Academic Reading — Test 87

Reading passage

As the global population continues to expand and arable land grows scarcer, agricultural scientists have turned their attention to an unconventional question: what if crops could be grown upwards, in stacked layers, rather than spread across vast horizontal fields? This idea lies at the heart of vertical farming, a method of cultivation in which plants are raised indoors on shelves arranged one above another, often inside warehouses, disused factories or purpose-built structures in the centre of cities. By stacking growing surfaces vertically, a vertical farm can produce a far greater quantity of food from a given area of ground than a traditional field, making it especially attractive in densely populated regions where outdoor farmland is limited or prohibitively expensive. Two technologies make this form of agriculture possible. The first is hydroponics, a technique in which plants are grown without soil. Instead, their roots are bathed in or regularly fed with a water-based solution that contains all the mineral nutrients a plant requires, such as nitrogen, potassium and phosphorus. Because the grower controls the exact composition of this solution, plants receive a precisely balanced diet and rarely suffer from the nutrient shortages that can affect crops in ordinary fields. A further advantage is that hydroponic systems recirculate their water, capturing and reusing whatever the plants do not absorb. As a result, a hydroponic farm may use as little as a tenth of the water consumed by conventional agriculture to produce the same harvest, an efficiency that becomes increasingly valuable as fresh water grows scarce in many parts of the world. The second essential technology is artificial lighting. Plants depend on light to drive photosynthesis, the process by which they convert carbon dioxide and water into the sugars that fuel their growth. In an enclosed building, sunlight cannot reach crops stacked on inner shelves, so growers rely on light-emitting diodes, commonly known as LEDs. These devices are well suited to indoor farming for several reasons. They consume relatively little electricity, generate little heat, and can be positioned very close to the leaves without scorching them. Perhaps most importantly, LEDs can be engineered to emit specific colours of light. Research has shown that plants make greatest use of red and blue wavelengths, and many vertical farms therefore bathe their crops in a distinctive pink or purple glow produced by combining these two colours. By tuning the light in this way, growers avoid wasting energy on wavelengths that contribute little to growth. The controlled nature of the indoor environment offers benefits that extend well beyond water and light. Because the building is sealed, temperature, humidity and the concentration of carbon dioxide can all be regulated to suit the particular crop being grown. Pests and plant diseases, which thrive in open fields, are largely excluded, which means that vertical farms can often dispense with chemical pesticides altogether. Crops are also shielded from droughts, storms, frosts and other unpredictable weather, so production continues steadily throughout the year regardless of the season outside. Many operators harvest the same crop dozens of times annually, a frequency that would be impossible in a conventional field bound by the rhythm of the seasons. Vertical farming is not without its drawbacks, however, and the most serious of these is energy. Replacing the free light of the sun with electric lamps, and running pumps, fans and climate-control equipment around the clock, consumes a great deal of power. Where that electricity is generated by burning fossil fuels, the environmental advantage of saving water and land may be partly offset by the carbon emissions produced. The high cost of energy, together with the considerable expense of constructing and fitting out a suitable building, means that vertical farms have so far concentrated on fast-growing, high-value produce. Leafy greens such as lettuce, spinach and herbs are the typical crops, because they mature quickly and command good prices, whereas staples like wheat and rice remain uneconomical to grow in this way. Critics argue that until the price of renewable electricity falls further, the contribution of vertical farms to feeding the world will stay modest. Even so, the appeal of the technology is clear. Because the farms can be built within cities, food can be grown only a short distance from the people who will eat it, cutting the cost, fuel and spoilage associated with transporting fresh produce over long distances. Supporters envision a future in which a portion of every city's vegetables is grown in the very neighbourhoods where it is consumed, harvested in the morning and sold the same afternoon. Whether vertical farming becomes a central pillar of the food supply or remains a specialised niche will depend largely on the progress of cheaper, cleaner energy in the years ahead.