Gardening in Clay

• By Ralph Morini
• /
• July 2018 - Vol.4 No.7
• /
• 4 Comments

We all have a sense that soil plays a key role in our gardening success. In fact, different mineral content, textures, and structures offer very different growing conditions and may be more or less suitable for specific plants. In our locale, the Virginia Piedmont, the native soils typically have a shallow sandy loam surface with a clay-based subsoil, colored red or yellow-red from oxidized iron weathered from native minerals. As good as clay is for bricks and flower pots, it is problematic for gardeners. The good news is that with regular amendment and smart management, its high moisture holding ability and mineral content provide the basis for building clay into a productive growing medium.

Most of us recognize clay by its stickiness and clumpiness when wet and concrete-like hardness when dry. Let’s go a little deeper into soil composition generally and clay specifically, discuss actions we can take to make it a more productive soil, and finally identify some clay tolerant plant options.

Soil Types and their Differences

Soil is a living, breathing natural entity comprising solids, liquids and gases. It performs multiple functions including providing a habitat, recycling wastes, filtering water, and is a medium for plant growth that offers structural stability, while retaining and supplying nutrients.

The ideal soil is roughly 50% pore space and 50% solids with the pores filled with equal parts air and water. Activities like tilling increase pore space while compaction tends to reduce it. The solids half is ideally 45% mineral and 5% organics. The solids are a blend of mineral particles derived from weathered rock and organic matter and are broken into three size classes:

• Sand: the coarsest particles, .05-2.0 mm in diameter
• Silt: mid-size particles: .002-.05mm in diameter
• Clay: the finest particles: less than .002mm in diameter.

Soils contain a blend of these particles and clay properties become evident in soils with higher than 20% clay content. A desirable blend is something like 40% sand, 40% silt and 20% clay.

This link explains simple ways to estimate your soil composition.

Clay: pluses and minuses

Clay soils are fine-textured, heavy soils, with very small particles, predominantly mineral and little organic matter. They are “sticky-smooth” when wet, and form balls and ribbons when shaped by hand. A smoother feel indicates a higher proportion of clay particles. A gritty feel indicates more of a blend of particle types and sizes. A red color indicates good aeration. Grayish color shows poor drainage.

Wet clay is sticky and moldable when wet.
Photo courtesy of HGTV.com

Clay’s small particles increase pore space but also promote aggregation and are easily compacted when wet. It drains slowly when wet and hardens to the point where it may be untillable when dry. Because it drains slowly, it warms more slowly than well-drained soils in spring.  

On the positive side, clay’s high water and nutrient-holding capacity give it good growing potential, with a bit of gardener intervention.

The red clay indicates iron content and good aeration.The gray clay is fine grained and poorly aerated.

Photo credits to geokkhim.blogspot.com and soilquality.org, respectively.

Clay Fix-it Tactics

There are a number of straightforward steps that the home gardener can take to improve the productivity of clay soil:

        Start with a soil test:

• Clay soils vary in pH and may require correction to get into the commonly recommended pH range of 6.3 to 6.8. Agricultural Extension soil tests will also provide guidance for optimizing levels of phosphorus, potassium and micronutrients.  You can pick up a kit for collecting a soil sample at the local Extension Office, 460 Stagecoach Road in Charlottesville.

       Avoid compaction:

• Minimize or eliminate walking and equipment travel over planting areas. Creating permanent pathways that allow access to planting areas without treading on them is a good idea.
• Never work clay when wet. It will clump and defy cultivation, requiring a re-do as it reaches a workable moisture level or presenting a big problem if it dries into said clumps. On the other hand, don’t over work it or you destroy its structure.
• Don’t overtill. Turning the soil over completely will reduce porosity, adding to soil density, increasing runoff, slowing drainage, and reducing nutrient availability to plants. It will also tend to compact the layer beneath the tilled section. Using a spade or rotary tiller only as deep as 6-8 inches — just to loosen the planting zone —  is recommended. Smooth out only the soil surface.

       Add organic material:

• Apply 3-4 inches of compost, well-rotted manure, leafmold, etc., on top of the beds and work it into the soil 4-6 inches deep, in the fall or prior to planting in the spring. This will improve soil structure and reduce compaction, promoting better water infiltration, drainage, and retention, as well as nutrient availability.
• Mulch with organic materials such as bark, sawdust or wood chips and allow it to work into the soil naturally over time
• Plant cover crops such as clover, timothy hay, hairy vetch or borage. The cover crop roots act as a living amendment. Legume crops increase nitrogen availability and protect against erosion over the winter. Plant a month before the first killing frost. Work into the soil before it goes to seed and allow it to decompose for a couple of weeks before planting.
• Adding organic material and tilling in the fall allows time for decomposition prior to planting, and freezing-thawing during the winter can help to loosen the soil and reduce compaction without overworking it.
• Prior to planting, break up large surface clods and rake level. Small seeds germinate best in fine, smooth surface soil. As noted above, avoid pulverizing deeply, which can harm soil structure.
• Manage runoff. Building a garden of lighter soil surrounded by heavy clay can create a drainage issue if the surrounding clay traps water in the garden. Diverting runoff so it doesn’t enter the garden is a good idea to prevent a potentially damaging drainage issue.
• Be patient. Over time, soils tend to return to their native state. It takes some years of amendment to create a significant improvement and ongoing commitment to maintain it.

                                         Raised beds offer a way to sidestep problematic clay soils
Photo courtesy of saga.co.uk

Consider raised beds 

• Raised beds offer a way around problems with a clay soil environment, or any problem soil for that matter. Build beds to dimensions that allow working them without standing on the soil. Loosen the base clay under the beds to reduce potential drainage issues from the beds. Fill them with garden soil from a reputable source and follow the soil improvement steps mentioned above for continuous improvement of your soil and growing conditions.
• In addition to providing a quicker fix to difficult soil conditions, raised beds can make it easy to utilize intensive planting techniques, simplify weeding requirements and reduce water consumption.
  

Trees and Landscape Plants for Clay Soils 

Modifying clay soils for vegetable gardens or flower beds is one thing. Tree and landscape planting however is more a matter of choosing plants that can do well in the native soil and planting them in a way that makes success most probable.

The recommended way to plant a tree or shrub is a bit different for clay than for well-drained, looser soils:

• Prepare the plant. Remove it from its container and if, as is likely, its roots are curling around the root ball, cut the roots with a knife, from top to bottom of the root ball, every 4-6 inches. This releases the roots to grow out from the plant and prevents girdling problems after planting.
• Dig the hole just large and deep enough to fit the root ball. Use the existing natural soil for backfill, without amendment. Having a consistent soil type will help the plant adapt and prevent the hole from becoming a water pool that is unable to drain into the surrounding clay.
• Elevate the top of the root ball about 2 inches above the soil line. The water needs to drain away from the plant, again to minimize pooling issues and prevent root rot.

For plants other than trees and shrubs, matching pH preferences with your soil and avoiding plants that don’t like excessive moisture is a good start. Some common suggestions are switchgrass, asters, Russian sage and hostas. These links include suggestions for various plant types from the University of Maine Extension (“Trees and Shrubs for Clay Soil”) and www.missouribotanicalgarden.org/ProblemSolver Plants for Clay Soil – Perennials.

Take Heart

Clay is a hard soil to love. Can’t work it when wet, can’t even penetrate it when dry, and it rarely seems to be “just right”. But, with appropriate management and amendment, a gardener can take advantage of its mineral content and water/nutrient holding capacity and make it a very satisfactory growing medium. Like life, our soil is what we make of it.

References:

https://content.ces.ncsu.edu/extension-gardener-handbook/1-soils-and-plant-nutrients

https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/426/426-313/426-313PDF.pdf

https://durham.ces.ncsu.edu/files/library/32/UNH%202.PDF

https://www.thespruce.com/understanding-and-improving-clay-soil-2539857

https://content.ces.ncsu.edu/extension-gardener-handbook/1-soils-and-plant-nutrients

Source for featured photo: gardeningknowhow.com