FAQ - Frequently Asked Questions on the Topic of Soil

Candler asked some questions about soil and its importance: 

Dear Dr. Dirt, 

My name is Candler. I am in 3rd grade and I am doing a research project for school on soil. 

1.) What is soil?

 - http://soils.usda.gov/education/facts/soil.html
 - http://school.discovery.com/schooladventures/soil/down_dirty.html

These websites give a definition of soil. The one below probably gives a few more ideas about what is in soil.

Nathan had a similar question: What different particles does dirt contain? How much of each particle are in 1/2 a cup of dirt? 
Soil particles are defined as less than 2 millimeters in diameter. Particles larger than that are called gravel or rocks.
Soil particles come in three sizes: 

  • Sands are the largest, and are coarse and gritty. They allow water and air to move into and through the soil because there are large pores (holes) between them.
  • Silts are the middle-sized soil particles, and are smooth. They feel much like flour or baby powder. They hold water that plants can use.
  • Clays are the smallest soil particles, and are fine and sticky. They hold a lot of water, but do not give as much to the plants as silts. They also hold nutrients. They help hold soil together in clods which scientists call aggregates. Clays can be molded and shaped, much like modelling clay or Play-Dough.
  • An ideal soil would have equal amounts of sand, silt, and clay particles. But most soils are not ideal. Some are mostly sand, others are mostly clay or silt, depending on where they     are found. The about equal mixtures of  soil particles are called loams.


2.  How is soil made?

This web site answers some of these questions plus many more. The part you should read is the part that talks about parent materials, topography, climate, biological organisms, and time.


3.  How can we protect soil? /  How does soil make plants grow?

Most of these answers can be found at the following website. It has a list of questions similar to yours. Click on the right question and you will find the answer.


4.  What is the most important thing about soil?

This is the most challenging question. "Most important" is always answered from the perspective of values.

  • To many farmers, the most important thing about soil is that it produces crops that provide their livelihood.

  • To environmental engineers, the most important thing about soil is its ability to purify itself.

  • To engineers who build things, the most important thing about soil is its ability to support weight or loads. (The leaning Tower of Pisa leans because it was built on soil that could not support its weight.)

  • To people concerned about wildlife, the most important thing about soil is its role in providing habitat (food and shelter) for many wild animals and several endangered species.

Soil plays an important role in both air and water quality. Poor quality soil that is not covered with plants or residues blows with the wind, causing dust storms. It will erode with the rain, adding nutrients to streams, rivers and lakes, and makes them muddy. Soil that has good quality resists wind and water erosion. The rain that falls on good quality soil moves into the ground to be used by plants, or to recharge aquifers. The water is purified as it moves through the soil because the soil is a natural filter that removes pollutants in three ways: chemically, physically, and biologically. Clay particles in soil have a negative charge, and they attract positively charged pollutants and hold them in the soil. Most heavy metal pollutants have positive charges and so are chemically trapped in soils so they do not reach water tables.

The pores that exist between soil particles are very small. Large pollutants get trapped in these tiny pores, so the soil acts like a sieve, physically trapping large pollutants. Many pollutants are organic compounds that are biodegradable. The organisms in soil are able to decompose most of these compounds before they cause problems in the environment.

So, Dr. Dirt thinks the most important thing about soil is its ability to protect the environment. Problems only occur when we add more pollutants to soil than it has the ability to handle.

This weblink provides 10 important things about soil: ftp://ftp-fc.sc.egov.usda.gov/NSSC/Educational_Resources/sellsoil.pdf.


Maribeth asked if the materials on ocean floors are considered soils:

My daughter's homework requires her to identify if the resource SOIL is found in ocean waters or on the ocean floor.  We have found that the ocean floor contains sediments, and  decayed animal/plant material. Is this considered to be SOIL?  Any help would be appreciated. Thanks so much!    Maribeth


I encourage you to visit the website above for the definition of soils. Every definition I know defines soil as part of a terrestrial (land-based) system, rather than an aquatic (water-based) system. The materials on the ocean floor may be future sedimentary rocks that could, after an uplift or a recession of the sea level, eventually weather to become parent materials in which a soil forms, but they are not soil.


Sarah asked a similar question: Is dirt/soil only on the surface of the earth? How deep down does dirt/soil go?

Yes, soil is only found on the earth's surface. The depth of a soil depends mostly on how much rain falls on the soil, and how old the soil is. Some soils are only a few inches (or centimeters) thick - on mountains, in some deserts, and in arctic regions. Other soils can be more than 6' feet (two meters) deep. Though sometimes soils are deeper, we usually do not consider them as soil because roots of most plants are concentrated in the top 2 or 3 meters.  Topsoil is usually the top 15 to 30 centimeters of soil. The subsoil then may go down to 2 or 3 meters.


Hope asked: What is the biggest animal that lives in dirt?

There are many animals that make homes or dens in the soil, starting with many too small to see with your eye. Many worms and insects spend part or all of their lives in the soil. Many rodents (mice, rats, gophers, prairie dogs, groundhogs, rabbits and hares) live in the soil, as do many reptiles (snakes, lizards, toads and tortoises). Several predators dig dens in the soil (foxes, badgers ...) and some live under rocks or in caves (bobcats, coyotes, cougars, wolverines, and even bears, to name a few). Worms are probably the largest animal that spend their entire life in the soil - there are some earthworms in Australia that are 2 m long! Most of the larger animals sleep in a den in the soil, but spend much of their time roaming around looking for food.


Dakota and several others have asked questions about the age of soil:

Where did the first soils come from?

The first soils began to form as soon as parent material was available, either through deposition of particles by wind, water, ice, or gravity, or through weathering of rocks. One of the most important processes in topsoil formation is the accumulation of humus (organic matter) that occurs as plants grow, die, and are decomposed, and as earthworms, termites, ants, and other organisms rework the material. A by-product of this process is that individual particles are joined together to form aggregates. Aggregates are groups of individual soil particles held together by organic matter or other forces, and are the foundation of soil structure. At first, these changes may be rapid, but they tend to slow as time progresses. Soil scientists would expect more dramatic differences in the first 10 years after deposition, than in the next 10 years, than in the last ten years of the first century after the deposition, etc.


How long does it take to make a foot of soil?

This is an excellent question, but unfortunately, a difficult question to answer. 

Soil formation is a continuous process influenced by the parent material (the stuff in which the soil forms), the climate (precipitation, temperature, etc.), the organisms (plants and animals) that live in the soil, the topography (shape of the landscape and position on the landscape), and time. The USDA-NRCS website has more information on this: http://soils.usda.gov/education/facts/formation.html.

No one is old enough to have observed how long it took to form a foot of soil. With the right conditions, a foot of topsoil may form in less than one hundred (100) years. These conditions would include a fresh deposit of loose material (as might happen during or after a flood) in which the soil could form, a stable landscape position so that soil formation is not interrupted, and a favorable climate that encourages the growth of grass (plants with fibrous root systems form topsoil faster than trees with taproots). When the original material is bedrock, like sandstone or limestone, that must first weather into small particles before the soil can form, and the process is much slower.

Lava flows of different ages in Western New Mexico and Eastern Arizona (and in other regions) provide opportunities to observe the rate of soil formation from solid rock in arid regions. The most recent lava flows are about 300 years old. There is little weathering of the lava, and few plants are growing. In flows about 500 years old, grass is growing, and shallow topsoil has formed. As one moves to progressively older flows, the depth of soil formation increases, and the vegetation changes from grass to trees.
This does not give a definite answer to your question because there is not one absolute answer. The best answer is that the time required for soil to form depends on the parent materials, the climate, the topography (landscape position), and the organisms that grow and live in the soil. Even on the same landscape, the soil on the slopes will form more slowly than soil on an upland position, which will probably form more slowly than the soil in the lower positions.


We would like to know how you determine how old soil is?

Soil scientists agree that soils form over time as the climate (temperature, rain, wind, etc.), topography (shape of the landscape), parent material (weathered rocks or stuff deposited by wind, water, ice, or gravity), and living organisms (plants and animals that live in the soil) interact. So, when soil scientists want to know how old a soil is, they look for clues. Though the climate affects how fast weathering and erosion occur, it is usually does not give many clues about how long a soil has been in a given spot.  The landscape, parent materials and trees provide better clues. A soil cannot be younger than the oldest trees growing on it. It cannot be older than the materials in which it forms or the landscape on which it is found. Soil scientists work with geologists to determine how old the landscape is, and how long the parent materials have been there. Since most deposition of parent materials occurred before written history began, geologists make educated guesses, estimating the age of the landscape (and materials in it) relative to periods of known glaciation, volcanic activity, floods, and similar events. Soil scientists know that certain positions on the landscape tend to form more rapidly than others. More soil formation occurs on flat landscapes in upland positions than on slopes. Erosion on the slopes limits the rate of soil formation. Soil formation in lowland positions may be slowed by deposition of new materials on the surface by floods or gravity.


Is there any way we can determine the age of soil at our school?

Scientists in many countries have developed soil classification systems. The five factors of formation are used at the highest level (soil orders) in the United States soil classification system, Soil Taxonomy. The youth of a soil is suggested in two of the orders: Entisols (youngest soils, that have only topsoil) and Inceptisols (soils at the beginning of subsoil formation). The oldest soils, Oxisols, have a lot of weathering, and are typically found in the humid tropics. For more information on soil taxonomy, and the 12 soil orders, visit this site:http://soils.ag.uidaho.edu/soilorders/. It has some description of each of the orders with United States and world distribution maps. This poster, http://soils.usda.gov/technical/soil_orders/, also provides information on the soil orders.

One way to learn about local soils is to use the Web Soil Survey tool: http://websoilsurvey.nrcs.usda.gov/. It allows one to search by county and state, or with latitude/longitude coordinates and select an area of interest. Once the area of interest is defined, Web Soil Survey provides a list of the soils and their characteristics. Most counties (or parishes) in the United States have a USDA-NRCS office (United States Department of Agriculture, Natural Resources Conservation Service), with staff who could provide more information about local soils. 


Heather asked some questions about soil scientists.

What kind of daily work do soil scientists do?

Some soil scientists are involved in the daily activities of food production. They test the soil in which the crops are grown and make fertilizer recommendations, as well as managing irrigation scheduling in regions where irrigation is practiced.  They also work to conserve the soil resources, developing methods to protect the soil from wind and water erosion. In dryland regions, soil scientists help develop management systems (tillage and crops) that store rainfall in the soil for future crops.

Other soil scientists study the physics of soil, especially the way water moves through soil, and how temperature changes in soils when they are heated. Some soil scientists are chemists, and study what happens to nutrients and other elements in the soil. Some are biologists who study all the microorganisms that decompose the organic residues (dead leaves and other stuff) that are added to the soil. Many of these soil scientists also work on environmental issues to improve the quality of the soil, water, and air in the ecosystem.
Other soil scientists study how soils formed, where they occur on the landscape, and work to group and classify them. The soils contests in which my students compete require them look at the layers (horizons) of soils in pits to describe the color, texture, and structure of each layer so that they could classify the soil. 

Many soil scientists work for the government in the Natural Resources Conservation Service, or for the Department of Environmental Quality, or similar state organizations. There are also a lot of soil scientists who work for universities doing research and teaching. Other soil scientists are consultants either for the agriculture industry, for environmental protection industries, or for engineering firms.


What personal qualifications would be helpful for a person who wanted to be a soil science?

I think most soil scientists enjoy being outdoors, but some of the soil chemists, physicists, and biologists I know spend most of their time in the lab. 

A soil scientist needs to have an interest in science, and also needs to pay attention in math classes. 

A soil scientist takes the information from math, chemistry, physics, and biology, and applies it to soil.


What school did you go to to earn a degree in soil science and how long did it take?

I have a bachelor's degree (B.S.) in agriculture and a master's (M.S.) degree in Plant Science from West Texas State University (now known as West Texas A&M University, where I now teach). My Ph.D. (doctoral) degree is in soils and is from Iowa State University. As you might imagine, it took quite a while to get all those degrees (almost 13 years).

Soils degrees are usually only available at land grant universities that have agriculture programs, though universities with environmental science programs are developing soils programs now, too. 

A recent job announcement with the USDA Natural Resources Conservation Service had the following definition of a soil scientist: a degree in soil science or closely related discipline that included 30 semester hours or equivalent in biological, physical, or earth science, with a minimum of 15 semester hours in such subjects as soil genesis, pedology, soil chemistry, soil physics, and soil fertility.

So, you do not have to have a Ph.D. to become a soil scientist. There are a few soil scientists who only have a bachelor's degree, but most soil scientists probably have a master's degree, or classes beyond a bachelor's, to meet that definition. 


Tranissa asked similar questions about being a soil scientist:

What are some pros and cons of being a soil scientist?

My wife regularly accuses me of being a kid (actually she says I'm a 12-yr old in a 40-something body), but I think it is great that I get paid to play in the dirt.

I get to travel all over the USA and to foreign countries (Canada, Mexico, Japan, China, and counting) to study soil (play in the dirt there, too). I am an associate professor at a university, so my main responsibilities include teaching college students and doing research. I enjoy the lab activities most because we go outside and work with and/or in the soil.

One of the other really fun things I do is meet with school children, usually fourth or fifth graders, and talk about why soil is important to their lives (food, clothes, etc.), and to the environment. I am what I want to be when I grow up - in other words, I enjoy my job. 

The drawbacks of a job are usually related to people's personalities and expectations. If you enjoy being around people, then you probably would not enjoy soil survey activities in the Grand Canyon or in Alaska, where you might not see another person for 2 or 3 weeks.
If you enjoy the outside part of your job, you might not enjoy bringing the samples back to the lab and doing the analysis on the samples.
If you enjoy the intricate details of laboratory work, you might not enjoy coming out and presenting the results to other people.
If you teach, you might not enjoy grading papers.


What is the average salary? 

I am not sure what the average salary of a soil scientist is, as that probably depends a lot upon the region of the country in which they live. Most beginning soil scientists would probably start with a salary of about $30,000 (give or take $4,000) for regional differences in the USA.


What is some of the technology you use when you are working?

Technology? Wow! What a question! Soil scientists work with many types of technology, from extremely simple (hands, picks, and water bottles) to extremely complex. Some of the advanced technologies include: geographic positioning systems, geographic information systems, laser surveying systems, infrared thermometry, multispectral imagery, X-ray spectroscopy (mineralogists), several types of spectrophotometers (atomic absorption, plasma emission, etc.; fertility and environmental specialists), respirometers (biologists studying the activity of soil organisms), time domain reflectometry and various types of psychrometry (soil physicists studying water in the soil), ...

The list goes on, I know of people who have done cat scans and MRIs of soils (just like doctors use to diagnose problems in humans).


John asked similar questions about being a soil scientist:

What is your daily routine?  How many hours a day/week do you work? Do you ever have to take work home or work outside of the workplace?

My daily routine is varied. As a professor, during the semesters when I am teaching 3 classes and 2 or 3 labs, most of my time is consumed in preparing for lectures or labs, and conducting the lectures and labs. Often about 16 to 20 hours a week is spent with students in classes or labs. Another 10 hours a week is usually spent preparing for them. Additional work is involved in preparing assignments and exams and grading them.  I also have work that I do for the university, serving on committees and things that takes an hour or two a week. I a few hours each week with undergraduate students answering questions and helping them understand things they did not grasp during the lectures, and with graduate students, providing guidance on their research projects. I probably average 50 to 60 hours a week when all those things are combined. Sometimes I take work home, usually it is grading papers, or reviewing a manuscript (the scientific report of a research project). In addition to working with schools occasionally (usually about 500 plus students per year), I also give presentations on soil for local meetings, usually for farmers or for Master Gardeners groups.

In my spare time, I sometimes consult by providing guidance to businesses or individuals with questions about soil I can answer.  In the summer, I do little teaching, so my focus changes to research projects, and writings scientific reports of those research projects.


Is an internship required?  How in demand is your job?

We recommend that all students have an internship, usually after their junior year. Our Division of Agriculture does not require that students have an internship. However, the environmental science majors on our campus are required to have an internship to graduate.

The demand for soil scientists is growing faster than the supply of qualified college graduates. The growth in jobs is occurring both in agriculture (because fewer people are majoring in basic soil and crop sciences) and in environmental science (because the role of soil is becoming more important as the discipline grows).


Jack asked about the correct terminology for someone who studies soil:

The inclusive term for those who study soil is soil scientist. But there are terms for several of the soil disciplines and interests.

  • Someone concerned with the study of the formation, morphology and classification of soil is called a pedologist or morphologist
  • Someone concerned with soil as a medium for plant growth might be called an edaphologist or agronomist
  • Others studying soil are classified by their disciplines: soil chemist, soil physicist, soil biologist, microbiologist, ...