The Difference Between Dirt and Soil

Dirt and soil - they mean the same thing, right? Not to a scientist. My graduate school soils teacher had this to say about the topic: "Dirt is dead. Soils are not."

 Soils are actually pretty complex. According to Brady and Weil, in their fine volume The Nature and Properties of Soils:

The four major components of soil are air, water, mineral matter, and organic matter.

What does that mean exactly? Let's break it down.

Air in soil is contained in pore spaces. Different soils have varying amounts of pore space. A well-aerated soil is more likely to grow crops, since it is easier for plants to stretch out their roots. If a soil is highly compacted, nothing is going to grow there. Think of tight packed clay, for example.

Water in soil is pretty easy to visualize. A sandy desert soil, for example, might retain very little water, even after a rain. But a soil higher in organic content, like a loam, will hold more water.

Mineral matter simply refers to the type of bedrock from which the soil was derived. Bedrock is sometimes referred to as a soil's "parent material." A soil that came from granite will have different properties than a soil derived from limestone. (According to this article in Soil Science and Plant Nutrition, red limestone-based soils have higher levels of trace elements than soils derived from other types of bedrock.) 

Organic matter in soil consists of plant or animal residues along with soil-dwelling critters like earthworms. This is probably the biggest difference between dirt and soil, and is the origin of the comment above from my professor: soils are living ecosystems.  

Soils are probably the most underrated of all ecological resources. If you think about it, everything we do depends upon the soil. We rely upon soils to filter the water we drink. We need soils to grow crops for food, lumber for housing, cotton and hemp for clothing. How well a soil compacts - or doesn't - affects our ability to build homes and businesses. In short, we shouldn't treat soil like dirt! 



This soil from Illinois is high in organic matter. Photo credit: Natural Resources Conservation Service Soil Health Campaign, via Flickr

This is National Lead Poisoning Prevention Week!

What a busy week this is! In addition to being National Chemistry Week in the U.S. and Waste Reduction Week in Canada, my friends at the Coalition to End Childhood Lead Poisoning have reminded me that October 18-24, 2009 is also National Lead Poisoning Prevention Week.

According to the U.S. Department of Housing and Urban Development, "childhood lead poisoning is considered the most preventable environmental disease among young children" and yet it still affects over one-quarter of a million of kids in the United States. The Coalition to End Childhood Lead Poisoning says that "[l]ead poisoning is the number one environmental hazard threatening children throughout the United States."

Why is lead still a problem? I thought we banned it years ago.

Historically, lead was used in just about everything, including pesticides, pipes, gasoline, paint, and batteries. But two sources — leaded gasoline and lead-based paint — caused most of the remaining exposure risk in the U.S. today.

The U.S. government's ban on leaded gasoline in motor vehicles didn't fully go into effect until 1996. Prior to that time, exhaust from cars released lead onto roadways and adjacent soils.

Lead-based paint was banned from U.S. residential use over 30 years ago. But homes built before 1978 often contain lead-based paint. Home renovations can disturb lead paint lurking just below the surface. It is difficult to safely remove it. Traditional paint removal techniques, like dry sanding, are not recommended. Once released, lead dust can spread throughout your house, contaminating everything.

The U.S. Environmental Protection Agency recently passed a new rule requiring that contractors be certified in lead-safe practices before renovating or repairing buildings that may contain lead paint. This rule goes into effect next year.

How can exposure to lead affect me?

Our bodies mistake lead for the beneficial (and chemically similar) elements of calcium and iron. The human body can store lead in bones and teeth in place of calcium. Lead can be found in the bloodstream, substituting for iron. The effects of lead on the human body are most pronounced in the central nervous system.

The more lead in your system, the greater your risk for having adverse health effects, like cognitive impairment, headaches, irritability, stomach upset, learning disabilities, and seizures. These effects are most pronounced in children. Pregnant women exposed to lead can suffer from stillbirths and miscarriages.

I think my family may have been exposed to lead. What should I do?

Visit your doctor. Request a blood-lead test to put your mind at ease. Fortunately, the test is simple, involving a simple finger prick or blood draw.

Feed your family a good, nutritious diet high in calcium and iron. The more calcium and iron in your system, the less likely that your body will take up the lead.

Maintain your home. If you think that your home contains lead-based paint, damp-mop areas that may contain lead dust, like windowsills or doorways.

Keep dirt outside. Have family members remove their shoes when coming in from the outdoors. Wipe the paws of your pets before they come inside. Wash your hands after gardening or playing in the dirt.

Avoid cheap costume jewelry. Some items, simply put, are frequent offenders of the lead paint ban. It is best to avoid giving them to children. If you have concerns about a toy, check with the Consumer Product Safety Commission to see if it has been recalled.

For more information:

Visit The Coalition to End Childhood Lead Poisoning for more tips on how to protect your family.

The Centers for Disease Control and Prevention offer these Lead Poisoning Prevention Tips.

The U.S. Environmental Protection Agency has an entire section of their website devoted to the topic of lead. They also have a nice page detailing different Lead Prevention Week activities scheduled across the country.


Portions of this post previously appeared in Blood Level Basics: What You Really Need to Know in the October 2006 issue of Washington Parent magazine.

Website of the Week: Dig It! The Secrets of Soil

The Smithsonian National Museum of Natural History is currently featuring a new exhibit called Dig It! The Secrets of Soil.

What is Soil? Here's a handy definition I learned in graduate school -- Dirt is dead. Soil is alive. Soils are dynamic ecosystems filled with living things like insects, worms, bacteria, and fungi.

How do soils form? Check out this handy chart to learn more about the five soil formation factors: Climate (Is the surrounding area rainy or dry? Salty or devoid of minerals?) , Biota (Which plants and animals live and use the soil?), Slope (Did the soil form on a mountainside or in a valley?), Bedrock (What kind of rock has contributed to the soil?) and Time.

How many different types of soil are there? "Scientists recognize about 70,000 soil types in the United States alone ..." according to Paul Reich of the Natural Resources Conservation Service of the USDA, as quoted in this exhibit. His Global Soil Regions Map is shown on the NRCS Soils Website.

Check out the Educators Activity Page for fun ways to learn about soils both at the museum and at home. Learn More about soils by visiting these websites recommended in the exhibit.

(My thanks to Soils Professor Dan Richter for letting me know about this exhibit!)