Soils and Crop Growth

What soil is made of

Soil is the layer of weathered rock and decayed plant and animal material at the surface of the land, and it is the medium in which almost all of our food is grown. A soil is not one substance: it is a mixture of four things - mineral particles, organic content, gases and water. The mineral particles come from the rock that lies beneath the soil and from material carried there by rivers, wind or ice, and they make up most of the solid part of the soil. Their sizes, and the way they are arranged, decide how the soil behaves.

The organic content has two parts, and both are examinable. The first is the living organisms: plant roots, insects, worms, fungi and bacteria. Earthworms pull dead leaves down into the soil, mix them with the mineral particles and leave tunnels that act as pores for air and water; bacteria and fungi break down dead material and release the nutrients locked inside it; and bacteria living in the root nodules of legumes such as beans, clover and lucerne take nitrogen from the air and add it to the soil. The second part is the organic matter from decomposition - plant and animal remains in every stage of decay, ending in humus, the dark, sticky material that holds water and nutrient ions and glues particles together into crumbs.

Gases and water share the pore spaces, which are the gaps between the solid particles. Soil air is broadly the same mixture as the atmosphere, mostly nitrogen and oxygen, but it contains more carbon dioxide and less oxygen than the air above, because roots and soil organisms respire and use oxygen while releasing carbon dioxide. Soil water is the medium in which mineral ions are dissolved, so it is the route by which nutrients enter a root.

The gases and the water compete for the same space. After heavy rain the pores fill with water and the air is pushed out; as the soil dries, water drains away and air returns. That is why a soil can be too wet as well as too dry, and why drainage matters to a crop just as much as rainfall does.

• Mineral particles: sand, silt and clay, produced by the weathering of rock.
• Organic content: living organisms (plants, animals, fungi and bacteria) plus organic matter from decomposition.
• Gases: mostly nitrogen and oxygen, with more carbon dioxide than the atmosphere because soil organisms respire.
• Water: held in the pore spaces, carrying dissolved nutrient ions into the roots.

The soil profile and its horizons

Cut a vertical pit into ground that has not been disturbed and you see a soil profile: a series of layers, called horizons, lying one above another. At the surface is the O horizon, the litter of dead leaves and plant material. Below it lies the A horizon, or topsoil, which is dark because it contains most of the soil's humus, and which holds most of the roots, most of the soil organisms and most of the nutrients. Next comes the B horizon, or subsoil, with little organic matter but a great deal of the clay and iron washed down from above. Below that is the C horizon, made of weathered fragments of the parent rock, and finally the solid parent rock itself. Crop roots reach mainly into the A horizon, and that is the layer a farmer works, feeds and, all too often, loses.

Soils form where rock is broken down in place or where material is deposited by rivers, wind or ice, and is then colonised by plants. Weathering produces the mineral particles: heating and cooling, and the freezing of water in cracks, break rock into fragments; slightly acidic rainwater dissolves minerals out of it; and roots and burrowing animals prise it apart. Organic matter is added as plants and animals die and decompose, and water moving down through the profile carries clay and dissolved ions from one horizon to another - a process called leaching when the material is carried beyond the reach of the roots. A few centimetres of topsoil can take many human lifetimes to build, while a single storm can strip it away, so soil is best treated as a resource that renews itself extremely slowly.

The profile matters to crop growth because the horizons do different jobs. The A horizon supplies the roots with nutrients, water and air. The B horizon stores water that moves upwards later in the season, and can hold nutrients that have been leached down out of the topsoil. The C horizon and the parent rock supply the mineral particles and, over a very long period, the nutrients released by weathering. A deep soil with a thick, humus-rich A horizon sitting on a well-structured B horizon therefore supports crops far better than a shallow soil lying directly on hard rock.

Soil texture: sand, silt and clay

Soil texture is the relative proportions of mineral particles of different sizes in a soil. They fall into three groups. Sand particles are the largest, up to about 2 mm across, and they feel gritty between your fingers. Silt particles are intermediate in size and feel smooth and floury when dry. Clay particles are the smallest, less than about 0.002 mm across, and feel sticky and plastic when wet; a moist clay soil can be rolled into a ribbon that holds together. Almost no soil contains only one of the three, and the proportions vary from field to field, but the textural class of a soil tells you a great deal about how it will behave.

Texture matters because the size of the particles fixes the size of the pore spaces between them. A sandy soil has relatively large pores, so water drains through it quickly: it is well aerated, it warms up early in spring and it is easy to work, but it also dries out quickly and the ions dissolved in its soil water are easily leached out of reach of the roots. A clay soil has tiny pores and holds on to water and to nutrient ions strongly, but it drains slowly, so it is often cold and airless in spring, heavy and sticky to plough, and liable to waterlogging in a wet season.

Soil structure is a different idea from texture and is frequently confused with it in exams. Structure is the way the particles are bound together into crumbs or aggregates, and it is created mainly by humus, roots, fungi and worms. Good structure gives a mixture of pore sizes: large pores between the crumbs for drainage and for air, and smaller pores that hold water against gravity. Structure can be destroyed by driving heavy machinery over wet soil, by cultivating the same soil year after year, and by losing organic matter, and it can be rebuilt by adding organic matter, by growing deep-rooted plants and by keeping off the land when it is wet. Texture cannot be changed; structure can.

Humus and the nutrient ions a crop takes up

The organic content feeds the crop as well as the soil organisms. Humus and the organic matter that is still decaying release nutrients slowly as bacteria and fungi break them down, and because the release is gradual, the crop is supplied all through the growing season instead of in a single dose. Humus also holds nutrient ions on its surfaces so that they are not washed out of reach of the roots, and it improves structure and water-holding. That is why soil that has been cropped for years without any return of organic matter becomes hard, dusty and unproductive.

Plants take their nutrients up as ions dissolved in the soil water, not from the solid particles themselves. Nitrogen is taken up as nitrate ions (NO3-) and is needed for leaves, stems and the green pigment chlorophyll, so a shortage leaves the older leaves pale yellow and the plant stunted. Phosphorus is taken up as phosphate ions (PO4 3-) and is needed for roots, flowers, seeds and the transfer of energy inside the plant, so a shortage holds root growth back and delays ripening. Potassium is taken up as potassium ions (K+) and controls water movement and the opening and closing of the stomata, so a shortage leaves the leaf edges scorched and the stems weak.

Where do those ions come from? Some are released as the minerals in the parent rock weather, some are released from organic matter as it decomposes, and the rest are supplied as fertiliser. Most of the ions in a soil are held on the surfaces of clay particles and humus, both of which carry a negative charge that attracts positive ions such as K+. That is why a sandy soil loses nutrients to leaching far faster than a loamy or clayey one and needs smaller, more frequent applications of fertiliser, and why applying more fertiliser than the crop can take up is both a waste of money and a source of pollution once the surplus is leached into rivers and groundwater.

• Nitrogen: taken up as nitrate ions (NO3-), needed for leaves, stems and chlorophyll.
• Phosphorus: taken up as phosphate ions (PO4 3-), needed for roots, flowers, seeds and energy transfer.
• Potassium: taken up as potassium ions (K+), needed for water regulation, the stomata and disease resistance.
• Clay and humus particles hold ions on their surfaces, so a heavy or humus-rich soil retains nutrients far better than a sand.

pH, pore spaces, gas content and drainage

Soil pH measures how acidic or alkaline a soil is, on a scale running from below 7 for acidic soils, through 7 for a neutral soil, to above 7 for alkaline soils. Most crop plants grow best in a soil that is slightly acidic to neutral, because that is where the nutrient ions they need are most soluble and therefore most available; outside that range a crop can suffer even though the nutrients are present in the soil. In a strongly acidic soil, phosphate becomes locked up in compounds the roots cannot take up, aluminium and manganese may dissolve in quantities that poison the roots, and the bacteria that release nitrogen from organic matter work more slowly. In a strongly alkaline soil, iron, manganese and phosphate become unavailable, and the soil is often a heavy clay that is hard to cultivate.

Crops differ in what they tolerate, and this is worth a mark. Potatoes, tea and blueberries do well in acid soils, and the tea gardens of Assam in India and of Kenya are grown on soils too acid for most other crops. Brassicas such as cabbage, and asparagus, prefer slightly alkaline soil. Roots also take up and release ions in a way that tends to make the soil around them more acidic over time, so under continuous cropping a soil drifts towards acidity unless it is managed.

Farmers manage pH mainly by liming: spreading ground limestone (calcium carbonate) raises the pH towards neutral and adds calcium at the same time. The amount needed has to be worked out from a soil test, because too much lime is as damaging as too little and can make other nutrients unavailable, and liming is a cost that must be repeated as the soil re-acidifies. Where a soil is too alkaline, adding organic matter or a sulphur-containing material can lower the pH slowly, but neither is quick and neither is cheap.

The pore spaces decide the gas content and the water content, because both have to occupy the same gaps. In a well-structured soil the large pores drain within hours of rain and let air back in, while the smaller pores hold water against gravity and release it to the roots between rainfalls. If the pores stay full of water, oxygen cannot reach the roots, respiration stops, growth stalls and the roots begin to rot; carbon dioxide released by respiring roots and decomposing organic matter also builds up because it cannot escape. Very free-draining soil creates the opposite problem: water passes through so quickly that the crop goes short between rains and irrigation has to make up the difference.

Drainage is therefore as important as water supply. Fields are drained by cutting ditches or by laying subsurface pipes of clay or plastic, and raised beds can lift a crop's roots above a high water table; where the soil dries out too readily, irrigation supplies water directly. Both are management strategies with a price. Drainage costs money to install and maintain, and it moves water off the land faster, which can make flooding and low flows worse downstream; irrigation uses water that other users also need, and if it is mismanaged the water table rises and waterlogs the soil or, in dry regions, salts accumulate in the topsoil as the water evaporates - a process called salinisation.

Why loam is a good medium for crop growth

A loam is a soil in which sand, silt and clay are blended together in a good balance, so that no single particle group dominates. Because it is a mixture, it keeps the advantages of each and cancels many of the drawbacks. The sand fraction and the larger pore spaces let excess water drain away and let air back into the soil; the silt and clay hold enough water to carry the crop through a dry spell and hold nutrient ions on their surfaces; and the humus normally present in a loam binds the particles into crumbs, so the soil holds together without setting like concrete.

The reasons a loam suits most crops all follow from that mixture. It has an ideal combination of mineral particles and pore spaces, with large pores for drainage and aeration and small pores for storing water. It retains moisture, so the crop is not exhausted by a short dry spell. It contains organic content and the inorganic ions the crop needs, because a loam supports the worms, fungi and bacteria that build humus and holds the ions they release against leaching. It allows excess water to drain, so the roots keep their oxygen even after heavy rain. And it is easy to cultivate, because it can be ploughed, drilled and lifted over a much wider range of moisture contents than a clay, which is unworkable when wet and brick-like when dry, or a sand, which has too little structure to hold together.

Loam is not a free pass, and examiners reward candidates who say so. The same soil can be compacted by heavy machinery, and once its large pores are squeezed out it drains and aerates as badly as a clay. A loam can also be ruined by losing its organic content, through burning or exporting crop residues or by growing the same crop year after year, so the structure that made it valuable disappears. And no soil is ideal for everything: root crops such as carrots and potatoes are often grown on lighter, sandier soils that are easier to lift and less likely to waterlog, while wheat and brassicas do well on heavier soils that hold more water and nutrients. Matching the crop to the soil is part of good management.

• An ideal combination of mineral particles and pore spaces: large pores to drain and aerate, small pores to store water.
• Retains moisture, so a short dry spell does not exhaust the crop.
• Contains organic content and the inorganic ions the crop needs, held against leaching.
• Allows excess water to drain, so roots are not starved of oxygen after heavy rain.
• Easy to cultivate over a wide range of moisture contents, unlike clay or sand.

Weather, wet and dry seasons, and the conditions for photosynthesis

Weather is the day-to-day conditions of the atmosphere in a location: temperature, rainfall, humidity, wind, cloud and sunshine, changing from hour to hour and day to day. Do not confuse it with climate, which is the average pattern of the weather at a place over many years. Weather decides whether a crop can be sown today, whether the soil will be too dry for germination next week, and whether frost will kill the seedlings tonight, so it is the factor a farmer watches most closely and can influence least.

Many parts of the world have a marked wet season and dry season rather than rain spread evenly through the year. In a monsoon climate most of the year's rain may fall in a few months, and the cropping calendar is tied to that rhythm: land is prepared and sown as the rains arrive, growth is rapid while the soil is moist, and the dry season that follows is a time of irrigation, of fallow land or, where there is no water to irrigate, of bare fields at risk of wind erosion. A wet season that arrives late, or fails, can destroy a harvest. A wet season that is too heavy leaches nutrients out of the soil, waterlogs the roots and washes topsoil off sloping land. The Sahel, on the southern edge of the Sahara, is the extreme case: one short wet season, a long dry season, and yields that swing sharply with the rainfall.

Crop growth is driven by photosynthesis: carbon dioxide from the air and water from the soil are combined using light energy trapped by chlorophyll to make glucose, with oxygen released. The rate at which a crop can photosynthesise sets a ceiling on how much it can grow, and the conditions that raise the rate are the conditions a crop needs. Light must be strong enough and available for enough hours; the temperature must be warm enough for the enzymes that run the process, with many crop plants near their best between about 20 °C and 30 °C and slower above that, when respiration and water loss rise faster than photosynthesis; water must be available from the soil; carbon dioxide must be present in the air; and the crop must have the nutrient ions it needs to build new tissue. Whichever of these is in shortest supply becomes the limiting factor, and adding more of the others will not raise the yield until it is corrected.

Two more conditions complete the syllabus list. The length of the growing season is the part of the year when the temperature is high enough and water is available for growth. In the humid tropics it can run all year, allowing two or even three crops of rice to be taken from the same field; in high latitudes, or in a climate with a long dry season, it shrinks to a few months, so the crop must complete its whole life cycle inside that window and the yield is limited by it. Daylight hours decide how many hours of photosynthesis a crop can run each day, and changing daylength also switches some plants into flowering: short-day crops such as rice and soybean flower as the nights lengthen, while long-day crops such as wheat and barley flower as the days lengthen. Together, the growing season, the weather within it and the daylength decide how much of the theoretical maximum yield a field can actually reach.

• Photosynthesis: carbon dioxide + water -> glucose + oxygen, using light energy trapped by chlorophyll.
• Length of growing season: the part of the year warm and moist enough for growth; a long season gives larger crops and allows several harvests.
• Optimum weather: enough warmth (near 20 to 30 °C for many crops), enough rain or irrigation to keep the soil moist without waterlogging, and no damaging frost, storm or drought.
• Daylight hours: more daylight means more hours of photosynthesis each day, and daylength also controls flowering in short-day and long-day crops.
• Limiting factors: the factor in shortest supply sets the rate, so correcting that factor, not adding more of the others, is what raises yield.