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Native Plant Synoptic Garden
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proposed forested ridge area. According to Steven Van Hoven’s thesis, the soil series of the
Washington Park Arboretum is that of an Alderwood series. An Alderwood soil series is
characterized by reasonably well-drained gravelly sandy loam soils with hardpan twenty-four to
forty inches below the surface. For the proposed synoptic garden area, a hardpan was found at a
depth of eight to twelve inches, and as seen in Figure five, the textures differ in the lower portions
of the area.
Texture
FIGURE 5:
GROUP SAMPLE DATA
SAMPLE BULK DENSITY TEXTURE GRAVIMETRIC WATER
(g/cm 3 ) CONTENT (% MOISTURE)
A 1.04 Silty Clay 34%
Loam
B 1.17 Sandy Clay 31.40%
Loam
C 1.06 Silty Clay 35.50%
Loam
From field analysis, it was determined that the soil textures of both soils within the
proposed valley bottom area were that of a silty clay loam. According to the “Soil Textural
Triangle,” a silty clay loam has roughly between 50-70% silt content, 0-20% sand, and 30-40%
clay content (Craul, 1999; Harris et la., 2004). A silty clay loam can be satisfactory for planting
depending on the structure (which can be partly determined by bulk density); structure determines
pore space, water and air movement, and water and nutrient holding capacity (Craul, 1999). A
silty clay loam is considered a fine-textured soil and can sometimes have slow drainage if
structured poorly or impacted by soil compaction (Harris et la., 2004). Aeration within this
particular soil type can also be constrained if poorly structured or compacted; soils high in silt and
clay have an increased likelihood for compaction (Harris et la., 2004). On the positive side, water
holding and nutrient storage are both fairly high in a silty clay loam because the water is held
more easily through adhesion to soil particles and cohesion amongst water molecules (Brady et
la., 2004). Also, since clay is negatively charged, soils higher in clay adsorb more positively
charged ions which include many of the essential nutrients needed by plants (Harris et la., 2004).
The soil sample from the forested ridge section was a sandy clay loam. Sandy clay loam
is similar in most ways to a silty clay loam, however some differences exist depending on the
percent of each type of particle (sand, silt, clay) in the soil and soil structure. One difference
would be sandy soils by themselves are more permeable by water and have greater pore space