WHY ARE MY PLANTS HUNGRY?
UNDERSTAND THE REASONS WHILST PONDERING THE SYMPTOMS
NOT a week goes by at TAFE Horticulture where we aren’t presented with a plant problem for us to ponder, with a variety of specimens coming from students, eager to include them in their pest and disease assignments, and a good smattering coming from the general public as well.
After a pretty thorough grilling of the plant owner via a series of well-aimed questions, there’s a fair chance that we’ll arrive at a conclusion that may include such problems as attack by fungal or bacterial diseases, damage through herbicide drift, environmental extremes or mechanical injury (whipper snippers are the usual suspects!), or sometimes a malaise caused by a deficiency of one or more essential plant nutrients.
Part of the training a horticulturist receives is to recognise the symptoms of a number of “classic” plant nutrient deficiencies, taking into account such signs as variations in plant size (stunting or elongation) and flowering times, malformation of leaf margins, early fruiting, distorted fruit, changes in leaf colouring (yellowing or chlorosis, other colour changes), right down to whether the problem occurred in the newer growth or in older foliage.
In many cases, nutrient deficiencies occur as a result of some underlying problem that may include such causes as changes to physical or chemical (usually pH related) soil conditions, variations of temperature, moisture or light, even an oversupply of another plant nutrient.
At this point it’s worth noting that an experienced and competent horticulturist will continue with their investigations to discover, and remedy, the root cause of the nutrient deficiency.
Unfortunately, there are many “experts” who will simply recommend the application of extra quantities of the missing nutrient, which, at the best of times, will only temporarily relieve the symptoms, and may lock the unsuspecting gardener into applying “bandaid” solutions for a very long time.
Let’s investigate some common and obvious nutrient deficiencies, their possible causes, and suggested remedies.
Probably the most talked about deficiency amongst Facebook gardening groups, and on “guru” style TV shows, is magnesium.
It usually appears on a plant’s older foliage, as a triangular wedge of green, wide at the base of the leaf and narrowing towards the tip, or as yellowing between the veins on some plants.
Citrus trees can display classic magnesium deficiency symptoms, usually during and soon after winter, as they struggle to take up a range of nutrients in cool soils. Many gardeners also believe that gardenias can often suffer this deficiency as well.
Now I may ruffle a few gardeners’ feathers here, but my belief is that citrus, gardenias, and many other plants displaying classic magnesium deficiencies are actually lacking in a range of nutrients, and that magnesium is the most dominant symptom to be displayed.
In many cases, an application of a complete fertiliser (NPK plus trace elements) will go a long way to resolving the problem. However, there are many “experts” who will prescribe a dose of Epsom’s salts (magnesium sulphate) as a cure for these plants, which, in my mind, is purely a stop-gap, short-term fix which doesn’t address the underlying problem.
Cool soils, constantly wet soils, and an extreme rise or fall in soil pH are usually the culprits here.
Another common deficiency symptom, especially prevalent in plants that prefer to grow in acidic soils, is yellowing between the veins of young leaves and new shoots, indicating a lack of iron in that part of the plant.
Azaleas, gardenias, magnolias, rhododendrons, blueberries, and camellias all prefer to grow in soils of a pH around 5 to 6.5, but if planted in an alkaline soil (pH above 7), they will most likely display the above symptoms.
Grevilleas and banksias, indeed most plants in the Proteaceae family, can also suffer from iron deficiency, but this is usually caused by an over-application of fertilisers containing a high level of phosphorus (keep the % of P around 2% or lower, as in most “native” fertilisers)
Now it’s a fact that any iron in the soil, even if there’s ample supplies, becomes less available to plants once the pH rises above 7, so adding more in the form of iron chelates (recommended by 8 out of 10 gurus), is more often than not a temporary fix only. Most times, an effective and long-lasting remedy is to lower the pH of the soil or potting mix, usually by adding sulphur.
A final word of warning for many gardeners who may need to apply gypsum to help open up their heavy clay soils to improve structure and drainage.
Gypsum is calcium sulphate, and while it has little effect on soil pH, the extra calcium (especially if one is heavy-handed in its application) can quickly reduce the availability of many other nutrients (namely potassium, iron, magnesium, boron, zinc, phosphorus and manganese).
Plants lacking all these nutrients will generally be stunted, yellow, lacklustre, prone to disease and insect attack, and flower prematurely.
The key to using gypsum is to understand your soil texture, and only apply the recommended amounts.