Making the Most of Your Garden’s Soil

— Written By

With spring in the air and leaves bursting out of their buds, it’s easy to focus on all of the exciting things happening above ground. But don’t forget that it’s also a great time to set your plants up for success by taking a minute to consider where they’ll be getting their nutrients and support from. Whether you’re new to Durham or a longtime Durhamite new to gardening, a quick crash course in managing our soils will get you (and your garden!) off to a good start.

Where does all of this clay come from?

Just like plants and animals, there’s actually a scientific way to classify soils. Knowing which of the 12 soil orders your soil belongs to can go a long way in helping you know how to plan your garden. Most of Durham County, and more broadly the southeastern part of the United States, has soils referred to as Ultisols. Ultisol soils are produced when heavy rain leaches many nutrients from the soil and warm weather prevents organic matter accumulation. The soils created under these conditions are highly weathered, acidic (low pH), and rich in clay (1).

Clay is the smallest of the three major particles of soil, with silt being slightly larger, and sand having particles large enough to see with your bare eyes. Relatively speaking, if sand particles were as large as basketballs, many clay particles would be roughly the size and shape of grains of rice. This helps explain why clay holds water so well – not only are the particles very small, but they also fit together closely because of their flattened shape (imagine draining water through a barrel of basketballs versus a bucket of rice). The proportion of clay to silt to sand is described as the soil’s texture.

Our clay soils make gardeners work a bit harder than gardeners in other parts of the country. When wet, the clay is slippery muck; when dry, it cracks and hardens into concrete. The clay isn’t all bad though: clay holds many plant nutrients extremely well, meaning our plants are well set up if we can just improve our soil structure.

The image on the left shows a close-up of sand particles which appear grainy as seen by the naked eye. The right shows the platelike texture of clay visible only under a microscope.

The image on the left shows a close-up of sand particles which appear grainy as seen by the naked eye. The right shows the platelike texture of clay visible only under a microscope. From the North Carolina Extension Gardener Handbook.

Tips for managing clay soils

Our soil needs organic matter, and lots of it, to help the soil particles form good structure and increase drainage. Established beds will need yearly additions of soil amendments because our warm temperatures allow soil organisms to break down organic matter quickly.

Our soil erodes easily, so it should never stay exposed. It takes nature hundreds of years to make 1 inch of topsoil, but rain can erode 1 inch in a week. Protect soil from erosion by always keeping it covered with plants and mulch. Covering soil also helps keep annual weeds at bay and can reduce the need for watering.

Our soil is strongly acidic. The natural soil pH for this area is 4.8 to 5.3 (neutral is 7.0), but most lawns, vegetables, and landscape plants prefer the 5.8 to 6.5 range. Plants cannot use nutrients that are already present if the soil is too acidic, so applying lime regularly can be just as important as fertilizing. Lime is relatively inexpensive and probably the best garden value you will find. Always have a soil sample analyzed before applying lime, especially because in certain cases, typically around recent construction, soil pH can be much higher than expected.

**note: The N.C. Department of Agriculture and Consumer Services Agronomic Lab is not currently accepting routine soil samples, but keep soil testing in mind for the future.

The Many Benefits of Organic Material

The top 8-10 inches of soil, where plants’ roots live, determine the success of your gardening efforts. Plants need oxygen just as people do. An ideal soil from a plant’s perspective is 50% soil matter and 50% open, or pore space (which should be half-filled with water and half left open for air). Clay soil, and soil near recent construction, is often too compact to allow roots to “breath.” Adding organic matter to the soil lightens soil structure (also called “tilth”). Soil structure is how the individual clay, silt, and sand particles are arranged, and is impacted by factors such as organic matter. By adding organic matter and improving overall soil structure, you can help discourage compaction, add nutrition, improve drainage and aeration, moderate soil temperature, and provide pore space which is essential for plant growth. Clay without organic matter is like a flattened deck of playing cards. Adding organic material jumbles the cards, permitting water and oxygen to enter the soil.

Do not work clay soil when it is wet. Mowing wet lawns or working in wet flower and vegetable beds compacts the soil and spreads fungal diseases.

Mulch

A 3-4 inch thick layer of mulch on top of your garden will benefit the soil in many ways. Mulch keeps the soil surface from eroding, prevents soils-borne diseases from splashing onto leaves and flowers, keeps roots cool, conserves moisture in the soil, keeps the soil surface from baking into a crust, reduces weed germination, decays to provide a steady supply of organic materials and nutrients, and is an attractive finish for a planting area. Many local landscape supply centers can easily deliver mulch to your house after taking an order over the phone or online, providing contactless delivery.

Soil Amendments

Soil amendments are materials that are worked into the top several inches of soil, to improve drainage and loosen the soil where plants grow. Organic soil amendments can make a world of difference in heavy clay soils. Organic matter breaks down fast in our warm climate, so beds will need regular additions of soil amendments to maintain good soil structure and fertility.

Well-rotted compost is an excellent organic amendment for our clay soils. Just about any plant-derived material is fine as a starting material. Well-rotted compost is black and crumbly, and smells fresh and loamy. It is decomposed enough when you can no longer well what most of it used to be. Finely-ground pine bark (sold as “pine bark soil conditioner”) is also an excellent organic soil amendment. A pea-sized grind (¼ to ½ inch) is ideal. Pine bark is a native and renewable resource.

Peat moss and sand are NOT recommended for improving clay soil, and purchased topsoil is frequently no better than your existing soil. Purchased topsoil is useful for creating raised beds, but for most landscaping needs, it is better to amend the existing soil with organic materials. If you do bring in topsoil, be sure to mix it thoroughly with the native soil. If new soil is just spread over clay, plants will not root into the clay, and the plants will dry out in hot weather. Be aware that unsterilized topsoil may contain weed seeds.

For some of the best, most reliable organic material, start your own compost pile. Your environment, your plants, and your landfill will all benefit. Detailed information on composting is available through your N.C. Cooperative Extension, Durham County Center.

Examples of granular soil structure (looks like chocolate cookie crumbs) in topsoil.

Examples of granular soil structure (looks like chocolate cookie crumbs) in topsoil. From the North Carolina Extension Gardener Handbook

But it still seems too wet…

Drainage in soil means the rate at which water moves through the soil. Poor drainage is responsible for many of our planting failures. Wet soils “drown” plants because their roots don’t get enough oxygen. Wet soils can also foster fungal diseases and root rot.

If you are concerned about wet spots in your yard or garden, you can test your drainage by doing a “perc test” when the soil is neither extremely dry nor extremely wet. Dig a hole in your yard about 12 inches deep, and fill it with water. Let the water drain completely and fill the hole again. Check it regularly for the next 24 hours.

Time to empty            means…

3-4 hours                     Congratulations! You have good drainage.

5-12 hours                   Moderate drainage. Some plants will tolerate this situation, but many will drown.

12-24 hours                 Poor drainage. Your soil definitely needs work!

Drainage can be improved by tilling or spading the soil 8-10 inches deep, and by adding organic soil amendments. You may also need to plant “high,” that is, set the plants in a mound of soil above the level of the surrounding grade. Raised beds work on the principle of planting high, as they put the majority of plant roots above the level of the surrounding, non-raised soil.

Want to learn more about managing your very own slice of Durham soil? Check out these excellent resources:

North Carolina Extension Gardener Handbook – This excellent publication from NC State University was written specifically for North Carolina, and has chapters on almost every topic a gardener could want. Learn more about Soils and Plant Nutrients in Chapter 1, and Composting in Chapter 2.

North Carolina Extension Gardener Plant Toolbox – Looking for the perfect plant for a particular soil condition? The “Find a Plant” tool on the Plant Toolbox allows you to search by soil texture, pH, and drainage conditions (as well as by many other traits of interest).

NC State Extension Composting Portal – Learn all about composting (including with worms!) from the homeowner to industrial scale. Updated regularly with new interesting information.

Adapted from Durham County Urban Horticulture Note #2: Durham’s Soil

(1) Brady, N, Weil, R. (2010) Elements of the Nature and Properties of Soils, 3rd Edition. Upper Saddle River, New Jersey: Pearson Education, Inc.