by Chris Smith, PhD
We here in King George and Westmorland Counties (and many other counties up and down the east coast) are living on ancient coastal plain marine and riverine sediments. Where we stand now was once occupied by prehistoric fish and other mostly extinct creatures. Sea level has risen and fallen many times since the sediments were first deposited about 275 million years ago as the Appalachian Mountains, pushed up by the collision of the Eurasian plate (twice), began eroding and depositing sediments into an ancient sea. As the sea dropped, rivers carved at the sediments and formed alluvial plains like the one Route 3 goes through as seen between Sealston and Port Conway road. Today’s natural soil surface ranges from sandy loam to clay loam with clay loam or clay subsoil textures. Where textures are loamy or clayey, the area was in a quiet bay-like setting when the deposits were put down. Where I live (Hopyard Farm), the surface is lower and more level than north across Highway 3 and east across Port Conway Road where the terrain is hillier. Those hills are remnants of what’s called the upper coastal plain. When the Rappahannock Riverbed was higher and flowed faster, it meandered across the coastal plain and carved out what is now a river terrace. We have upper, middle and lower coastal plain surfaces in King George and Westmorland Counties besides the wetland areas. Think of these surfaces like stair steps that formed at different stands of the sea. The upper plain has an elevation of about 50 to 100 feet, middle: 20 to 50 feet, and lower: 0 to 20 feet. These counties are mainly on the upper coastal plain.
The soil survey of the counties show that most of the soils are of the Altavista, Dogue, and Wickham soil series on the riverine terraces and Galeston, Sassafras, Aura, Bladen and Caroline soil series on the coastal plain surfaces. One can find provide data and interpretations of our soils by searching “Web Soil Survey” for your address or county. You can also contact the local Resource Soil Scientist through your local USDA service center. Soil textures in the surface layer range from fine sandy loam unless eroded. Subsoil texture range from fine sandy loam to clay. Clay content increases with depth.
For farmers and gardeners, there’s good news and bad news. The good news is where uneroded, loamy surface textures are relatively easy the cultivate and many landscapes are level to gently sloping. On the other hand, many textures are moderately to highly erosive and are subject to erosion where sloping. Farming and home construction equipment can compact the soil which reduces the infiltration rate, increasing runoff and erosion, reducing soil water replenishment, and limiting root growth. These soils are not naturally fertile and can hold only limited amounts of nutrients. They have gone though a long development in a warm, humid environment that has resulted in low charge clays and leached nutrients also resulting in high acidity. The native and introduced forest species have slowly captured nutrients from rain and the air and recycled them through their canopy, then litter fall, releasing to the root zone and root uptake to repeat the cycle. Once the forest is cut, grasses and brush can retrieve some of these nutrients but bare soil between crops can allow leaching and loss as well as organic matter volatilization. Organic matter and a healthy microbe population is important to soil health. I highly recommend adopting soil health management practices whether gardening or farming. There are several good web sites to read up on soil health.
For lawns of newer houses, we don’t have the option of breaking up the compaction which existed before the sod was placed, but there are management practices we can use to improve the soil structure. One is to add organic matter to our soil. One of the best ways to do this is through frequent, light applications of compost. Leaving mower cuttings on the lawn is the most practical way to add organic matter.
Grass and other lawn species mostly have fine fibrous roots. They do little to break up the compaction, but, by leaving cuttings, the organic matter in the soil will eventually increase. Organic matter helps our soil in two ways, first it helps to break up the compaction of the soil and increase its permeability, and secondly organic matter is needed to support an adequate microbe population in the soil. Microbes are needed to convert most nutrients into useable forms for plants.
Another way to better manage our lawns is to have a lawn that is multi species. In my yard, I’ve counted up to 7 species of plants. Two examples in the mix are clover and spurge. Clover, which many of you know is a legume, fixes nitrogen (N2) out of the air and associated bacteria convert the N2 to ammonium (NH4+). This is then converted to nitrate (NO3+) by other microbes which greatly helps the lawn. Spurge that has long, fine woody roots that penetrate the dense clay layer below the sod. As those roots die and decompose, they leave behind root channels that allow water to infiltrate into lower depths of the soil. Other plant roots can also use these channels and are therefore able to extract the stored water. That little wiry rooted plant not only helps to reduce runoff but increases drought tolerance of the lawn. We all know how costly those water bills can get in peak summer.
It takes years to improve the soil. But by making small changes in our practices, such as encouraging a multi species lawn, we can find that less fertilizer is required and even herbicides are generally no longer needed (I dig out the Dandelions). In addition to this, your lawn stays green longer. I know we are trained to want to have that beautiful “golf course-looking” lawn. If you must have a mono species lawn at least think about watching the weather before any fertilizer and herbicide is applied in order to minimize storm runoff. The oysters, crabs and the rest of the ecosystem in our nearby marsh, our beautiful Rappahannock River, and the Chesapeake will thank you.
A final practice to improve the lawn is the addition of lime to raise the pH of our acid soil. Most of our plants are acid tolerant, but they still may benefit from a little calcium and magnesium. The addition of about 60 lbs. for the average yard should be all that is needed. You should use dolomitic lime which contains magnesium in addition to calcium.
In order to better understand the characteristics of your soil it is a good idea to get it tested by the Virginia Cooperative Extension. They will provide sample boxes and instructions for sampling your soil. There’s an office here in King George, 10087 Kings Hwy, 540-775-3062. Nine nutrients and pH are analyzed, and interpretations are given for fertilizer and lime applications. All of this for $10.
So, if you renew the contract for your lawn service, maybe you can be prepared with some questions for them regarding what they are doing to improve the structure and quality of your soil. Do they know what the specific nutrient needs are for your lawn (You can be prepared with your soil test results as noted above)? Are they applying the nutrients you really need in the correct amounts? Are they adapting their fertilizer and pesticide applications with the weather in mind to reduce runoff? You may find this will save you money in the long run and create a more beautiful and sustainable lawn.
I hope that you will fin this information useful and interesting.
Chris Smith, Ph.D, Senior Soil Scientist, USDA, Natural Resources Conservation Service, retired