Soils and Vegetation at Soldiers Delight

Summary:  Vegetation types at Soldiers Delight occur on different types of serpentine soil.  The transition from one soil type to another is very narrow resulting in distinct sharp boundaries between vegetation types.  Grassland vegetation is on gravelly sandy loam averaging only 3 inches deep.  Oak savanna occurs on the deeper silt loam, averaging about 3 feet deep in one study area.  Oak forest is on the deepest silt loam, measuring over 5.5 feet deep in one study.  These soil types are easily distinguished by the gravelly surface and black color of sandy loam, and the reddish-brown color of the gravel-free silt loam.  Sandy loam is about two-thirds sand, while the silt loam is about two-thirds silt.  Gravelly sandy loam soil is too shallow and drought-prone to support trees.  In contrast, the deeper silt loam soils have adequate rooting space and moisture for trees.  Although very different in physical characteristics, soil chemistry is similar for these soil types.  In particular, Mg:Ca ratios are similar and pH is high for both (5.8 - 6.7).  After recently discovering that the gravelly sandy loam is a new soil series, scientists of the U. S. Department of Agriculture, National Resources Conservation Service excavated two pedons (surface to bedrock soil pits) for detailed analysis and description, along with the silt loam soil type.  In addition, they produced a block diagram of serpentine geology catena; i.e., the sequence of distinct soils which vary with slope from the same bedrock.

Discussion:  Plant communities at Soldiers Delight occur on different types of serpentine soil.  The transition from one soil type to another is very narrow resulting in sharp boundaries between vegetation types.  Grassland vegetation is on a gravelly sandy loam averaging only 3 inches deep (Tyndall 2012).  Oak savanna occurs on the deeper silt loam, averaging about 3 feet deep in one study area.  Oak forest silt loam soils are the deepest, measuring over 5.5 feet deep in one study (Rabenhorst et al. 1982).  These soil types are easily distinguished by the gravelly surface and black color of sandy loam, and the reddish-brown color of silt loam.  Sandy loam is about two-thirds sand, while the silt loam is about two-thirds silt.   

            These differences in soil texture and depth result in very different growing conditions and, therefore, different vegetation.  Tree and shrub establishment is severely limited on the gravelly sandy loam due to very shallow soil depth and high rock fragment content.  Shallow stony soils have limited rooting volume, nutrient capacity, and available water, plus extremes in maximum temperature and drought (Lutz and Chandler 1946, Poesen and Lavee 1994, Wesemael et al. 2000).  In other words, the gravelly sandy loam soil produces a hostile environment for woody plants.  In contrast, the deeper silt loam soils, have adequate rooting space and moisture for tree development.

            Although very different in physical characteristics, soil chemistry is similar for these soil types (Tyndall 2012).  In particular, Mg:Ca ratios are similar and pH is high for both (5.8 - 6.7).  (Soil pH is high because the Mg-rich bedrock results in the dominance of Mg²⁺ on the exchange complex (Rabenhorst et al. 1982).)   

            Although woody plant establishment and growth are severely inhibited on gravelly sandy loam, pre-settlement survey descriptions (Marye 1955) suggest that oak shrubs were present before settlement and, therefore, are expected to slowly reestablish where soils are not too shallow.  Exemplary descriptions of Soldiers Delight include, “… one half barrens the rest small saplings and bushes soil thin…”, “… 200 acres of sapling Land 300 acres of Bare Barrens the Rest small bushes…”; and “Soil of Both Bushy and barren Land is very thin and both Hilley, and Stoney, the soil of the Sapling Land is Middling…”.  The descriptor “barren land” may have referred to gravelly sandy loam too shallow for woody plant development.  “Bushy … land” may have been deeper gravelly sandy loam as well as patches of shallow silt loam within large units of gravelly sandy loam.  “Sapling Land” and “Middling” soil may have referred to small trees on deeper silt loam.  Definitions of these descriptors were not provided in historical publications.

            After the gravelly sandy loam soil was first analyzed (Tyndall 2012), soil scientists of the U. S. Department of Agriculture, National Resources Conservation Service, David Verdone and Aaron Friend, determined that it was a new soil series which had never been classified.  That finding led to official USDA descriptions of the two main soil types, with the excavation of two pedons (surface to bedrock soil pits).  The following is a summary of their findings (Soil Survey Staff, National Resources Conservation Service, United States Department of Agriculture 2013), beginning with a block diagram of serpentine geology catena; i.e., a sequence of distinct soils which vary with slope from the same bedrock.  The gravelly sandy loam described in Tyndall (2012) is in a still nameless soil series, referred to as "Fine-silty, magnesic, mesic Lithic Hapludoll" in the Taxonomic Class, Loamy, Mesic Lithic Hapludolls.  The silt loam in Tyndall (2012) is in the Chrome soil series, in the Taxonomic Class, Fine, Mixed, Active, Mesic Typic Hapludalfs.  The Travilah soil series (associated with intermittent streams) was not sampled in their study.      

Serpentine Geology Catena

Fine-silty, magnesic, mesic Lithic Hapludoll ["gravelly sandy loam" in Tyndall (2012)]

This unique well drained soil has yet to be established, but can be found in the serpentine geology on crests and nose slopes in the northern piedmont region. The profile is less than 25 cm to an aquitard of serpentine bedrock, high in OM throughout, extremely low Ca:Mg ratios (e.g. 0.25:1) and high in Ni and Cr. This thin profile has a very low AWC which drastically limits vegetative growth in the dry summer months. These edaphic properties create a unique grassland forb ecosystem that has drawn international attention.

Chrome ["silt loam" in Tyndall (2012)]

This well drained soil is found on broad convex interfluves and linear side slopes. It has many of the same chemical properties as the Lithic Hapludoll, but is 50 to 100 cm to serpentine bedrock. The silty clay loam textures found at 25 cm creates an aquitard, which results in a low Ksat and moderate AWC. This soil becomes very dry during summer months. The vegetative community is still very unique with slightly more tree species.

Travilah

This somewhat poorly drained soil can be found on base slopes, head slopes and concave areas within interfluves of serpentine geology. These are discharge areas for seeps, and accumulate overland flow during precipitation events. This soil has silty clay loam textures at 25 cm that act as an aquitard with a representative soil depth between 50 and 100 cm to bedrock. The profile has a moderate AWC that is sustained into the summer months by ground water discharge from higher elevations. This prolonged ground water table allows for more water demanding vegetation such as trees to persist.

SDNEA Pedon Description: NRCS sample number: S2012MD005001 TAXONOMIC CLASS: Loamy, mesic Lithic Hapludolls ["gravelly sandy loam"] Location Description:  About 620' northwest of the intersection of Deer Park road and Ward Chapel road; 950' North of Ward Chapel road and 375' West of Deer Park road along west side of BGE power lines in Soldiers Delight Natural Environment Area, in Baltimore County, Maryland; Latitude: 39 degrees 25 minutes 17.50 seconds north; Longitude: 76 degrees 50 minutes 30.00 seconds west; WGS84

Oe--0 to 2 centimeters; loose; clear smooth boundary.                                            

A1--2 to 7 centimeters; black (10YR 2/1) very gravelly loam, very dark gray (10YR 3/1), dry; 22 percent clay; moderate fine granular structure; very friable, nonsticky, nonplastic; many fine roots throughout and common medium roots throughout and many very fine roots throughout; 45 percent nonflat subangular strongly cemented 2- to 75-millimeter serpentinite fragments; neutral, pH 6.6, pH indicator solutions; clear smooth boundary.                                                   

A2--7 to 15 centimeters; black (10YR 2/1) gravelly loam, dark gray (10YR 4/1), dry; 22 percent clay; moderate fine subangular blocky structure parting to weak fine granular structure; very friable, nonsticky, nonplastic; many fine roots throughout and common medium roots throughout and many very fine roots throughout; 29 percent nonflat subangular strongly cemented 2- to 75-millimeter serpentinite fragments; neutral, pH 6.8, pH indicator solutions; clear smooth boundary.          

BC--15 to 21 centimeters; dark yellowish brown (10YR 3/6) gravelly clay loam; 28 percent clay; weak

fine subangular blocky structure; very friable, nonsticky, slightly plastic; many fine roots throughout and common medium roots throughout and many very fine roots throughout; 15 percent nonflat subangular strongly cemented 2- to 75-millimeter serpentinite fragments; neutral, pH 7.0, pH indicator solutions; abrupt smooth boundary.                                                  

R--21 to 46 centimeters; few very fine roots in cracks