Don't Call It Dirt: A Little Bit About Soil

Art project demonstrating soil layers "The soil is the great connector of our lives, the source and destination of all." - Wendell Berry

"To be a successful farmer one must first know the nature of the soil." - Xenophon, 400 B.C.

"The nation that destroys its soil, destroys itself." - Franklin Delano Roosevelt

Why Do I Need To Learn About Soil?

Put simply - your plants will thank you. An understanding of basic concepts in soil science will make all of your garden ventures much more successful. Build good garden soil, and your plants will thrive. Neglect your soil, and your plants won't produce as much and will be much more likely to have disease and insect problems.

Soil Structure

What we call soil is made up of tiny grains of mineral matter called soil particles, organic matter, and the space in between the soil particles and organic matter, called "pore space," which is filled with both water and air.

There are different types of soil particles. Sand particles are the largest type, and they are gritty and coarse when you feel them between your fingers. Silt is smaller and smoother with a floury feel, and clay particles are the smallest, with a sticky feel.

Soils that contain a lot of sand drain very quickly when water is poured through them. Clay soils drain more slowly and hold more water. Most soil in Utah has a lot of clay - which makes it stick together when you squish it in your hands. It can be very hard to dig in clay soil when the soil is dry, and you may observe cracks along its surface due to the shrinking and swelling properties of some clays.

Soils with a good mix of sand, silt and clay are best for gardening. Because the size of the soil particles is different, there are irregularly-shaped spaces between them. It is in these "pore spaces" that air and water are held - the exact things your plants' roots need in order to grow.

People who have been trained to do this can estimate the texture of a soil - how much sand, silt and clay it contains - using a simple "feel" test. While I recommend that you have your soil tested by a laboratory to be sure, you can try this test at home (it's fun!) by following these simple directions offered by West Texas A&M University.

Soil Compaction

When people walk often in the same place or when heavy machinery or cars travel over the soil, it squishes the soil particles together. Eventually, the soil becomes hard and doesn't absorb water very well. Without as many pore spaces for water and air, the roots of your plants have nowhere to grow. This is called "soil compaction." Soil compaction kills trees and other plants, and can destroy otherwise great garden soil.

An extreme example of soil compaction occured in a garden where I used to work - The Arnold Arboretum in Boston. There was a beloved tree that received many visitors. Over time, so many people walked over its roots that eventually, the tree died. "Twenty-two schoolchildren from the Winsor School in Boston were perched in the tree, posing for a photograph, when the weight of the group caused the tree to crack audibly and collapse. The children hastily climbed down, with no one suffering injury." This is an extreme example, but it illustrates why you should avoid walking in your garden bed and shouldn't park your car under your favorite tree.

Problems with soil compaction and erosion can also occur if we till our soil when it is still too wet in the spring.

What can we do if we already have hard, compacted soil in our garden? Try double digging or lasagna gardening - two different approaches that each can help, depending on your situation.

What Else Is In the Soil?

If you've ever taken a spade to the ground, you know that there isn't just sand, silt and clay in there. Aside from rocks, there's also organic matter - all sorts of once-living material that is breaking down over time - leaves, sticks, dead insects, etc. You can tell when your garden soil has lots of organic matter in it - it's dark in color and crumbles nicely in your hand.

Good soil is also teeming with life! Just one teaspoon of healthy soil contains some 5 billion bacteria, 20 million fungi and 1 million protocists! Not to mention all of the ants, spiders, beetles, earthworms, centipedes, millipedes, mites, slugs, snails, springtails and nematodes that call your garden home. That's why we practice organic gardening - because we know that the healthy garden is a system, and all of these little guys belong to that system. We could turn to pesticides for a quick fix - but we know that if we kill our little helpers, our whole garden would suffer.

Soil Properties - pH

There are some other important things to know about your soil that are not as easy to see as the soil structure, but they are important to know about.

pH - This is a measure of the acidity or alkalinity of a substance - in this case, we are talking about the pH of the water that is contained in the soil. Pure water has a pH of 7, which is considered neutral. Acids, such as lemon juice, have a pH between 0 and 7, and alkaline or basic liquids, such as bleach, have a pH between 7 and 14. Most plants prefer a pH that is close to neutral. pH becomes important to plant health, because as it becomes higher or lower, it changes the amount of those essential soil nutrients that are available to your plants.

Here in Utah, most soils tend to be on the alkaline side - with a pH between 7.8 and 8.2. This is because there is more calcium carbonate - lime - present in our soil. Luckily, this is the easiest problem to solve. As my soil science professor used to constantly say, "Life is acidifying!" What this means is that decomposing organic matter tends to be acidic. So, over time, adding compost to your soil will help to balance out the pH of your soil, unless it's extremely alkaline.

How do you know the pH of your soil? You'll notice that the pH of your soil may be too high or low will be if your plants aren't growing very well. If you see yellowing or reddish leaves, and your plants seem stunted, you need to have your soil tested for pH and nutrients. I recommend having your soil tested every few years by the Utah State University extension or another soil testing lab.

Knowing the pH of your soil will also help you select the best plants for your situation. For example, cranberries really need a very acidic soil to grow well. Knowing this, you can select a different type of fruit to grow instead that will be happier here in Utah.

Essential Nutrients and Soil Fertility

Plants need a number of nutrients in order to grow. The ones they need in the largest amounts are called macronutrients. The primary macronutrients that plants obtain from the soil are nitrogen (N), phosphorus (P), and potassium (K). These major nutrients are sometimes lacking from the soil because plants use large amounts for their growth.

There are other important nutrients, but they are needed in relatively small amounts. These are called micronutrients, and include manganese, boron, iron, chlorine, zinc, copper, molybdenum and nickel.

Now, to make our garden the most productive, we often think that we need to add some quick-acting chemical fertilizer to make everything grow at its best. But this is a mistake. Using all those chemical fertilizers can have negative effects on the soil and the environment as a whole. A few examples of this include:

the contamination of drinking water through the over-application of nitrogen fertilizers, leading to public health problems such as "blue baby syndrome;"
the contamination of surface water with phosphates, which can kill fish and cause algal blooms (called eutrophication);
and the loss of crop land due to soil erosion. In the U.S., the average acre of cropland is eroding at a rate of 7 tons per year because of conventional farming practices, including the use of chemical fertilizers.

By having your soil tested to see how much fertilizer you really need to add, and by using soil amendments that take time to break down, such as compost, you can prevent these problems, improve your garden and help protect the environment as well. And the nice thing about using organic amendments like compost, is that they not only contain the "big three" nutrients - nitrogen (N), phosphorus (P), and potassium (K) - but they also contain lots of important micronutrients as well.

"Feed the soil and it will feed the crop." - Robert Rodale

So, let's review here. We know that the goal of organic gardening is to build and maintain stable nutrient levels in the soil instead of just doing "quick-fixes" whenever you have a problem. We know that this is the best practice, for our garden and for the environment. But in practical terms, how do we do this?

Again, having a soil test performed periodically is always helpful, both as a starting point, and to help you track your progress.
Get your hands dirty and see directly how fertile your soil is. Take a shovel and dig right in! Do you see any signs of life? If there are obviously earthworms and centipedes, you are on the right track; if there are none, don’t worry, your soil will soon be full of life.
Start a compost pile! The best organic fertilizer is cheap and readily available. In addition to being nutrient-rich, good compost hosts a number of living organisms, all of which help break down and convert the organic matter in your soil to nutrients ready to be used by your plants.
You can also use composted manures and other organic fertilizers as needed.
Rotate your crops. If you grow a "heavy feeder" - a plant that takes up many nutrients from the soil, follow that with a plant in the legume (pea and bean) family. Legumes actually add nitrogen to the soil. These "green manures" are otherwise known as cover crops.
Share information and tips with fellow gardeners, read the resources at the end of this article, and stay informed by signing up for our monthly newsletter, which includes organic gardening tips.
Enjoy your garden!

An Important Note: Lead Contamination

Especially in the city, it is possible that your garden soil may be contaminated with lead or other heavy metals if your garden is close to busy streets and highways, near old buildings where lead-based paint has peeled or been stripped off, or if your garden was once an orchard because of the use of lead- and arsenic-based pesticides.

But don't panic. Just have a soil test performed and you will know if there is any reason for concern. The Utah State University Extension offers soil testing for a small fee. Call them for more information at 801-468-3170, or visit the Utah State University extension website. Make sure to tell them that you would like to make sure your soil is not contaminated with lead. You can collect soil samples for this test any time of year when the ground is not frozen.

If there is a high amount of lead in your soil, you can still garden - but you will need to garden in containers or in raised beds with soil you have purchased from a supplier. Keep the contaminated soil covered by growing grass or a ground cover, or by laying mulch or stones over it. Keep children from playing directly in the soil or eating it, and wash your hands thoroughly if you have been in contact with contaminated soil.

Recommended Reading

Brady, Nyle C., and Ray R. Weil. The Nature and Properties of Soils. (now in its 14th edition, this is the classic soil textbook for the soil-obsessed gardener.)
Montague, Fred. Gardening, an ecological approach to individual, community and global health, Mountain Bear Ink, 2009. http://www.mountainbearink.com/

Additional Sources

Del Tredici, Peter. Requiem for a Cork Tree. Arnoldia, Fall 1995. http://arnoldia.arboretum.harvard.edu/pdf/articles/422.pdf
Gale, Jody, Rich Koenig and James Barnhill. Managing Soil pH in Utah. Utah State University Extension Electronic Publishing, 2001. http://extension.usu.edu/files/publications/publication/AG-SO-07.pdf
Granstedt, Artur, and Lars Kjellenberg. Long-Term Field Experiment in Sweden: Effects of Organic and Inorganic Fertilizers on Soil Fertility and Crop Quality. Proceedings of an International Conference in Boston, Tufts University, Agricultural Production and Nutrition, Massachusetts March 19-21, 1997. http://www.jdb.se/sbfi/publ/boston/boston7.html
Hood, Ernie. The Apple Bites Back: Claiming Old Orchards for Residential Development, Environmental Health Perspectives, Volume 114, Number 8, August 2006. www.ehponline.org/members/2006/114-8/focus.html
Lynda Knobeloch, et al. Blue Babies and Nitrate-Contaminated Well Water. Environmental Health Perspectives Volume 108, Number 7, July 2000. http://ehp.niehs.nih.gov/members/2000/108p675-678knobeloch/knobeloch-full.html
Mahler, Robert L., Alex Colter and Ronda Hirnyck. Nitrate and Groundwater. University of Idaho College of Agricultural and Life Sciences, July 2007. http://info.ag.uidaho.edu/pdf/CIS/CIS0872.pdf
North Carolina Department of Agriculture and Consumer Services. Plant Nutrients. http://www.agr.state.nc.us/cyber/kidswrld/plant/nutrient.htm
Rosen, Carl J. Lead in the Home Garden and Urban Soil Environment, University of Minnesota Extension. www.extension.umn.edu/distribution/horticulture/DG2543.html
Sideman, Eric. Basics of Organic Soil Fertility. Maine Organic Farmers and Gardeners Association, 2006. http://www.mofga.org/Default.aspx?tabid=518
Sullivan, Preston. Sustainable Soil Management. ATTRA Publication #IP027/13. http://www.attra.ncat.org/attra-pub/PDF/soilmgmt.pdf
West Texas A&M University, PSES 3411L Exercise 2: Estimating Soil Texture by Feel. Available at: http://www.wtamu.edu/~crobinson/soils/unit_1/txtr_feel.html
U.S. Department of U.S. Department of the Interior/U.S. Geological Survey. Definition of eutrophication. http://toxics.usgs.gov/definitions/eutrophication.html