Ask the Plant Doctor! How to achieve optimal soil pH and deal with whiteflies

Ideally, aim for soil pH 5.5-6.5, but this may not be practical for large gardens. Photos:

Do you have a question about plants or how to maintain your garden? Send your questions to the Plant Doctor!

Email your questions to with "Plant Doctor" in the subject field. Questions may be edited for brevity and clarity.

Recently, I tested my soil using a soil tester. The result was that most of my soil was acidic, with a pH level below 3. In addition, I suspect the acidic soil has made my okra trees grow poorly, whereby burned leaves are often seen. Other than putting sulfur or ashes to make my soil alkaline, what are my other options? I used to add coffee grounds to my compost, but have since refrained from doing so. Kindly advise. – Vinnie Goh

Malaysian soils are inherently acidic, with pH levels typically between 4 and 5. More acidic soils do exist in this country. Acid sulfate soils, for instance, are predominantly found along the coastal areas, and they can have a pH below 3. Hence, that your garden soil has a pH below 3 is unusual. Prolonged and excessive application of nitrogen-rich fertilisers may also increase soil acidity.

High soil acidity may be why you see burn-like appearance on your leaves (called leaf necrosis) due to nutrient unavailability, particularly potassium. Your immediate task, therefore, should be to raise the soil pH, not to add even more fertilisers. Avoid coffee grounds, as its caffeine content is allelophatic (toxic) to soil life and plants.

Also avoid sulfur, as it will further acidify your soil! Instead, apply liming materials, such as limestone or calcite (calcium carbonate), dolomite (calcium and magnesium carbonate), quicklime (calcium oxide), and hydrated lime (calcium hydroxide) at a rate of about 400g per square metre per year. If possible, mix the lime with the soil to enhance its effectiveness, but expect to wait several months before a noticeable rise in soil pH. Applying organic matter to your soil would also raise soil pH, but it is not as effective in this task as liming materials.

Ideally, aim for a soil pH 5.5-6.5, but this may not be practical for large gardens. Moreover, many tropical plants have adapted and can thrive in acidic soils, so long as their nutrient requirements have been fulfilled, so achieving an optimal pH is often not mandatory.

Dealing with whiteflies is one of the tasks in planting chillies.Dealing with whiteflies is one of the tasks in planting chillies.

Most chilli plants attract small whitish flying pest under the leaves. How do we get rid of it? How can we prevent this pest from multiplying? – Yip Aarolyn

Whiteflies are probably the culprit, as these insects are a common pest of chili plants. These pests typically live on the underside of leaves and will swiftly scatter upon disturbance. Their sustenance comes from sucking the sap from leaves, that results in a sticky residue on leaves.

Over time, this residue can lead to the development of black molds. The damage caused by whiteflies includes yellowing of leaves, hindered growth, and an overall decrease in plant health and vigour.There are several strategies for mitigating the impact of whiteflies.

First, consider utilising yellow sticky tapes, which can be conveniently purchased online. These tapes lure and entrap whiteflies. Second, insecticidal soap is also a potent and eco-friendly option that can be found online. This soap envelopes the insects and disrupts their cell membranes, leading to dehydration and subsequent death.

Ensure the soap is sprayed on the leaf undersides, where the whiteflies usually live, and that the insects are fully coated. Regular applications may be necessary until the infestation is under control. Third, neem oil spray is another effective solution. It can be made by mixing 10 ml of pure neem oil (can be found online) with 1 litre of water, followed by adding roughly 10 drops of low-sodium detergent.

Ensure to mix thoroughly before spraying onto infected leaves. Last, you can introduce Nasturtiums as trap plants in your garden. Plant them near your chilli plants. Nasturtiums are known to attract whiteflies, diverting them away from your chili plants. The whiteflies on the Nasturtiums can then be exterminated collectively.

Do you need a green thumb to do the gardening? – Daria Morozova

We do not have to be born with innate gardening skills to succeed (and enjoy) in gardening. Like any other skills, gardening prowess can be nurtured over time. To enhance your gardening skills, consider the following strategies:

1. The Internet is replete with gardening resources, but prioritize resources from university extension services, as they provide the most accurate and evidence-based information. These sources contain extensive guides on different plants, their specific needs, common pests and diseases, and how to address them.

2. There is no substitute for actual hands-on experience. Start small. For instance, begin with easy-to-grow plants, then gradually take on more challenging projects as your confidence and skills grow. Any failures should not be viewed as setbacks, but as opportunities towards honing your abilities.

3. Join gardening communities, whether virtual or physical. These communities allow you to gain wisdom from seasoned gardeners, seek the advice from others, and be inspired. Some of these communities also provide hands-on training and guidance from experienced gardeners.

4. Find a gardening mentor, someone with experience who can provide personalized advice and guidance. This could be a member of a gardening community you have joined or a knowledgeable neighbor.

6. Stay curious. Gardening is a field where there is always something new to learn. Always be curious and open to new ideas and techniques.

7. Try visiting public gardens and nurseries. These places can offer inspiration for your own garden and provide opportunities to learn about different plants and their care.

8. Experiment, experiment, and experiment. Try different plants, apply varied techniques, and experiment with unique arrangements. Each experiment, regardless of the outcome, brings you one step closer to becoming a proficient gardener.

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