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Green Thoughts: The ins and outs of fertilizers

Today I bought some plant fertilizer. Hardly an earth-shattering event. But certainly an earth-improving one. True, there are agents which actually alter the very structure of your soil for the better – natural substances such as peat, coconut fibre or compost or more sophisticated agents such as vermiculite or volcanic perlite. All these so-called ‘amendments’ are named on account of their capacity to improve texture and aeration in your garden’s soil. Thus, in terrain that is light and sandy, they will be needed for better water retention: in flower beds composed of heavy clay, amendments will assist drainage and friability. Horses for courses.

Green-Thoughts
By Patrick Campbell

Sunday 6 June 2021, 02:00PM


ags of fertilizer are easily found at any small garden centre.

ags of fertilizer are easily found at any small garden centre.

I digress. We were about to talk about fertilizer – and artificial stuff at that. Why chemically based ones you may ask? After all, they are not environmentally friendly and research suggests that most applied to cultivated land will eventually end up in rivers and oceans causing eutrophication. That in turn can create algal blooms and other toxins in our seas.

Unfortunately, the very fact that artificial fertilizers are more concentrated is part of their appeal. Moreover, they are normally available in granular form which is easier to apply, and in Phuket at least, easier to find. Especially in regard to your container plants. Bulky organic matter may be difficult to fork into a root-filled pot; on the other hand it is easy to sprinkle a light dressing of granules on its  surface soil. And light is the word. Never over-feed. A dessert-spoonful is enough for a big 15- or 20-inch pot.

Let’s take a look at the ingredients in these fertilizers. A bag of this chemically balanced food will contain roughly equal amounts of potassium or potash (K), phosphate (P) and nitrogen (N).

The only essential plant nutrient not present in plant tissues, potassium is key to helping your plants avoid stress, caused by extremes of hot or cold, by drought or the depredations of pests. It also helps ensure that regular supplies of water are absorbed by the root system and thence to the hollow tubes (xylem) which transport water to the visible parts by a process known as osmosis. Essential for healthy growth. 

The tell-tale signs of potash starvation are yellowing of the foliage (chlorosis) and scorching at the edges of leaves. Left unattended, the plant will begin to wither. It may eventually die. However, you probably won’t have to worry about its lack in Phuket: it is far more likely to affect cool-climate veggies such as cabbages, tomatoes and potatoes. In general, there are adequate supplies of potash in your local soil, partly because its clayey nature means that naturally occurring potassium has not been leached out by torrential rain of the kind the tropics perennially experience.

Phosphate is usually extracted from rock minerals in the form of salts – super-phosphates are big business around the world. It not only promotes flowering and fruiting, but is essential for the transference of energy from one part of the plant to another. But take care. Too much phosphate interferes with the absorption of other trace elements such as magnesium and calcium, leading to stunted growth. 

The third key element required by garden plants is nitrogen. It is the macro nutrient most likely to be deficient in your garden’s soil. On the other hand, too much can make everything lush and over-leafy, short on blossom and more prone to attacks by foliage-sucking aphids. The main nitrogen-based fertilizer is ammonia (NH3), often in the form of readily available ammonium sulphate or nitrate or urea. Nitrogen is often used as a foliar feed, sprayed directly on the leaves of fruiting shrubs or trees. Used on its own, it has a rating of 34-0-0. It is produced naturally by the roots of legumes, such as peas and beans.

A few tips. Talking of ratios, most common are balanced foods with equal amounts of all three chemicals – eg 15-15-15 or 16-16-16. If you want to be on the safe side, use one with these concentrations. To promote flowering, try a 3-1-2 formula (15-5-10) or something similar. Container plants tend to require more feeding, since the potting mix is likely to be leached of minerals by manual over-watering or excessive rain. You can buy bags of granular fertiliser from stores such as Super-Cheap: better still, go to a government store which stocks a wide range of styles and sizes. And remember that organic fertilizer in the form of mature manure or compost is a more environmentally friendly alternative – if you can get it…


The information in this article and much more is to be found in my book “The Tropic Gardener”. Described in one Bangkok review as the best book on Thai gardening for 50 years, it is available for B500 (half price) to personal callers from 59/84 Soi Saiyuan 13 in Rawai (Tel: 076-61227 or 085-7827551].

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