How much do you know about dirt?

A hand letting soil fall to into a pile
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How much do you really know about the ground under your feet? Test your knowledge with this quiz.

Question 1 of 17

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Layers of soil and grass roots
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In the time it took nature to form one inch of the soil that exists on the Earth today, which of the following happened?

It's all of the above! Nature takes 500 years to form one inch of soil.

Question 2 of 17

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Hilo soil inspected in Hawaii
Photo: Mike Kolman/Wikimedia Commons
Every state has a state soil.

Soil varies greatly from state to state. Pictured here is hilo soil, the state soil of Hawaii. Visit this site to read about your state soil.

Question 3 of 17

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Plants attempt to grow in dry, cracked soil
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What percentage of the world’s soils is estimated to be currently degraded?

According to the United Nations, a third of global soil is estimated to be moderately to highly degraded through erosion, salinization, compaction, acidification, chemical pollution and nutrient depletion.

Question 4 of 17

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Tillage rake in soil
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Regularly tilling your vegetable garden helps keep open pores at the soil's surface and increases water filtration into the soil.

Hoeing (or tillage/plowing) actually reduces the soil's capacity to receive and hold water because it destroys soil aggregates and the biologically produced glues that hold soil aggregates together. This results in the collapse of those aggregates and the pores between them, which can lead to compaction and crusting. The alternative is a "no-till" method that involves cutting a very narrow slice into the soil — resulting in minimal soil disturbance — and planting seeds or transplants into the slice.

Question 5 of 17

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Freshly harvested root vegetables still covered in soil
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What percent of our food comes from soil?

It is estimated that 95 percent of our food is directly or indirectly produced in our soils. Healthy and good quality food can only be produced if our soils are healthy. A healthy living soil is a crucial ally to food security and nutrition.

Question 6 of 17

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Root nodules containing bacteria
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In general, microorganisms in the soil in your vegetable and flower beds harm plants.

Most microorganisms are beneficial to plants in many ways. For example, they decompose residues, and so are able to build soil aggregates. They also make key nutrients available to plants, such as nitrogen, phosphorous and potassium, through nutrient cycling that is the result of a dynamic soil food web involving soil microorganisms like bacteria, fungi, protozoa and nematodes. Beneficial organisms can also help fight disease organisms. Those tiny organisms are so important that many plants exude substances through their roots to attract them to live in their root zones.

Question 7 of 17

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Wheat seedlings grow in rich soil
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The world's growing population will increase the demand for food, feed and fiber by what percentage by the year 2050?

The Earth's population is expected to exceed 9 billion by 2050, and such a large population will need to be fed somehow.

Question 8 of 17

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Soil and sprout in a tablespoon
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There can be more living organisms in a teaspoon or two of healthy soil in your lawn and garden than there are people on the globe.

It's true, and those living organisms play a critical role in decomposing crop residue into soil organic matter that enhances nutrient cycling and a soil’s available water holding capacity.

Question 9 of 17

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Layers of soil in a shale
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Which layer of soil contains the most amount of organic matter?

Most of the organic matter is located in topsoil. Soil organic matter is a substance which is vital for air, moisture and nutrient retention. It is critical to sustainable agricultural production because it plays an important role in sustaining soil fertility and improves the physicochemical and biological properties of the soil.

Question 10 of 17

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Stages of life of a sapling receiving water
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Organic matter buffers the soil against big changes in moisture and temperature.

Organic matter provides insulation from variations in temperatures and it provides greater water holding capacity. Increasing the soil organic matter by 1 percent can add approximately 1 inch of water to the soil profile.

Question 11 of 17

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Pile of soil in a petri dish near a microscope
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Soils host how much of the world's biodiversity?

Soils host a quarter of our planet's biodiversity. Soil is one of nature's most complex ecosystems. It contains a wide variety of organisms that interact and contribute to the global cycles that make all life possible.

Question 12 of 17

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Raised garden beds
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To keep your soil in flower and vegetable beds healthy, you should allow it to rest from time to time — that is to lie bare without growing plants.

Having plants growing all the time allows more solar energy to be converted into carbon to feed soil microbial populations and improve soil health. When living roots grow in the soil throughout the year, they continuously feed soil organisms and build porous soil structure (through root and earthworm channels, decomposing residues to create soil organic matter, and producing biotic glues). Root-fed microorganisms also help make nutrients available to crops.

Question 13 of 17

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Planting a new tree sapling
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Where is most of the world’s organic carbon stored?

Most of the world's organic carbon is stored in the Earth's soils. For example, forest soils store a quantity of carbon equalling that of the global forest biomass, about 45 percent each. Soils thus provide a prospective way of mitigating the increasing atmospheric concentration of CO2.

Question 14 of 17

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Small mushrooms growing in soil
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Plants in your garden don't need fungi for healthy plant growth.

Arbuscular mycorrhizal fungi (abundant in healthy soils) have a symbiotic relationship with almost all agricultural plants. They grow into the inside of plant roots to tap into the sugars and carbohydrates transported from the plant leaves. In turn, the fungal hyphae (filaments) that grow out from the roots bring water and soil nutrients back to the plant, and they can even help protect roots from pathogens.

Question 15 of 17

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Rotten vegetables in a compost pile
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What percentage of the world's agricultural land grows crops that are wasted?

It is estimated that 28 percent of the world's agricultural land grows crops that are wasted. In the process, 250 km³ of water goes to waste and the carbon footprint of food produced and not eaten is estimated at 3.3 gigatonnes of CO2 equivalent.

Question 16 of 17

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Earthworms in soil
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Earthworms are more abundant when you till or plow the soil in your vegetable and ornamental beds.

Tillage damages earthworms directly, and also damages the habitat earthworms need to survive. It stimulates drying the surface soil and creates wide day/night temperature fluctuations. Tillage also brings earthworms to the surface where they are subject to predators such as birds. Total earthworm populations in long-term no-tilled fields are typically at least twice those of clean-tilled fields.

Question 17 of 17

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Leonard DiCaprio at Cannes in 2013
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Leonardo DiCaprio wrote, "We know more about the movement of celestial bodies than about the soil underfoot."

It was the other Leonardo — Leonardo da Vinci — and he's probably right. Though we're learning more about the soil's complex and miraculous ecosystem every day — the more we discover, the more we realize how much more there is to still learn about that amazing universe beneath our feet.

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