Part Two: Soil Moisture and Temperature

This section introduces material common to three standard protocols and a fourth optional protocol for advanced students. The protocols are all related to soil moisture and temperature. To begin, Students will use a simple procedure to measure soil moisture. They will weigh a soil sample, dry it out, and weigh it again. The difference in weight is the moisture in the soil that was dried out. An optional protocol for advanced students involves the use of gypsum blocks and a soil moisture meter to take daily readings of soil water content. Two new protocols measure other important soil properties. The rate water flows into the soil (infiltration) is measured using two concentric cans. Soil temperature is measured using a short dial or digital probe thermometer.

The soil moisture site should be in the open, with no canopy overhead, and within 100 m of the Atmosphere Study Site or a supplemental Atmosphere Study Site with at least a rain gauge. Depending upon which sampling protocol is used (see below) you will need an area 10 m in diameter characterized by low slopes, homogeneous soil characteristics and natural soil moisture and sunlight conditions. It is useful to make soil characterization, soil temperature and infiltration measurements within the same homogeneous 10 m area.

Your Soil Moisture study site(s) should be:

Unirrigated. Since we want to investigate the soil's response to the sun and natural precipitation, it is important that your site be unirrigated.

Uniform. Soil moisture can vary significantly across short distances. The challenge is to find an area where the soil moisture is representative of your site. Look for a relatively flat site that has uniform soil properties and vegetation.

Relatively undisturbed. Sample soils at least three meters from buildings, roads, paths, playing fields or other sites where the soil may be compacted or heavily disturbed by human activity.

Safe for Digging. Check with local utility companies and site maintenance staff to ensure that you do not dig into or disturb a utility cable, buried pipe, or sprinkler irrigation system. You will not be digging below one meter.

Measure soil moisture at regular intervals, twelve times per year.. Select a period during which you would normally expect the soil at your study site to undergo significant moisture changes. Observations of soil moisture should not be made when the ground is frozen . Weekly measurements during the beginning of the dry season at your locale are appropriate. Monthly observations throughout the year or measurements every three weeks during a nine or ten month school year will provide insight into the climate of your area.

Make your observations at the same time every day and avoid early morning when dew is present. Soil moisture changes slowly so the time of observation is not critical.

Measure soil temperature once per week and on the same date and at the same location as your soil moisture measurements. If your school is not measuring soil moisture, make soil temperature measurements within 10 m of you Atmosphere Study Site using the star sampling pattern. These measurements should be made within one hour of local solar noon. Every three months, preferably during March, June, September and December, take readings of soil temperature every two hours during the daytime for two consecutive days to study the diurnal variation at your site.

Measure soil infiltration three times during the course of your soil moisture investigation, ideally around the beginning, middle and end of that observation period, and on the same day you sample soil moisture. If you measure soil moisture monthly, measure infiltration seasonally.

Materials and Tools

GLOBE Science Notebooks and pencils
Compass and 10 meter tape
25 cm ruler, meter stick
Rope with knots made every 5 m.

Collecting in a Star Pattern (6 containers, trowel)

Measurements are made in a star-shaped pattern with data for each date obtained at different locations on the star. Soil moisture samples will come from a depth between 0 to 5 cm and at a depth of 10 cm. Each time, two additional samples should be acquired for quality control purposes within 25 cm of the primary measurement. Three soil temperature measurements should be made at depths of 5 cm and 10 cm within 25 cm of the sampling point following the Soil Temperature Protocol.

Layout a simple star two meters in diameter by using a meter stick to locate four points approximately one meter north, south, east and west from a central reference marker. Locate four more points halfway between these points along an imaginary circle connecting these points. You now have eight points on your star. Four more points should be located 25 cm from the reference marker along the north - south, east - west lines. Every year, select a new reference marker within ten meters of the previous year's star and repeat this pattern. It should take less than five minutes to collect each surface soil moisture sample using a trowel.

Collecting Along a Transect (13 cans, trowel, rope with knots tied every 5 m or 50 m tape)

Students with access to an open , natural field are encouraged to make measurements along a transect. The soil samples will come from the top 5 cm of soil. Each time , thirteen samples will be acquired - ten regular samples along the transect and one triplicate sample (1 sample along the transect and 2 additional samples within 25 cm of the first) for quality control purposes.

Layout your transect along a straight line 50 meters long across an open area within 100 m of a rain gauge, if possible. Measure soil moisture every five meters along this line. Place a permanent flag or marker at the ends of your transect. Use the knotted rope or measuring tape to locate these sampling points. Orientation does not matter but please record the orientation as metadata on the Study Site Definition Data Entry Sheet. The next time you sample the transect, shift each of your data collection points 25 cm to avoid the previously disturbed area. It might take an hour to layout and sample a transect, especially if students are sharing equipment and observing other surface and soil characteristics.

Collecting at Different Depths (5 cans, auger)

Students make measurements in a star-shaped pattern, with data for each date obtained at different locations on the star. The soil samples will be collected from the same hole from the top 5 cm using a trowel and at four different depths (10, 30, 60, 90 cm) using an auger. Unlike the previous two sampling strategies that are designed strictly for open areas, this one can be done in the open or under a canopy, depending upon what you want to compare the data to (soil evaporation or tree growth). Advanced students in areas where soils are not acidic are encouraged to consider using the optional Protocol Two: Gypsum Block Soil Moisture.

Lay out a star pattern as described above to locate the sampling holes around a central reference marker. If your auger strikes an obstruction, offset by 25 cm and try again. Depending upon conditions, a hole 90 cm deep might take 30 minutes to auger and sample.