Soil
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50 free Soil trivia questions with answers — science & nature quiz, new questions added Oct 2026.
Soil looks like the dullest thing on Earth until you learn what it does. Nearly everything we eat starts in it, it takes centuries to build and a single storm to lose, and a handful holds more living things than most people would ever guess. Whole civilizations have risen on good ground and stumbled when they wore it out. This quiz covers the subject the way a science class does. There are questions on how rock turns into soil, the layers you see in a freshly dug pit, the main soil types and textures, and the creatures and microbes that live underground. Others deal with what plants need from the ground, how farmers feed and protect their fields, and the scientists and disasters that shaped soil science. It suits students revising a soil science unit, gardeners and growers, and anyone who likes knowing how the world under their boots works. Every answer comes with a short explanation, and the questions can be printed for a classroom or club quiz.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Charles Darwin's last scientific book, in 1881, argued that which animals create the fertile top layer of soil?
Earthworms
Earthworms intrigued Darwin so much that he made an excursion to Stonehenge to see how its monoliths had been buried by their castings. The book sold thousands of copies within weeks.
Q 02Which disease, popularly blamed on rusty nails, is caused by bacteria whose spores live in soil and manure?
Tetanus
Tetanus comes from Clostridium tetani, and rust itself is harmless. A dirty nail simply punctures the skin and carries the soil-dwelling spores deep into the wound.
Q 03What name did two Australian scientists coin in 1964 for the earthy smell of rain falling on dry soil?
Petrichor
Petrichor joins the Greek word for stone with ichor, the golden fluid said to run in the veins of the Greek gods. Before 1964 the scent was known as argillaceous odor.
Q 04Many hydrangeas act as natural soil testers, producing flowers of which color when the ground is acidic?
Blue
Blue flowers appear because acidic soil makes aluminum ions available for the shrub to absorb, while alkaline ground turns the same plant pink.
Q 05Which Italian landmark began tilting during its 12th-century construction because the ground beneath was too soft?
Tower of Pisa
Tower of Pisa builders had only reached the second floor when it began to sink in 1178. Engineers finally steadied it between 1993 and 2001, trimming the tilt to just under four degrees.
Q 06Which waterlogged soil preserved Denmark's Tollund Man so well that his finders in 1950 suspected a recent murder?
Peat
Peat builds up where stagnant water starves dead plants of oxygen, and the same acidic bog water tanned the skin of a man who died around 400 BC.
Q 07On a bag of NPK fertilizer, the letter K stands for which plant nutrient, known to early chemists as kalium?
Potassium
Potassium takes its English name from potash, the residue left when wood ashes were soaked in a pot and the water boiled away. The numbers on the bag list nitrogen and phosphorus before it.
Q 08Which 1930s disaster followed drought and deep plowing on the Great Plains, raising storms called black blizzards?
Dust Bowl
Dust Bowl was a name coined by a news editor in April 1935, and its choking clouds traveled as far as New York City and Washington, D.C.
Q 09The largest known fungal colony, a honey mushroom spreading under 3.5 square miles of forest, lies in which US state?
Oregon
Oregon's Malheur National Forest hides the giant, which creeps from tree to tree on black underground cords nicknamed shoestrings. It may weigh as much as 35,000 tons.
Q 10Streptomycin, isolated from a New Jersey soil microbe in 1943, became the first antibiotic cure for which disease?
Tuberculosis
Tuberculosis treatment won lab chief Selman Waksman the 1952 Nobel Prize, although it was his PhD student Albert Schatz who first isolated the drug.
Q 11Which Illinois blacksmith built a polished steel plow in 1837 that cut cleanly through sticky prairie soil?
John Deere
John Deere remembered polishing needles with sand in his father's tailor shop, and reasoned that mirror-smooth steel would shed the earth that clogged older plows.
Q 12George Washington Carver urged Southern farmers to revive worn-out soil by rotating in which nitrogen-restoring crop?
Peanuts
Peanuts became Carver's signature crop, and his 1921 testimony to Congress about them made the Tuskegee professor a national name.
Q 13Which is the finest of the three soil particle classes, with grains even smaller than those of silt?
Clay
Clay turns plastic when wet and hardens when fired, which is why the stickiest ground underfoot is also the raw material of pottery.
Q 21Besides soil bacteria, which weather phenomenon converts nitrogen in the air into forms that plants can use?
Lightning
Lightning forges nitrogen oxides in the air it passes through, doing naturally what fertilizer factories and root-nodule microbes achieve by other means.
Q 22Which waterlogged ground, feared in adventure films, is actually too dense for a person to sink in past the waist?
Quicksand
Quicksand is about twice as dense as the human body, so a trapped person floats at around waist height instead of vanishing below the surface.
Q 23Centuries of irrigation ruined Sumer's farmland by leaving which substance behind in the soil as the water evaporated?
Salt
Salt built up until Sumerian farmers switched from wheat to hardier barley, and even that could not stop harvests and population from collapsing.
Q 14Gardeners prize which crumbly soil, a balanced blend of sand, silt and finer particles that drains without drying out?
Loam
Loam is about 40 percent sand and 40 percent silt by weight, a mix that holds nutrients and water while letting the excess drain away.
Q 15What name is given to ground that stays below freezing for two or more years, as across much of Siberia?
Permafrost
Permafrost lies under about 15 percent of the Northern Hemisphere's exposed land, and the oldest has stayed frozen for around 700,000 years.
Q 16The world's soils store roughly three times as much of which element as the entire atmosphere holds?
Carbon
Carbon held in the world's soils outweighs the carbon in the atmosphere and in every living organism put together.
Q 17Which seabird-dropping fertilizer was so prized that an 1856 US law let citizens claim unowned islands holding it?
Guano
Guano takes its name from a Quechua word for dung spread on fields, and Spanish colonial records say Inca rulers punished anyone who disturbed the birds with death.
Q 18Which soil-free growing method let Berkeley's William Gericke raise tomato vines 25 feet tall in mineral solution?
Hydroponics
Hydroponics got its name from Gericke in 1937, after the towering tomatoes in his backyard had already caused a sensation.
Q 19Which plant group, including peas, beans and clover, houses bacteria in root nodules that pull nitrogen from the air?
Legumes
Legumes leave some of that nitrogen behind in the ground for whatever is planted next, which is why farmers have long worked them into crop rotations.
Q 20Which dam, finished in 1970, ended the yearly Nile floods that had enriched Egypt's farm soil since ancient times?
Aswan High Dam
Aswan High Dam was built with Soviet help through the 1960s, and it swapped the summer flood and its natural nutrients for year-round irrigation.
Q 24Which powder, milled from chalk or similar rock, do farmers spread on fields to correct soil that is too acidic?
Lime
Lime works as a base that neutralizes acid in the ground, and the farm-grade powder needs only milling instead of burning in a kiln.
Q 25Which French word do winemakers use for the way a vineyard's soil and climate shape the character of its wine?
Terroir
Terroir comes from terre, the French for land, and the idea underpins the AOC rules that tie a French wine's name to the place it was grown.
Q 26What do gardeners call a layer of bark chips, straw or leaves spread over soil to hold moisture and smother weeds?
Mulch
Mulch also evens out soil temperature through hot days and cold nights, and the organic kinds feed the ground as they slowly break down.
Q 27Which building material of the American Southwest is made from sun-dried bricks of earth mixed with straw?
Adobe
Adobe is simply Spanish for mudbrick, a word that came from Arabic, and the straw is there to help the bricks shrink evenly as they dry.
Q 28Which stable, brown-to-black material is left in soil once dead plants and animals have fully decomposed?
Humus
Humus is the Latin word for ground, and soil scientists keep it apart from detritus, the name for matter that is still in the middle of rotting.
Q 29Terra preta, the fertile dark earth made by ancient Amazonian peoples, owes its black color to which added material?
Charcoal
Charcoal was worked in along with bones, broken pottery and manure, turning the Amazon's naturally poor soil into rich farmland.
Q 30A red color in airy, well-drained soil usually signals a buildup of which oxidized metal?
Iron
Iron that has built up and oxidized turns well-aerated ground red, whereas a dark brown or black tint usually points to plenty of organic matter.