Earth as a System: Energy, Matter, and Life: Class 9 Science · Chapter 13

In May the plains of north India bake under a fierce Sun. That heat helps pull moist winds in from the sea a few weeks later, and the monsoon arrives. The rain fills rivers, the rivers carry fresh soil to the coast, and the fish near the coast feed on what the rivers bring. Not one of these events stands alone. Each is a link in a chain that runs through air, water, rock, ice and living things.

The Earth as one system

The Sun supplies almost all the energy that keeps this chain moving. Two smaller sources add to it. One is heat seeping up from deep inside the planet. The other is energy given off when substances in air, water and rock react with one another. In your middle-school classes you met winds, the water cycle, seasons, nutrient cycles and pollution as separate topics. This chapter joins them up. Scientists picture our planet as one Earth system, made of five connected parts called spheres.

• The geosphere (भूमंडल) is the solid Earth: rocks, soil, mountains and plains, right down to the hot interior. The black cotton soil of Maharashtra and the old, worn Aravalli hills both belong to it.
• The hydrosphere is all the liquid water: the seas, rivers such as the Narmada, lakes such as Chilika, and the groundwater that a village handpump draws up.
• The cryosphere is water frozen solid: glaciers such as Gangotri, winter snow on the Pir Panjal range, sea ice and the polar ice caps.
• The atmosphere is the layer of air wrapped around the planet, from the humid monsoon air over Kerala to the thin, dry air above Spiti.
• The biosphere is every living thing and the place it lives: the Sundarbans mangroves, a wheat field in Punjab, coral reefs off Lakshadweep and the plankton drifting in the Bay of Bengal.

The spheres are not sealed boxes. A falling raindrop is part of the atmosphere. Once it reaches a stream it is part of the hydrosphere, and after a tree drinks it, part of the biosphere. Fig. 13.1 lets you pick out each sphere in one landscape.

Scale this up to the whole subcontinent. The monsoon draws much of its moisture from the Arabian Sea, so climate scientists watch its surface temperature closely. A warmer sea puts more water vapour into the air, and the rains turn harder to predict: a district may be swamped one week and parched the next. Rising air temperatures reach the frozen world too. As Himalayan glaciers and polar ice shrink, glacial lakes can overflow, and the oceans slowly rise and creep towards low-lying coastal towns. Every one of these changes also costs plants and animals their habitats.

13.1 Uneven Heating of the Earth

Does every part of the Earth get the same share of the Sun's energy? On the same June afternoon, people in Chennai swelter at 38 °C while sea ice still floats on the Arctic Ocean. To see why the heating is so uneven, we first need to know what sunlight is.

Sunlight is electromagnetic (EM) radiation. EM waves carry energy and need no medium: they cross the near-empty space between the Sun and the Earth at the speed of light, $3\times {10}^8\mathrm{m}\mathrm{/}\mathrm{s}$. Sound is different. In Chapter 10 you saw that sound is a mechanical wave, so it cannot travel through a vacuum at all.

EM waves come in a huge range of wavelengths, called the electromagnetic spectrum (Fig. 13.2). The shorter the wavelength, the higher the frequency and the more energy each wave carries. Gamma rays and X-rays sit at the short end; they carry so much energy that they can damage living cells. Radio waves sit at the long end.

Of everything the Sun sends out, only three bands carry much energy to us: ultraviolet (UV), visible light and infrared (IR). Between them they hold about 99 % of the energy in sunlight. The rest hardly matters for heating the planet: solar gamma rays and X-rays are soaked up far above our heads, and the Sun's microwaves and radio waves are too feeble. The three important bands do very different jobs on their way down:

How much solar energy actually arrives? Two numbers answer this. Imagine a solar panel parked in space just above the atmosphere, turned to face the Sun squarely: every square metre of it would catch about 1.4 kilowatts. That figure is the solar constant. The solar energy that finally lands on the Earth's surface, per square metre per second, is called insolation (सूर्यातप).

Every calculation of the Earth's energy budget, and so of climate, begins with the solar constant. Think of it as the Sun's full delivery arriving at the gate. On the way down to the ground, air molecules, dust and clouds absorb or scatter part of the load, so even at noon on a cloudless day the insolation at the surface tops out near $1\mathrm{k}\mathrm{W}\mathrm{/}{\mathrm{m}}^2$. Most of India lies between about 8° N and 37° N, and most parts of the country receive 4 to 7 kWh of solar energy per square metre every day (Ministry of New and Renewable Energy). That sunshine heats the land that sets off the monsoon, ripens the crops and offers a vast supply of clean electricity.

Insolation is a power per area. To find the energy collected, multiply by the area facing the Sun and by the time:

13.1.1 Interaction of solar radiation on the Earth's surface

Walk barefoot at noon from a white marble floor onto a black granite step and your feet notice the difference at once. Park a white car and a black car side by side in May, and the black one is far hotter to touch. You already know from Grade 7 that land and water warm up at different rates; colour and material matter just as much. Pale, shiny surfaces send most of the sunlight back, while dark ones soak it up.

Scientists put a number on this. The albedo of a surface is the fraction of the incoming sunlight that it reflects (the word comes from albus, Latin for white). Fresh snow, with an albedo near 0.85, throws back most of the light and stays cold even in bright sunshine. A tarred road, near 0.05, keeps almost all of it and turns scorching.

The Earth as a whole has an albedo of about 0.3. Follow 100 units of sunlight arriving at the top of the atmosphere in Fig. 13.3. About 30 units are reflected straight back to space by clouds, air, ice and the ground. About 23 units are absorbed by the air and clouds. The remaining 47 units are absorbed by land and sea, and that is what warms the surface.

Every warm object also gives off heat, as invisible infrared. Sit on the stone steps of a river ghat an hour after sunset and you can still feel the day's heat flowing out of them. Brick and concrete buildings store heat in the same way and release it through the night, which is why top-floor flats stay stuffy long after dark. Homes with thick mud walls, a thatched roof or plants on the roof take in heat more slowly and stay more comfortable.

13.1.2 Latitude and Earth's shape

Shine a torch straight down onto a table and you get a small, bright circle. Tilt the torch and the same light spreads into a long, dim oval. Sunlight on the round Earth behaves the same way (Fig. 13.5). Around the equator the noon Sun stands high, so its rays land almost straight down and pack their energy into a small area. Towards the poles the Sun never climbs high; its rays arrive at a low slant and the same energy is spread over a much larger area. So the tropics stay warm through the year and the polar lands stay icy. This difference in heating between the equator and the poles is the engine behind the planet's winds and ocean currents (Section 13.2).

The tilt of the Earth's axis adds a second effect. As the Earth goes round the Sun, the latitude where the noon Sun is overhead moves between about $23.4\mathrm{°}$ N and $23.4\mathrm{°}$ S. In June it is overhead near the Tropic of Cancer, which passes through Gujarat, Madhya Pradesh and West Bengal. North India then gets steep rays and long days: summer. In December the Sun is overhead far to the south, its rays reach India at a slant and the days are short: winter. Try it in the simulation below.

13.1.3 Role of the atmosphere

Gravity holds a layer of air around the Earth: the atmosphere. Dry air is about 78 % nitrogen and 21 % oxygen. Argon makes up almost 1 %, and carbon dioxide only about 0.04 %. Water vapour varies from almost none over the Thar in May to about 4 % in sticky monsoon air.

The atmosphere is arranged in layers, and each layer behaves differently (Fig. 13.7). The bottom layer, the troposphere (क्षोभमंडल), is on average about 12 km thick: roughly 16–18 km over the equator but only 8–10 km over the poles. It is heated from below by the warm ground, so the air gets colder as you go up, by about 6.5 °C for every kilometre. Warm, moist air near the ground rises, cools and forms clouds. Almost all weather, from a light breeze to a cyclone, happens in this layer.

From about 12 km to 50 km lies the stratosphere, which holds the ozone layer. Ozone absorbs UV from the Sun, so here the air gets warmer with height. Warm air sitting on top of cooler air does not rise, so the stratosphere is calm and stable. It acts like a lid that keeps the weather below it. Long-distance jets cruise near the top of the troposphere or just above it, where the ride is smoother. Higher still come the mesosphere, the thermosphere and the exosphere, which fades into space. These high layers hardly affect the climate at the surface. About 100 km up is the line usually taken as the edge of outer space.

The atmosphere does two jobs at once, like a sun umbrella and a quilt. As an umbrella, it takes the edge off the incoming sunlight: ozone soaks up most of the UV, and gases, dust and clouds absorb about a quarter of the sunlight before it reaches the ground (Fig. 13.3). As a quilt, it slows the escape of heat. Sunlight warms the ground, and the warm ground radiates infrared upward. Molecules of carbon dioxide, methane and water vapour absorb that infrared and give it out again in all directions, partly back towards the ground. Trapping heat in this way is called the greenhouse effect (हरितगृह प्रभाव), shown in Fig. 13.8.

Without this quilt, the Earth's average surface temperature would be about −18 °C and the oceans would freeze; with it, the average is a mild 15 °C or so. Adding more greenhouse gas thickens the quilt. Burning coal, oil and gas has pushed the amount of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ in the air up by about a third since 1960, and the planet is now roughly 1.4–1.5 °C warmer than in the late 1800s (Table 13.5). This extra heating is global warming. Left unchecked, it could make large regions of the Earth very hard to live in.

The atmosphere is never still. Besides sunlight, it is struck by streams of charged particles from the Sun and by fast particles from deep space called cosmic rays. High up, these make some layers electrically charged. Those layers bend radio signals back to the ground, and a strong solar storm can disturb radio, GPS and satellites. Together, all these layers control how energy flows into and out of the planet, shaping weather, climate and life. No other planet we know has an atmosphere quite like ours.

13.2 Uneven Heating Causes Wind and Ocean Currents

Air moves from where the pressure is high to where it is low, and we feel that movement as wind. The pressure differences come mostly from uneven heating. Air over a warm patch of ground expands, becomes lighter and rises, leaving lower pressure behind. Cooler, denser air sinks and raises the pressure. You have already used this idea for sea breezes by day and land breezes at night. The same idea explains winds that blow across a single valley and winds that circle the whole planet.

13.2.1 Local winds

Spend a clear day in a Himalayan valley and you can feel the wind change direction twice. By late morning a breeze blows up the hillsides; paragliders in Himachal ride it. After dark the air flows the other way and pours down the slopes. Why? The sunlit slopes face the Sun more directly than the deep valley floor, so by mid-morning the ground on them, and the thin layer of air resting on it, is warmer than air at the same height over the middle of the valley. Warm air is less dense, so it creeps up the slope and valley air is drawn up behind it: a valley breeze. At night the slopes radiate their heat to the open sky. The air touching them chills, becomes denser than the air beside it, and drains downhill into the valley like water: a mountain breeze (Fig. 13.9).

Because cold air drains downhill at night, the valley floor is often the coldest place by dawn, and in winter that is where frost and fog settle first. Hill farmers in Himachal and Kashmir know this: they often plant apple orchards on the slopes rather than on the valley floor, where a late spring frost could kill the blossom. These daily breezes also carry moisture up and down the valley, which affects how quickly the soil dries and where clouds form on summer afternoons.

13.2.2 Planetary winds

On the scale of the planet, the big temperature difference is between the equator and the poles. This sets up broad belts of high and low pressure that circle the Earth, and the winds that blow steadily between them are the planetary winds (Fig. 13.10).

• At the equator, strong heating makes warm, moist air rise. This leaves the equatorial low-pressure belt, a region of frequent thunderstorms.
• High up, the risen air spreads towards the poles, cools and sinks again around 30° N and 30° S. The sinking air piles up at the surface as the subtropical high-pressure belts. Many of the world's great deserts, including the Thar, lie near these latitudes, because sinking air is dry.
• Most of the surface air from the subtropical highs flows back towards the equator, completing one loop. Part of it flows the other way, towards the poles.
• Around 60° N and 60° S that poleward air meets very cold air from the polar regions and is pushed upward, forming the subpolar low-pressure belts.
• Over the poles the bitterly cold, dense air sinks, forming the polar highs. Air flows out from them towards the subpolar lows, completing a third loop.

If the Earth did not spin, these winds would blow straight from high pressure to low pressure, along the lines of longitude. But the Earth rotates, and moving air is deflected: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. So air heading for the equator from 30° N arrives from the north-east, as the north-east trade winds. Air heading poleward between 30° and 60° bends into the westerlies, and air leaving the poles becomes the polar easterlies. In India, the northeast trade winds are the dry winds of winter; in summer the monsoon reverses them.

13.2.3 Ocean currents

The oceans move too. An ocean current (महासागरीय धारा) is a steady flow of a huge mass of sea water in one direction. Several things drive it:

• Wind. The planetary winds blow over the sea for thousands of kilometres, and friction drags the surface water along with them.
• Temperature. Water warmed at the equator is lighter and spreads poleward along the surface. Cold polar water is denser; it sinks and creeps back towards the equator along the deep ocean floor.
• Salinity. Saltier water is denser than less salty water. Very salty water sinks; water freshened by rivers or melting ice stays near the top.
• The Earth's spin deflects moving water just as it deflects wind, and the continents block currents and steer them sideways.

Together, the winds and the spin make each ocean basin's surface water circle round in a giant loop called a gyre. Gyres turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere (Fig. 13.11).

Here is a puzzle. Hammerfest in Norway lies at about 71° N, inside the Arctic Circle, yet its harbour stays open all winter. Churchill, on Canada's Hudson Bay, lies much farther south at about 59° N, but its port is locked in ice for more than half the year. The difference is an ocean current. The Gulf Stream carries warm water out of the Gulf of Mexico and across the North Atlantic, and its extension, the North Atlantic Drift, bathes the coast of Norway. All over the world, currents work like a conveyor belt for heat: warm water flows poleward and cooler water flows back, narrowing the temperature gap between the tropics and the poles.

Currents also feed the sea. Where winds push surface water away from a coast, cold water wells up from below and brings nutrients up from the deep. During the southwest monsoon this happens along the coast of Kerala. The plankton that bloom in the cool, rich water feed the huge shoals of oil sardines that Kerala's fishers depend on.

13.3 Biogeochemical Cycles

A banyan tree takes carbon dioxide from the air, water and minerals from the soil, and energy from sunlight. When a leaf falls, fungi and bacteria break it down and hand those materials back. Living things (the biotic part of the Earth) are always trading matter with the non-living air, water, soil and rock (the abiotic part). Because the Earth gains almost no new matter from space, the same atoms of carbon, nitrogen and oxygen have to be used again and again.

The repeated journey of an element between living things and their surroundings is called a biogeochemical cycle: bio for life, geo for the Earth, chemical for the elements. Energy flows in from the Sun and leaves as heat, but matter goes round and round. These cycles link ecosystems to one another. That linking is also what lets a damaged forest or a polluted lake recover in time, as long as the damage is not too great.

We will follow four of these cycles: water, carbon, nitrogen and oxygen. For each one, notice which spheres it passes through, what normally keeps it steady, and where human activity is pushing it off balance.

13.3.1 Water cycle

You already know the steps of the water cycle (Fig. 13.12). The Sun's heat evaporates water from seas, rivers and lakes, and plants give off water vapour from their leaves by transpiration. The rising vapour cools, condenses into tiny droplets and forms clouds. The water returns as precipitation: rain, snow or hail. Some of it runs over the surface into streams and back to the sea (runoff). Some soaks into the soil and rock (infiltration) and collects as groundwater.

Along the way, water works as a delivery service. Rain trickling through the soil dissolves minerals such as calcium and potassium, which roots then take up. Rivers carry dissolved nutrients and silt all the way to the coast, fertilising the estuaries and shallow seas where many fish breed. No plant or animal on land could survive without the fresh water this cycle hands out.

Warming is turning up the speed of the water cycle. For each 1 °C of warming, air can hold about 7 % more water vapour, so storms have more water to drop. The result is a cycle of extremes: more cloudbursts in the hills and city streets turned into rivers after a single day's downpour, but also longer dry gaps between rains. A hard downpour on dry ground mostly runs off, carrying topsoil with it, instead of soaking in to refill the wells, so farmers have less groundwater to fall back on in April and May. High in the Himalaya, shrinking glaciers first swell the rivers that drain them and will later leave them thinner in the dry season. Their meltwater, together with melt from the polar ice sheets, is slowly lifting the sea along India's long coastline. Can you spot all five spheres in this paragraph?

13.3.2 Carbon cycle

Look at any molecule that matters to life, whether a sugar, a fat, a protein or DNA, and you will find a skeleton of carbon atoms. Carbon is stored in all four non-living spheres as well as in living things. It is in the air as $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, and in plants, animals and soil. It is in rocks such as limestone and in fossil fuels such as coal, oil and natural gas. It is also in the oceans, dissolved in the water and in the shells of sea creatures (Fig. 13.14). The cycle has a fast lane and a slow lane.

The fast cycle takes days to years. Green plants use sunlight to turn carbon dioxide and water into glucose in photosynthesis. Animals get that carbon by eating plants or other animals. Plants, animals and microbes break glucose down again in respiration and breathe the carbon back out as $\mathrm{C}\mathrm{O}{\phantom{X}}_2$. When organisms die, decomposers feed on them and release the rest of their carbon as $\mathrm{C}\mathrm{O}{\phantom{X}}_2$.

The slow cycle takes millions of years. Now and then, dead plants and animals are buried in swamps or on the sea bed before decomposers can finish them off. Pressed and heated deep underground over ages, their remains slowly turn into coal, oil and natural gas. Power stations, vehicles and factories burn these fuels, and carbon that took millions of years to lock away returns to the air in moments.

The sea works like a giant carbon sponge. Carbon dioxide dissolves at the surface, especially in cold water, and reacts to form carbonate and bicarbonate ions. From there, life takes over. Phytoplankton, the microscopic algae drifting near the surface, draw on this dissolved carbon to photosynthesise, while corals, oysters and chalky plankton turn carbonate into shells. When they die, a steady 'snow' of shells and organic specks drifts down into the dark. Carbon that reaches the deep sea floor can stay out of touch with the air for centuries or longer, and over millions of years the shells can harden into limestone and chalk.

People have upset this balance. Burning fossil fuels and cutting forests have pushed the amount of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ in the air up year after year. It has been measured continuously since 1958 at the Mauna Loa observatory in Hawaii. There the yearly average rose from about 317 parts per million (ppm) in 1960 to about 427 ppm in 2025, a rise of roughly 35 % (Fig. 13.15). Ice cores show that it has not been this high for at least 800 000 years. Within each year the level also rises and falls by a few ppm. It dips every northern summer, when the vast forests and farms of the Northern Hemisphere are growing and taking up $\mathrm{C}\mathrm{O}{\phantom{X}}_2$.

If plants need $\mathrm{C}\mathrm{O}{\phantom{X}}_2$, why worry? Because the trouble is the rate. Forests, soils and oceans take up only about half of the carbon dioxide that people release each year, so the rest piles up in the air and thickens the greenhouse quilt (Section 13.1.3). What that means for glaciers, sea level, monsoons and farms is taken up in Section 13.4.

13.3.3 Nitrogen cycle

Every protein and every DNA and RNA molecule contains nitrogen, so no organism can grow without it. Nitrogen is all around us, 78 % of every breath you take, yet neither you nor a plant can use a single molecule of that gas. Nitrogen gas ($\mathrm{N}{\phantom{X}}_2$) is so unreactive that living things can only take up nitrogen that has been fixed: joined to hydrogen or oxygen in compounds that dissolve in soil water. Following nitrogen from the air through the soil, water and living things and back again traces out the nitrogen cycle (Fig. 13.16). It has five stages:

1. Nitrogen fixation. Nitrogen-fixing bacteria change $\mathrm{N}{\phantom{X}}_2$ from the air into ammonia ($\mathrm{N}\mathrm{H}{\phantom{X}}_3$). Some, like Rhizobium, live in small swellings called root nodules on legumes such as gram, peas and moong. Others, like Azotobacter, live free in the soil. Lightning also fixes a little nitrogen, as nitrogen oxides that rain washes into the soil.
2. Nitrification. Two groups of soil bacteria work in turn: Nitrosomonas changes ammonia into nitrite ($\mathrm{N}\mathrm{O}{\phantom{X}}_2{\phantom{X}}^{-}$), and Nitrobacter changes nitrite into nitrate ($\mathrm{N}\mathrm{O}{\phantom{X}}_3{\phantom{X}}^{-}$).
3. Assimilation. Plants absorb nitrates (and some ammonium) through their roots and build them into proteins and nucleic acids. Animals get their nitrogen by eating plants or other animals.
4. Ammonification. When organisms die, or animals pass urine and dung, decomposers such as bacteria and fungi break down the nitrogen compounds and release ammonia back into the soil.
5. Denitrification. In waterlogged, airless soil, denitrifying bacteria such as Pseudomonas change some nitrate back into $\mathrm{N}{\phantom{X}}_2$ gas, which returns to the air and closes the cycle.

13.3.4 Oxygen cycle

Oxygen is the most abundant element in the Earth's crust, nearly half its mass: rocks such as quartz and granite are largely oxygen bonded to silicon and metals. Only a small share of the planet's oxygen is free $\mathrm{O}{\phantom{X}}_2$ gas, but that share makes up 21 % of the air, and it is the part living things depend on. Oxygen is also built into water, carbon dioxide and every carbohydrate, fat, protein and nucleic acid. The oxygen cycle we follow here is the part that keeps the level of $\mathrm{O}{\phantom{X}}_2$ in the air steady (Fig. 13.17).

Three processes take oxygen out of the air: respiration in plants, animals and microbes, burning of wood, coal, petrol and gas, and slow oxide formation, such as iron rusting. Respiration and burning give back carbon dioxide and water. Oxygen is returned to the air mainly by photosynthesis, which uses sunlight to split water and releases $\mathrm{O}{\phantom{X}}_2$. Ocean phytoplankton make roughly half of it, and land plants most of the rest. As long as production and consumption stay in balance, the level of oxygen in the air hardly changes.

13.4 Human Impact on Earth's Processes

In Grade 8 you studied climate change and the crises it brings. Here we look at the mechanism: how our actions jam the cycles you have just traced. Burning fossil fuels adds carbon dioxide to the air faster than plants, soil and oceans can take it back. The extra $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ strengthens the greenhouse effect, and the warming that follows brings more extreme weather and pushes many species towards extinction.

The oceans have so far soaked up about a quarter of the carbon dioxide that people release, which has slowed the warming. But it comes at a price. Dissolved $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ makes sea water slightly more acidic: the average pH of the surface ocean has dropped from about 8.2 to about 8.1 since the 1800s. More acidic water makes it harder for corals, shellfish and tiny shelled plankton to build their skeletons, and that threatens whole marine food webs. At the same time, warmer water dissolves less gas, so a warming ocean becomes a weaker carbon sink just when we need it most.

Forests and soils are the other big carbon sinks, and every hectare of forest that is cleared weakens them. India's own path shows both sides of the problem: coal still supplies the largest share of our electricity, yet solar capacity has grown more than thirty-fold in about ten years. The table below collects some of the Earth's vital signs, with their sources.

Fertilisers disturb the nitrogen cycle. Sometimes farmers spread more urea or other nitrogen fertiliser than their crops can take up. Rain then washes the extra nitrate into ponds, lakes and rivers, and eventually into coastal seas. There it feeds a sudden, thick growth of algae called an algal bloom. When the algae die, decomposer bacteria feeding on them use up the oxygen dissolved in the water, and fish suffocate (Fig. 13.18). This over-enrichment with nutrients is called eutrophication (सुपोषण). Sewage and detergents rich in phosphate cause it too. It kills lakes and damages coastal fisheries.

Clearing a forest shows how one action reaches every sphere at once:

On still, sunny afternoons in a busy city, nitrogen oxides and unburnt fuel vapours from vehicle exhausts are cooked by sunlight into a brownish haze called smog. One of its ingredients is ozone, the very gas that shields us high in the stratosphere. Down here, in the air we breathe, it irritates the lungs and eyes, worsens asthma and stunts crops.

Why did the world manage to heal the ozone layer but not to curb carbon dioxide? CFCs were made by relatively few companies and safer substitutes were ready, so the Montreal Protocol could phase them out quickly. Carbon dioxide comes from almost everything we do, from power stations and vehicles to cement kilns and farms, so cutting it means changing whole economies. The Kyoto Protocol and the Paris Agreement set targets, but global emissions have kept rising.

India is working on several of these at once. Solar capacity has passed 100 GW, large tree-planting drives are under way, and Soil Health Cards tell farmers how much of each nutrient their fields actually need, so they can avoid overusing fertiliser.