SOIL MOISTURE FIELD STUDYVolumetric water content (VWC) is a dimensionless value used to express the ratio of water volume to the fullvolume of a soil sample. Mathematically it is expressed as:

θ =Vw

Vt

where θ is soil moisture, Vw is the volume of water, and Vt is the total soil volume. Total soil volume = soil volume(Vs) + water volume (Vw) + void space from air (Va).

Soil moisture plays an important role in soil science, hydrology, and agricultural studies. It is essential to plantgrowth and soil stability. The soil moisture for a given area is dependent upon many factors, including the state ofthe water table for that area, the composition of the soil profile, and the porosity of the soil. Compacted soil tends tohave less space between soil particles, which in turn decreases the space available for water to fill. This has anegative effect on soil-dwelling organisms and plant growth but is suitable for use in construction.

There are three main types of soil: sand, silt and clay. Of the three soil types, sand has the largest-sized grains, in therange of 2 mm to 0.05 mm. Silt has medium-sized grains in the range of 0.05 mm to 0.002 mm. Clay has thesmallest-sized grains, in the range of less than 0.002 mm. The size of the grains determines how tightly soil particlesfit together in a given volume of space. Smaller grains allow a tighter fit between particles. This leaves less air spaceavailable for water to fill. Organic matter and other items incorporated into soil also change the amount of spacebetween soil grains.

Figure 1. Sand, silt, and clay determine soil types

Sand, silt, and clay combine to form loam in a ratio of about 2:2:1. The proportions of each type of soil vary so thatthere are many sub-types such as sandy clay or silty clay loam. See Figure 1 for the ways different proportions ofsand, silt, and clay determine different soil types. Sand can generally absorb large volumes of water, but water drainsquickly. Clay can also absorb large volumes of water but the water drains slowly. Once saturated, clay soil remainssaturated for a long time.

NAME PERIOD DATE

PASCO 1

Figure 2. Soil horizons

Soil Identification Reference Table

Sandy (sand, sandy loam, sandysilt)

Silty (silt, silty loam, silty clay,silty clay loam)

Clay (pure clay, sandy clay, siltyclay)

Texture Gritty: Grain size is large anddiscernible.

Somewhat smooth and silky: Grainsize is not discernible.

Fine and slick: Very slippery whenwet.

StructureLoose: The more sand in the mix,the less the soil will clump, and themore unconsolidated it will be.

Cohesive: Forms into a ball, whichcould be crumbly if silty loam orstrong if silty clay.

Moldable and cohesive when wet.Hard as rock and brittle when dry.

Color Tan to light brown. Black to dark brown to gray brown. Light tan to gray to greenish brown.

Porosity High: Water does not pool. Good to slow: Water pools briefly. Poor: Water pools for a long time.

Use the Soil Identification Reference Table to help you identify soil types in the field. Grain size consistencydetermines soil texture. Clay has the finest texture, and wet clay feels pasty. Silt has a slightly gritty texture, andsand has a pebbly texture. Soil texture and structure is complex because most soil is a combination of all three soiltypes. Soil composition and water content contribute to soil structure, its ability to clump, and how easily it can becultivated. Soil type variations due to texture and structure causes variation in the rate at which soil absorbs water,known as the infiltration rate. Slow infiltration rates can be due to high clay content or an already saturated ground.Very fast infiltration rates can indicate high sand content.

Digging down into soil reveals horizontal layers (horizons) with differentcomposition, texture, structure and porosity. Porosity is most importantwhen determining how well a soil profile will drain or retain water. Poresize, continuity, and orientation all control the flow of water through thesoil horizon. As water moves down through soil layers, it draws anddissolves salts out of the soil in a process called leaching. Leached saltsand minerals accumulate in clay layers within the soil profile.

As shown in Figure 2, the types of soil horizons include:

• O – Organic matter (0 to 5 cm in depth)

◦ Not shown but if present: P – Heavily organic layer between Oand A horizons; forms under waterlogged conditions

• A – Surface soil (topsoil), 5 to 25 cm thick with accumulated organicmatter; typically silty; known as loam (mixture of sand, silt and clay)

◦ Not shown but if present: E – Mineral-leached layer of sand or siltbetween A and B horizons

• B – Subsoil containing high concentrations of minerals and clay (25 to76 cm in depth); in some areas this layer is also known as hardpan

• C – Substratum; partially weathered parent material (76 to 122 cm in depth); typically rocky and sandy

• R – Parent material (bedrock) that cannot be excavated; extends to the water table

While the A, B, and C horizons are the most common classifications, location and plant cover can add the otherlayers. Forested areas may have an obvious O horizon that supports plentiful soil biota, while boggy areas may startwith a P horizon. In many areas, construction companies that build large housing developments have removed thetopsoil layers down to the B horizon, and then replaced the topsoil in a more homogenous way. In other areas,natural processes have removed the topsoil altogether, and the clay-laden subsoil is the surface soil. All of thesefactors (soil type, texture, structure, porosity, and the composition of the horizons themselves) will affect the abilityof the soil on the surface to retain water.

SOIL MOISTURE FIELD STUDY | STUDENT HANDOUT

2 PASCO

Objectives• Evaluate soil porosity based on infiltration rate and volumetric water content (VWC).

• Relate VWC and infiltration rate to soil type.

Materials and Equipment• Mobile data collection system with on-board GPS sensor

(Parts 1 and 2)

• PC or Mac computer with Internet access (Part 3)

• Soil moisture sensor

• Infiltration ring

• Digging tool such as a spade or trowel

• Empty plastic bottles with cap and fill line (2)

• Distilled water or rainwater (~1.3 L)

• Plastic wrap, 2 ft2 piece

• Stopwatch or timer (separate from mobile device)

• Wood block, 2" × 4"

• Rubber mallet

• Stainless steel table knife

• Metric ruler

• Water-resistant gardening gloves (optional)

• Paper towels (2)

• Hand sanitizer or sanitizing wipes

SafetyFollow these important safety precautions in addition to your regular classroom procedures:

• Follow your instructor's directions regarding safe practices when working in the field.

• Check the weather forecast ahead of time and dress appropriately; bring sunscreen and drinking water. Wearsturdy close-toed shoes.

• The inside edge of the top or bottom of the infiltration ring may be sharp. Use caution to avoid cuts whendriving the ring into the ground.

Procedure

Part 1 — Surface versus depth moisture

Rinse two plastic bottles and fill each with distilled water up to the marked fill line. Seal the bottles.

Gather all materials and bring them with you to the field location.

Once you arrive at your field location, open SPARKvue and select Sensor Data. Walk to an open location awayfrom buildings and trees. Connect the soil moisture sensor to your device. Once connected, the Field SoilMoisture file will appear under Quick Start Experiments. Select the file to open it.

NOTE: If the Quick Start file is not available, move the slider for the On-board GPS Sensor to switch fromdisabled to enabled . Make sure only these measurements are checked: %VWC (Loam), %VWC(Sand), %VWC (Clay), Latitude, and Longitude; then choose the Digits template.

Follow your instructor's directions to determine a sample location and for working safely in the field.

Set your mobile device on top of the sample site.

Start collecting data. Record the latitude and longitude of your location in Table 1.

Set your mobile device aside. Remove vegetation from the sample site if necessary. Use the Soil IdentificationKey and any other available resources to classify the surface soil type. Enter your results Table 1.

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SOIL MOISTURE FIELD STUDY | STUDENT HANDOUT

PASCO 3

Figure 3. Probe buried 1 cm below surface

Figure 4. Ring in soil up to dotted line

Gently loosen the top few centimeters of soil. Push aside the first 3cm of soil to make a depression for the probe. Set the probe sidewayswith the probe body perpendicular to the soil surface as shown inFigure 3. Replace the soil to bury the probe 1 cm below the surface.Tamp around the probe with your fingers to remove air pockets andto maximize contact between the probe and soil.

Your SPARKvue data display shows measurements for loam, sand,and clay. Decide which measurement is most appropriate to usebased on the soil type identified. Monitor the soil moisture readings.When the reading stabilizes, record the result in Table 1.

Remove the probe from the soil and wipe it clean with a dry papertowel.

Dig a hole approximately 16 cm deep, 20 cm long, and 10 cm wide.When you get to a depth of 14 cm, loosen the last two centimeters ofsoil and leave it in place at the bottom of the hole. Take a small soilsample from the loosened soil and classify it. Record the result inTable 1.

Set the probe sideways with the probe body perpendicular to the soilsurface at 16 cm depth. Gently pack the loosened soil around theprobe to maximize soil contact, leaving no air pockets. Try to mimicthe natural compaction of the soil as much as possible.

Fill in the hole with the soil you dug out, matching the natural compaction as much as possible.

Monitor the soil moisture reading at 16 cm, and record the stable value for the appropriate soil type in Table 1.

Gently dig up the soil moisture probe and wipe it clean with a dry paper towel. Do not pull it by the cord. Refillthe hole and tamp the soil down.

Part 2 — Water Infiltration Rate

Use caution when working with the ring as there are sharp edges.Handle the ring from the top. Make note of the dotted line along theoutside of the infiltration ring.

Find a flat spot next to the sample site from Part 1. Remove vegetationif necessary.

Dig a circle into the soil with the infiltration ring by rotating it back andforth in place. Avoid making the circle any wider than necessary.Continue rotating the ring until it will no longer easily enter the soil, oruntil the dotted line is level with the soil. If you reach the dotted line,skip the next two steps.

Remove the ring from the soil by pulling it straight up. Use the mallet todrive the table knife about 8 cm into the soil along the ring to make itdeeper.

NOTE: If the soil contains immobile rock fragments and the knifecannot be inserted up to 8 cm, insert the knife into the soil until ithits a rock fragment and use this depth consistently around the ring,or choose a different nearby location.

Align the infiltration ring with the soil cutout. Use the piece of wood and mallet to drive the sharp end of theinfiltration ring into the soil to a depth as close to the dotted line as possible. Move the wood block around thering to apply force evenly so all sides of the ring enter the soil evenly as shown in Figure 4. Do not loosen ordisturb the soil inside the ring. If the ring begins to deform, stop and move on to the next step.

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SOIL MOISTURE FIELD STUDY | STUDENT HANDOUT

4 PASCO

Figure 5. Pour water on top of plastic

Gently compact the soil along the inside of the ring to fill gaps.

Set the plastic wrap inside the infiltration ring to completely cover thesoil and ring as shown in Figure 5.

Pour all of the water from one bottle into the infiltration ring.

Carefully and quickly remove the plastic wrap and begin timing theinfiltration of water into the soil; set the plastic wrap aside.

When the water has completely infiltrated the soil and the surface is justglistening, stop timing and record the elapsed time (in seconds) in thespace provided following Table 1.

NOTE: If the soil surface inside the ring is not flat, stop timingwhen about half the soil surface is glistening and half of the waterhas not yet infiltrated the soil.

Leaving the ring in place, repeat steps 7-10 for a second infiltration test.

Clean up and return materials according to your instructor's directions. Sanitize your hands once finished.

The measured volume of water in each bottle reaches a height of 2.5 cm inside the ring. Calculate the infiltrationrate at your site for each run in centimeters per second. Enter the results in the spaces provided.

Part 3 — Import Data into ArcGIS

From this step forward, you will need to work on a PC or Mac computer with Internet access. Your instructorwill share a spreadsheet with you. Enter your data according to your instructor's directions.

When all groups have entered their data, export or download the spreadsheet as a .CSV file.

Go to arcgis.com and select the Map tab at the top of the page.

When the map opens, choose Open in Map Viewer Classic at the top-right.

Keep the browser window open but reduce its size so the browser does not occupy the full screen.

Locate the downloaded .CSV file on your computer. Select, hold, and drag the file on top of the ArcGIS map inthe browser. Release the file on top of the map.

ArcGIS will automatically create data points for each group’s data based on information in the .CSV file. Usethe drop-down menu on the left to explore visualizations of all the data. Investigate each parameter listed (exceptfor group) and look for patterns in the data. Keep the map open while answering questions.

Data CollectionTable 1. Soil Location and Moisture Content

GPS LatitudeCoordinate (°)

GPS LongitudeCoordinate (°)

Surface SoilClassification

Surface Moisture(%VWC)

Depth SoilClassification

Depth Moisture(%VWC)

Time elapsed (s), Run 1:

Time elapsed (s), Run 2:

Infiltration rate (cm/s), Run 1:

Infiltration rate (cm/s), Run 2:

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SOIL MOISTURE FIELD STUDY | STUDENT HANDOUT

PASCO 5

Questions and AnalysisReview class data. Are there significant differences between the surface soil moisture and depth soil moisturereadings in your area?

Did soil classification play any part in differences between the soil moisture reading at the surface versus thereading at depth? Support your answer with data.

Review class data. Do surface versus depth volumetric water content (VWC) measurements and infiltration ratesappear to be predictable according to soil type? Explain your answers.

What infiltration rate patterns did you observe at your site?

In the United States, rainfall is often reported in inches (2.5 cm = 1 inch). The first infiltration rate youcalculated is called a first inch rate. The steady-state infiltration rate is reached where the infiltration rate test isrepeated until the infiltration rate neither decreases nor increases as more water is added to the soil. How mighteach of these rates be useful?

What are some ways to improve soil infiltration rate?

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SOIL MOISTURE FIELD STUDY | STUDENT HANDOUT

6 PASCO