The free beans on offer will be string or green beans (tua fak yao) cut into manageable lengths, and if you are lucky, winged beans (tua tak yaw). And beans are on our menu today because they are delicious, easy to grow and, as we shall see later, a source of nitrogen.
String beans come originally from Mexico, but are now widely grown throughout the tropics and are commonly seen climbing wooden supports in local small-holdings. The pods are extremely long – up to 18 inches – and are a staple of Thai food. They have a crunchy texture, and a slightly sweet taste which adds to their popularity with Thais. Particularly rich in vitamin K (as well as A and C), they are an ingredient in stir-fries, curries and much, much more.
Winged beans are the aristocrats of the family, with a tongue-twisting name to match: psophocarpus tetragonolobus. The pods are an attractive bright green, and shaped with four raised ‘winged’ edges that give them their name. They are easy to find as packeted seeds, grow quickly, especially in the shorter days when there is less sunlight, and will climb happily over any fence or large shrub. The author grew a couple of plants from seed which rapidly enveloped an entire avocado tree. Pods are produced after about 55 days.
We could go on to consider bean-like okra or lady’s fingers (kra siap), but they have been re-classified botanically and are now, believe it or not, included as members of the hibiscus genus.
These beans are, needless to say, delicious, but have to be picked young. Left too long on the vine, they become fibrous and inedible. With their delicate flavour, they are best eaten raw, but can be used in spicy nam prik or as a stir-fry vegetable. Mixed with prawn pate, they become a favourite dish called sambal. As is the case with many Thai edible plants, it is a pity that they are not more widely cultivated, since all parts of the plant can be eaten. The cooked leaves taste like spinach, the flowers like mushrooms. Even the nodulous roots can be eaten. Apparently they taste nutty…
And that, by a roundabout route, brings us to nitrogen, the last of our three key soil requirements, along with potash and phosphate. For where beans and indeed all legumes, possess added value is that they help fix nitrogen in the soil. Without getting too technical, certain micro-organisms inhabit their roots, and help produce nitrogen which is stored in root nodules. Left in the ground, these nodules add substantially to the available nitrogen in the surrounding soil. Indeed, it is common Western horticultural practice to alternate legumes such as clover, alfalfa, soybean or peanuts with other crops so that the follow-on crop benefits from the richer levels of nitrogen in the earth.
Nitrogen is a paradox. It is the largest constituent of the very air we breathe, and every Thai rai contains thousands of pounds of the stuff. It is essential for plant cell growth, it helps produce chlorophyll (the green agent needed for photosynthesis), and is present in the DNA that enables cells to reproduce. It is a constituent (3-4%) of plant tissue, and exists in such organic matter as compost, urea and manure. Yet it is the commonest deficiency in plants since it is not always in an available form. Succulent green foliage indicates a healthy supply; pale green or yellow leaves indicate it is in short supply. Weak roots may also be a sign of its absence.
So don’t forget your beans. They not only taste good: in your garden, they do good as well…
Patrick’s book ‘The Tropic Gardener’, the culmination of 13 years of writing about tropical plants and their cultivation in Phuket, is now available. Promotional price of B900, including post and packing. Dr Patrick Campbell can be contacted at his home Camelot, located at 59/84 Soi Saiyuan 13; Rawai; Phuket 83130. Tel:66 076613227 (land line), 0655012326 or 0857827551 (mobile).