The Dirt on Dogwood

Young field geologists quickly learn that although using a rock hammer to whack a chip off a rock outcrop and examining it through a field magnifying glass is an important investigative step, so is stepping back to take a good look at the overall landscape, horizon to horizon. In my field geology training days (so many years ago I don’t care to reveal the number) I learned to generally survey the field area with a broad 360 degree view before bending down to hammer on rocks, squirt hydrochloric acid on them to see if the liquid effervesces or scratch around for fossils. Taking in the big picture first may reveal changes in vegetation within the field area that give clues to the geology. Such changes could be that one type of tree dominates the hilltop while another dominates the hillside or a certain grass thrives on one side of a gully and scrub brush on the other. These vegetation changes, if naturally occurring, are due to some change from one micro-environment to another – usually soil composition or water availability.

We see similar micro-environments in the soils at Dogwood Ranch and how they impact the vegetation. Visiting Dogwood Ranch for a walk or drive through the property is the best way to appreciate these changes but the next best way is to look at an aerial photograph with a soil survey map overlay.

The National Resources Conservation Service (“NRCS”) division of the United States Agricultural Department (“USDA”) runs the Web Soil Survey, a free online service that will instantly generate a soil map of a designated area up to 10,000 square acres. http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm.

This website also provides valuable information about each soil type on the map, including information about irrigation, surface water sources, water table depths, chemical composition of the soil, how different building material can be effected by the soil type, sanitation issues, natural resources available, wildlife suitability and a great deal more.

In my Web Soil Survey, I designated Dogwood Ranch as the Area of Interest (“AOI”) and then generated a soil survey map.

Dogwood has five soil types; although two are the same soil composition but have different slopes so the map puts them in different categories.

Soil #                    Soil Description                                 Acres in AOI       % of AOI
  8         Bosque clay loam, frequently flooded                      5.7                  12.2%

25         Crockett fine sandy loam, 1-5% slopes                  24.0                  51.3%

26         Crockett fine sandy loam, 5-10% slopes, eroded    7.4                   15.7%

32         Frelsburg clay, 3-5 % slopes                                   8.9                  19.0%

66         Tremona loamy fine sand, 1-5% slopes                   0.8                    1.8%

Soil 8 is on what we call “Old Dogwood”, the original 10 acre tract. Although the description says “frequently flooded” to our knowledge is has never flooded but during a heavy rain, ankle deep water runs from the adjoining pasture across area 8 to the creek.

Growing in Soil 8 is a great diversity in vegetation, including large trees such as red oak, water oak, American elm, cedar elm, pecan, hackberry, mulberry, Mexican plum and, of course, flowering dogwood. Notably, there are no live oak trees in this soil and the one pine “Christmas” tree was planted by the prior owners.

Soil 32 covers all of the open pasture and a little patch on Old Dogwood. This is the Freslburg clay which is basically the left over soil from weathering of calcareous shale and marl. There are no large trees growing area 32 but we did chop down a mesquite tree in the area. The clay is classic Blackland Prairie soil good for supporting grasses and forbs but not so great for mighty live oak trees. In the section of Soil 32 on Old Dogwood, there are 4 small live oak trees. I believe they were planted by the prior owners.

Soils 25 & 26 are both Crockett fine sandy loam, but only sandy in the top seven inches. From seven to 51 inches deep, the soil is clay, creating a claypan under the sand. This means that rain water can seep through the top seven inches of the sandy loam but then reaches what is effectively impermeable clay. The upper layer of sandy loam soil fills with water so if the water cannot seep into or below the claypan fast enough during a heavy rain, the water runs-off the surface onto Old Dogwood (Soil 8) and into the creek. The differences between Soils 25 & 26 is the slope and Soil 26 is more eroded than 25; the composition is the same. Nearly all of the live oak trees, some that must be close to 100 years old, are in Soil 25 & 26.

There is just a little patch of Soil 66 on Dogwood. It is the sandiest of the soils on Dogwood but still contains some clay. There are no trees in this area, only grass and forbs. This history of this general area of the property is that it was fenced years ago from the rest of the property as hay- storage area so perhaps any trees were chopped down years ago.

Careful consideration of the soil information from the Web Soil Survey will be necessary before any significant infrastructure projects begin as constructing a driveway or building on a concrete slab.

Just like the young field geologist using the vegitation patterns to get clues about the geology of the area, we can see the effect of different soil types on Dogwood Ranch by the plants that grow in each.  Come visit to see for yourself.