Soil classification

An understanding of physical properties of soil leads to an understanding of how other processes may take place. The majority of these properties also change very slowly, therefore, a small difference can mean a lot. Soil texture is determined based on the proportions of the soil that contains sand, silt, and clay. This effects specific surface area, water relations, tillage, erosion, agrichemicals relations, and environmental considerations related to soil. In particular specific surface area plays a key role in determining the interface or contact zone between particles and their surrounding environments. Organic content is important for soil fertility, water absorption, and nutrients for the soil. Water content is important as plants need water to grow, and it is also needed to help replace water in the water table. Color helps to determine what the specific context of the soil may be. For example, a reddish soil indicates exposure to oxygen and possibly iron. Lastly, pH affects the surrounding environment of the soil, and its ability to hold minerals such as calcium.

There are basically three schools of thought when it comes to classifying soils; engineering, soil science, and OSHA. I am going to go into the most detail with the soil science approach because it is what I am most familiar with.  Unified Soil Classification System (USCS) is the most common engineering classification system used in North America. It breaks soils down into three groups; coarse-grained soils, fine-grained soils, and high organic soils. Coarse-grained soils are sand and gravels, while fine-grained soils are things like clay and silt. High organic soils are peat soils, which I discussed in my wetlands overview post. These three groups are then further broken down. The coarse-grained soil classification is broken down into sand and gravel based on the particle size. Typically this is done using a stack of sieves that sort soil by particle size. The top sieve has the largest mesh, and progressively the mesh gets smaller and smaller. The stack can be shaken by hand or placed in a shaker.

laboratory_sieves_bmk

Occupational Safety and Health Administration (OSHA) requires soils to be classified to protect those working in environments with trenches and those doing excavations.  Soil is broken down into categories based on how stable it is and how likely slopes are to slide. Type A is the most stable and type C is the least stable. These types are determined by the number of tons per square foot (tsf) the soil can support. Type A can support 1.5 tsf or greater. Type B can support between 1.5 and 0.5 tsf. Type C supports less than 0.5 tsf. While these measurements are useful for workplace safety a soil science approach gives us a more holistic picture of the soil and collected data allows us to make conclusion about the soil, not just about how stable it is. Below is some data related to soil that I took and analyzed. It is important to note that soil naturally forms layers, and we call these layers horizons. Each of these layers has its own characteristics so it is important to keep this in mind while sampling. Here I sampled all of the horizons from two soil pits.

horizons

 

The objective was to gain an understanding of these procedures as they are important from an environmental standpoint. In addition, pit locations were chosen in areas where it was hoped that there would be some difference in the properties of each pit explainable by their locations. In particular, I choose pits located on a gradual hillside with one higher up the hill than the other. In theory, erosion of soil down the hill should cause there to be a difference in the soil profile. This study took place at Alfred University in Alfred, New York. Samples were collected on October 27, 2013. Samples were analyzed on January 30, 2014- February 4, 2014. Soil pits were dug in the woods by the Stull’s Observatory. Pit one was located at N 42° 15.050’ W 077° 47.104’ at an elevation of 1,972 feet. Pit two was located at N 42° 15.048’ W 077° 47.114’ at an elevation of 1,970 feet. Coordinates and elevations was obtained using a hand-held GPS unit. Surrounding trees included hemlocks, red maples, sugar maples, and spruce. During sampling it was raining and sleeting.

Particle size distribution was determined using bouyocous hydrometer analysis and plotting results on a soil texture triangle. Organic content was determined using gravimetric methods. Water content was determined using gravimetric methods. Color was determined using the Munsell Color Chart. pH was determined using a Fisher Accumet APG3 meter. Complete procedures can be found in Thien, Stephen J., and John G. Graveel. Laboratory manual for Soil science: agricultural & environmental principles. 8th ed. Boston, MA: McGraw-Hill, 2003. Print. These procedures are fairly easy to understand and do and are a great way to determine what type of soil you have on your property.

Table 1. Particle size distribution analysis. Alfred, NY, 2014

Measurement Pit 1 O Horizon Pit 1 A horizon Pit 1 B Horizon Pit 1 C Horizon Pit 2 O Horizon Pit 2 A horizon Pit 2 B Horizon Pit 2 C horizon
Soil weight in grams 40.14 40.05 40.50 40.08 40.43 40.11 40.18 40.05
Average 40-second hydrometer value g/L  7.75 24.00 22.00 29.00 20.00 22.00 26.00 23.00
Temperature of suspension at 40-sec, C° 22 21 23 24 23 21 24 24
Temperature corrected 40-sec hydrometer value, g/L 8.00 24.75 23.25 30.50 21.25 22.75 27.50 24.50
Two-hour hydrometer value, g/L 5 16 7 12 9 11 13 12
Temperature of suspension at 2 hour reading, C° 22 24 24 24 23 24 22 22
Temperature corrected 2-hour hydrometer reading, g/L 6.00 17.50 8.50 13.50 10.25 12.50 19.00 13.00
Grams of sand 32.14 15.30 16.95 9.58 19.18 17.36 12.68 15.55
Grams of clay 6.00 17.50 8.50 13.50 10.25 12.50 14.00 13.00
Grams of silt 2.00 7.25 15.05 17.00 11.00 10.25 13.50 11.50
Percent sand 80.00 38.20 41.85 23.90 47.44 43.28 31.56 38.83
Percent clay 14.90 43.70 20.99 33.68 25.35 31.16 34.84 32.46
Percent silt 5.10 18.10 37.16 42.40 27.20 25.55 33.60 28.71
Soil texture class Sandy Loam Clay Loam Clay loam Sandy clay loam Clay Loam Clay Loam Clay loam

The majority of the soils were classified as either a type of loam or clay (table 1). Most of the soils had a high composition of sand, and varying amounts of both silt, and clay. The type of texture in pit 2 was more stable than in pit 1, as the texture of pit 1 rapidly changed.

Table 2. Horizon depths, color, and pH. Alfred, NY, 2014

Parameter Pit 1 Pit 2
O horizon pH 5.40 5.28
O horizon color 2.5 y, value 3, chroma 1 2.5 y, value 3, chroma 2
A horizon pH 6.05 4.98
A horizon color 2.5 y, value 4, chroma 1 2.5 y, value 3, chroma 3
A horizon starting depth 2 cm 2 cm
B horizon pH 5.69 4.93
B horizon color 10 yr, value 3, chroma 2 10 yr, value 3, chroma 2
B horizon starting depth 8 cm 10 cm
C horizon pH 5.80 5.13
C horizon color 10 yr, value 4, chroma 1 10 yr, value 4, chroma 1
C horizon starting depth 28 cm 32 cm

pHs of the soils ranged from 4-6 (table 2). All of the colors of the solid were very similar. Pit 2’s B and C horizon was below that of pit 1.

Table 3. Moisture content analysis for pit 1. Alfred, NY, 2014

Horizon Wet mass (in grams) Dry mass (in grams) Percent of water
O 193.30 118.19 38.85 %
A 196.17 127.15 35.18 %
B 195.47 147.52 24.53 %
C 194.48 143.07 26.43 %

The general trend of moisture content in pit 1 was to decrease as you go down in the horizons (table 3).

Table 4. Moisture content analysis for pit 2. Alfred, NY, 2014

Horizon Wet mass (in grams) Dry mass (in grams) Percent of water
O 180.65 108.81 39.76 %
A 184.13 125.33 31.93 %
B 196.09 149.60 23.71 %
C 188.03 147.92 21.33 %

The general trend of moisture content in pit 2 was to decrease as you go down in the horizons (table 4). Pit 1 had more moist content in all of its horizons than pit 2.

Table 5. Organic content analysis for pit 1. Alfred, NY, 2014

Horizon Starting mass (in grams) Final mass (in grams) Percent of carbon content
O 5.00 4.07 18.6 %
A 5.00 4.58   8.4 %
B 4.90 4.60   6.1 %
C 5.01 4.79   4.4 %

Table 6. Organic content analysis for pit 2. Alfred, NY, 2014

Horizon Starting mass (in grams) Final mass (in grams) Percent of carbon content
O 5.00 4.23 15.4 %
A 5.00 4.35 13.0 %
B 5.00 4.54   9.2 %
C 5.00 4.67   6.6 %

Carbon content was generally higher in the upper horizons in pit 1 and 3 (table 5, table 6).Pit 1 had more carbon in the O horizon but the rest of the horizons in pit 2 had more carbon content.

The color of most of the soil samples was roughly the same making it hard to determine the horizons. The uniformity of color indicates that the area was most likely disturbed at some point which makes sense since the area was located near a building built-in the last 100 years. For the most part, the horizons were similar in their depths. Pit 2’s B and C horizon were below that of pit 1. This makes sense because the land was pit 1 was located was steeper than were pit 2 was located therefore it makes sense that the horizons would be thinner.

 

Most of the soils had a high composition of sand, and varying amounts of both silt, and clay. The type of texture in pit 2 was more stable than in pit 1, as the texture of pit 1 rapidly changed. Possible the area were pit 2 was more disturbed. The general trend of moisture content in pit 2 was to decrease as you go down in the horizons (table 4). Pit 1 had more moist content in all of its horizons than pit 2. As the samples were taken in the rain this makes sense as water flows downhill and most likely had not had time to flow down into pit 1.

The general trend of carbon content in pit 2 was there was more carbon content in the higher horizons (table 6). Pit 1 had more carbon in the O horizon but the rest of the horizons in pit 2 had more carbon content. As carbon is introduced into the soil by the O horizon it is fitting that this layer had the most carbon. Pit 2 mostly had more carbon because it was on a less steep area therefore it can build up more than pit 1. There appeared to be no general trend for the pH but it was fairly low compare to neutral pH. This is explainable by the high amounts of acid precipitation in this area. Notably there were difference in the pits with regard to horizon depths, carbon content, moisture content, and soil texture.

Disclaimer: I know that soil classification technically isn’t taxonomy but it is a type of classification that involves breaking the word down into logical parts.

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