The dirt on dirt
Beth DuFault sticks a spade into a flower bed in her north Fargo backyard and lifts out a heavy shovelful of slick, black soil.
"I don't need someone to tell me this is clay," says DuFault, an avid flower gardener. "It's very, very dense. It's like sticking a shovel into quick cement."
Like many who garden in the Red River Valley, DuFault has a love-hate relationship with the area's heavy, dense, sometimes-clumpy soil.
But experienced green thumbs have figured out how to work with what Mother Nature gave them.
DuFault, for instance, adds "Army-sized" bags of peat moss to the sticky clay soil. She also takes advantage of the city of Fargo's free compost program, in which residents can get a free yard of compost each spring and fall.
Thanks to her diligence over the last few years, the soils in her flower beds have lightened up. The plots are easier to work and the black dirt clumps less and drains more easily.
"The compost thing is the best thing since sliced bread," she says.
Types of soil
To understand the care and feeding of clay, it may help to learn a few basics about soil first.
R. Jay Goos, soil scientist at North Dakota State University, says the mineral portion of soil is primarily made up of sand, silt and clay. The different proportions of these soil components will determine whether a soil is loose, light and sandy, or dense and sticky.
Clay is so tiny that one particle is the size of a virus, and you would need an electron microscope to see it. Most of the clays from this area have a flat, sheet-like shape, which gives them a tremendous surface area per gram of material.
"That is why clays are so sticky and slick," Goos says. "There is just so much surface area to adhere to each other and everything else."
Clay soils - those containing at least 40 percent clay - are typically classified as fine-textured and heavy.
The Red River Valley is widely known for its clay soils - a result of Lake Agassiz, the immense, prehistoric glacial lake located in the center of North America.
"The center part of the Red River Valley is where the water from Lake Agassiz was the deepest and where the most clay was deposited," Goos says.
There are some excellent advantages to this heavy, fine-textured soil.
It has a high natural fertility for nutrients such as calcium, magnesium and potassium, Goos says.
Clay also will hold an impressive amount of "plant-available" water, that portion of water in the soil that can be absorbed by a plant.
But clay has its downside, too. It can become so dense and hard that it's extremely difficult to dig in or work. Come springtime, it can take forever to dry out and warm up. And in really clay-dense areas, the soil can form such heavy, sandwich-like layers that young roots can't squeeze through it. As a result, you get stunted root systems.
To determine if your soils are predominantly sandy, clay-filled or somewhere in between, try what Goos calls the "ribbon test."
Take a small amount of soil in your palm and moisten it. Roll it into a ball in your palm. Next, try to make the clay into a flat ribbon between your thumb and your first two fingers. If you can barely make a ribbon and can feel the sand grains easily, the soil can be classified as a light, sandy variety. If you can form a ribbon at least 1½ inches thick, it is a loamy, medium-textured soil. And if you can form a long ribbon with it, the soil probably contains more than 40 percent clay.
For a more detailed evaluation of your soil, you can try a home test kit or send samples to NDSU's soil-testing lab, (701) 231-8942.
But if you're confident its clay, here are some steps toward amending it:
- In gardens, till or deeply dig up the soil - but not so deeply that you bring up undesirable soil containing salts or carbonate minerals. Be sure to only do this when the clay is completely dry, or you will have to battle with large clumps throughout your growing season, says Todd Weinmann, Cass County Extension horticulturist.
- Mix in generous amounts of compost. Compost provides stabilized organic matter that will improve the physical properties of the soil. Additions of compost over time will boost the soil's organic carbon and humic matter content.
Goos prefers to collect tree leaves in the fall and store them in bags over winter. The next summer, he mixes the previous year's tree leaves with grass clippings to make a balanced compost. (The grass clippings provide nitrogen; the tree leaves provide energy and aeration.)
Then, late in fall, he spreads the compost on the garden and rototills it thoroughly.
"A good dose of compost causes the general microbial population of the soil to explode due to the new food source," Goos says. "This suppresses many plant pathogenic organisms, whose dietary needs are more narrow."
- Add peat moss, the partly carbonized remains of sphagnum moss and other mosses that grow in Canadian bogs and elsewhere. Peat moss is much lighter than mineral soil and has big pores so water and air will flow more easily through the soil, Goos says. Peat also tends to increase acidity in high-alkaline soils. (Note: When working with peat moss, Weinmann advises washing your hands afterward. Some people break out from it.)
- Think twice before adding sand. Some gardeners figure that the opposite of clay - sand - is what's needed to offset clay's density. Experts advise against this.
"Additions of sand to clay soil can create a cement-like creature that resists root growth and impedes the flow of air and water, writes gardener Keith Baldwin for Fine Gardening magazine.
For free compost
Each spring and fall, the city of Fargo offers residents one yard each of compost and wood chips. The spring giveaway is available from 3:30 to 5 p.m. every Wednesday and Thursday through June 3 west of the Household Hazardous Waste facility, 606 43½ St. N. Proof of residency is required. A small pile of compost is also available for those who wish to fill their own bags or cans. It's next to the scale house at the landfill, 4501 7th Ave. N.
Before and after the free giveaway, you can purchase compost ($10/yard) and wood chips ($8/yard) from the city. For information, call 701-282-2489.
Your guide to common soil problems
Patches of soil with high sodium levels may dry hard and, when watered, form pools quickly. Sodium prevents soil particles from combining, forming large monoliths with soil particles so tightly bound that roots can't poke through. Plants growing in these areas will require frequent watering yet fail to thrive.
Evidence: Poor plant growth, ponding and hard-to-till soil. The surface may contain white crust, and a soil test of the top 6 inches shows high levels of salt.
Remedies: Your first priority is better drainage. A soil test can provide a sodium absorption ratio (SAR), showing the relative balance of sodium to the desirable amounts of calcium and magnesium.
The higher the SAR reading, the greater amount of soluble calcium amendments you'll need to replace sodium on soil clays and flush them away with rainfall or low-sodium irrigation water.
Amounts required may range from 20 to 200 pounds per 1,000 square feet of gypsum.
The amendments will need to be tilled to the depth of the sodium problem.
The best solutions include growing grasses that are drought-tolerant, moving the garden or building raised beds with improved drainage and lower-sodium soils.
A few inches of good topsoil over poor-quality soil will result in plant roots growing in just the good stuff. This restricts the amount of soil used for water and nutrients, which means your plants will need more water and fertilizer.
Evaluation: Soil core samples indicate soil textural changes within the common rooting zone (about 6 inches for lawns,
18 inches for garden plants).
Remedies: For lawns, make sure the topsoil is at least 6 inches deep before seeding.
For gardens, amendments need to be tilled in deeply. Raised beds should be uniform. If raised beds are shallower than 18 inches, blending the original soil - if not contaminated with salts or undesirable materials - into the added soil material will help.
Cracking is common in dry soils with high clay content. These cracks will accelerate drying of subsoil and limit the soil's water-holding capacity during the heat of summer.
Remedies: Water lawns more frequently but less intensely. Maintain good grass coverage. In gardens, add organic amendments - like unmilled peat moss - to improve aggregation. Do not overwater, but don't allow soil to become overly dry.
Thatch - a surface layer of undecomposed organic material on established lawns - is beneficial for turfgrass up to ½-inch thick. But once it exceeds that, thatch can restrict water, air and nutrient movement into the root zone. Excess thatch can harbor insects and diseases, limit rooting and develop hydrophobic properties that repel water.
Evaluation: You'll notice an excessive amount of "cushiness" when walking across the lawn. Sod appears loose due to lack of rooting. Response to fertilizer, water and pesticide applications is poor.
Remedies: Thatch can be controlled through regular core aeration, power raking and selecting non-aggressive turf cultivars. It is not caused by returning clippings to the surface when mowing.
Source: "Evaluation, Preparation and Amending Lawn and Garden Soil" by Ron Smith and Dave Franzen of NDSU Extension
Readers can reach Forum reporter Tammy Swift at (701) 241-5525