxide - Ca(OH)2 to name a few. One more source of calcium is Gypsum. Gypsum - CaSO42H2O is a moderately soluble source of the essential plant nutrients, calcium and sulfur, and can improve overall plant growth. Gypsum amendments can also improve the physical and chemical properties of soils; Most of our agricultural use is from calcium carbonate and dolomite. In the soil, the lime reacts with carbon dioxide and water yielding bicarbonate (HCO3-), thus displacing or removing (H+) ions and raising the pH in the process. Consult with your Extension County Agent for the best selection.
As a general rule, in the winter, an indicator of poor over-all fertility in a lawn is the presence of clovers. Excessive weeds may indicate the pH is too acidic. Years of annual applications of Ammonia sulfate may result in this condition. Alternating alternative sources of nitrogen based fertilizer, such as urea, is recommended.
So, before you run to the garden center to purchase expensive fertilizer, first check your soil pH. Soil chemistry with special interest directed towards soil pH is very important. pH is “THE” controlling factor for nutrient uptake.