Experiment On Porosity,Percolation And Capillarity Of The Soil

Meaning Of Soil Porosity

Soil porosity refers to the amount of pore or open space between soil particles. Pore spaces may be formed as a result of movement of roots, worms, and insects; expanding gases trapped within these spaces by groundwater; and/or the dissolution of the soil parent material. Soil texture can also affect soil porosity.

There are three main soil textures: sand, silt, and clay. Sand particles have diameters between .05 mm and 2.0 mm (visible to the naked eye) and are gritty to the touch. Silt is smooth and slippery to the touch when wet, and individual particles are between .002mm and .05 mm in size (much smaller than those of sand). Clay is less than .002 mm in size and is sticky when wet. The differences in the size and shape of sand, silt, and clay influences the way the soil particles fit together, and thus their porosity.

Soil porosity is important for many reasons. A primary reason is that soil pores contain the groundwater that many of us drink. Another important aspect of soil porosity concerns the oxygen found within these pore spaces. All plants need oxygen for respiration, so a well-aerated soil is important for growing crops. Compaction by construction equipment or our feet can decrease soil porosity and negatively impact the ability of soil to provide oxygen and water.

Materials

• 3 100ml graduated cylinders per group (or a measuring cup and two clear plastic bottles)
• Fine, playground-style sand and coarse, aquarium-style gravel
• Blank piece of paper and something to write on
• Pencil or pen
• Ruler
• Metal spoon or gardening spade

Procedure

1. Divide into small groups. On a piece of paper, make a data table like the one below for each group.
   Soil particle type                    Volume of Water used (ml)
   gravel
   sand
2. With each group taking 3 graduated cylinders, fill one cylinder with 100ml of sand, one with 100ml of gravel, and one 100ml of water.
3. Repeat step 4 with the sand and record the amount of water used in the data table.
4. Discuss the experiment: Which substance has more pore space: gravel or sand? How did you make this decision?
5. Before leaving the classroom, though, refill two of the graduated cylinders with 100ml of water. You will also need paper, pens, and pencils to record observations. Draw the data table below for each group.
   Survey area                                                       Volume of Water used (ml)

 Soil Percolation Rates and Percolation Tests

The soil percolation rate indicates how quickly water moves through soil and helps evaluate the ability of the soil to absorb and treat effluent — waste water that has received preliminary treatment in a septic tank. The percolation rate is measured in minutes per inch (mpi). Soils with slower percolation rates, through which it takes longer for water to travel, need larger drain fields to handle a given amount of waste water than those with faster percolation rates. Soils with very slow percolation rates may be unsuitable for drain fields. In Nebraska, when soil percolation rates are slower than 60 mpi, consider installing a lagoon system if the lot is at least three acres. Otherwise, an engineer must design a specialized system.

The percolation rate is determined by conducting a percolation (perc) test. The percolation test measures the amount of time it takes for water in a test hole to drop 1 inch.

How to Conduct a Soil Percolation Test

1. Dig holes – Using a 4- to 8-inch hand auger, post hole digger, or shovel, dig at least three (preferably four) holes spaced evenly where the drain field is planned. If the soil type varies considerably, dig at least four (preferably six) holes, with two to three test holes per lateral or trench. Holes should be between 4 and 12 inches in diameter, and as deep as the proposed trench. A good average depth is 24 to 30 inches. Do not conduct percolation tests on disturbed soil or frozen ground.
2. Roughen Sidewalls and Bottom – Because tools may have smoothed, compacted, or sealed the sides and bottom of the hole during digging, roughen or scratch the sides and bottom of the hole with a knife, or nails driven into a board. Remove all loose material and place about 2 inches of 1/4- to 3/4-inch clean gravel in the hole to prevent bottom scouring.
3. Presoak Soil – Carefully fill the hole with clear water to a point at least 12 inches above the gravel, being careful to avoid washing soil into the hole. Continue to add water until the soil becomes saturated. For most percolation tests, maintain a 12-inch water depth in the hole for at least four hours, and preferably overnight, before measuring the percolation rate. In clay soils, keep the hole filled at least 12 hours to allow the soil to swell. Soils with moderately slow permeability and/or containing more than 30 percent clay in the testing zone will require several days of saturation when the soil is dry in order to get an accurate reading. In sandy soils, soaking is unnecessary; if, after filling the hole twice with 12 inches of water, the water seeps completely away in less than 10 minutes, proceed with the test immediately.
4. Measure Soil Percolation Rate – Measure the percolation rate the day after the saturation process, except in sandy soils as previously discussed. Record the readings on paper and store with the onsite system records. As shown in Figure 1, the general measurement strategy is to place a board horizontally across the hole and anchor it firmly in position. Add or remove water as needed to maintain a depth of 6 inches over the gravel. One way to measure the depth is to slide a yardstick or pointed stick straight down until it just touches the water surface. Immediately record the time and depth (or draw a horizontal line on the measuring stick), using the horizontal board as a guide and reference point.
5. If water remains in the test hole after overnight saturation, add water to a depth to 6 inches over the gravel. Measure the drop in water level during an approximate 30-minute period.
6. If no water remains in the hole after overnight saturation, add clear water to a depth of 6 inches over the gravel. Measure the drop in water level at 30-minute intervals over a four-hour period, refilling the hole to a depth of 6 inches as necessary after each 30-minute period. Calculate the percolation rate based on the drop that occurs during the last 30-minute period.
7. In sandy soils or other soils where the first 6 inches of water seep away in less than 30 minutes, even after the overnight swelling period, use a 10-minute interval between measurements, refilling the hole to a depth of 6 inches as necessary after each interval. Make six test measurements at 10-minute intervals. Calculate the percolation rate based on the drop that occurs during the last 10-minute period.
8. Calculate Percolation rate – The percolation rate is the average time in minutes required for water to fall 1 inch. Record the percolation test data from each hole. Divide the number of minutes elapsed by the drop in inches, using the last 30- or 10-minute reading taken for each hole. For example, the percolation rate for a hole where the water level drops 2 inches in 30 minutes is:

30 minutes
	=	15 minutes per inch (mpi)
2 inches

Calculate field percolation rate. Determine the percolation rate for the entire field by averaging the last percolation rates of all test holes. However, if percolation test results for individual holes vary more than 20 mpi, there is considerable variation in the soil type. Under these circumstances, do not calculate an average for the entire field. Instead, design the system based on the slowest rate, or consider using a different location with less soil variation.

Example: The following percolation rates were calculated from data collected at a site:

Hole #	Last Test
1	14.9 mpi
2	20.4 mpi
3	20.9 mpi
4	18.7 mpi

To determine the percolation rate for the site, add the individual percolation rates for the last test and divide by the number of test holes. Although only three test holes are required, the person conducting the test chose to use four holes to get a better idea of soil permeability at the site.

14.9 mpi + 20.4 mpi + 20.9 mpi + 18.7 mpi

4 holes

 

	74.9 mpi
=
	4 holes

=

18.7 mpi

Determine site suitability – If the percolation rate for the site is faster than 5 mpi, the soil is unsuitable for a drain field system. Waste water would travel too quickly through the soil to be treated properly. This could result in groundwater contamination, especially if the water table is shallow. When the percolation rate is faster than five mpi, dig the trench 1 foot deeper than the proposed trench depth. Using loamy sand soil with a percolation rate of 15 to 20 mpi, install a 1-foot thick soil liner in the bottom of the trench to improve soil characteristics. Base the trench size on the soil liner’s percolation rate.

Likewise, if the percolation rate for the site is slower than 60 mpi, it is unsuitable for a traditional soil absorption system. This soil has a high clay content, resulting in slow permeability. Clay generally swells when wet, reducing permeability even more. Waste water would travel too slowly through the soil and could pond on or near the ground surface, or back up into the house. These situations could result in system failure, causing odor and the spread of disease.

Keep soil percolation test data on the premises – If a construction permit is required, the percolation test results must be submitted to NDEQ along with the permit application, alternative system plans, specifications, soil evaluation, and soil boring information. When the system is registered, keep a copy of the registration form with soil percolation data.

Summary

Soil varies from one location to another, even within short distances. Therefore, before selecting a site for an onsite waste water treatment system, measure soil permeability using a soil percolation test performed by a certified onsite professional, registered environmental health specialist, or professional engineer. The percolation rate is used to select the most appropriate system, and to determine the proper size for the system.

Capillarity of Soils

Purpose

To compare the rise of water by capillarity in sandy, clayey, and loamy soil

Additional information

Capillarity is the phenomenon by which water rises in a cylindrical column. The narrower the column the higher the capillarity; similarly, the denser the substratum present in the column, the higher the capillary effect.

This is the reason why clayey and loamy soils are better options when it comes to growing healthy plants; along with being rich in nutrients it also retains moisture and helps water reach the transport channels of plants thereby helping them grow well.

 Required materials

• Sandy soil sample
• Clayey soil sample
• Loamy soil sample
• 3 Glass tubes (open at both ends)
• Water
• Beaker to hold the glass tubes
• Glass wool to plug one end of each of the glass tubes

Estimated Experiment Time

Approximately 10 minutes to set up the apparatus and 1-2 days to carry out the observations

Step-By-Step Procedure

• 1. Plug one end of the glass tubes using glass wool.
• 2. Pack 3 long glass tubes tightly with dry sandy, clayey and loamy soil; clearly label each tube.
• 3. Fill the beaker with water.
• 4. Immerse the tubes vertically in the beaker with the plugged end towards its base.
• 5. Make note of the levels of the water as it rises in the glass tubes containing each type of soil.

Observation

Initially the water rises fastest in the sand, followed by the loamy soil, and clayey soil. However after a day or two, the water fails to rise any higher in the tube containing sand, whereas in those containing clay and loam continue to rise until it reaches the top of the tube. The level in the loam may rise and drop but usually stabilizes very close to the water level in the tube containing clayey soil.

Result

Clayey soil has the highest capillarity, followed by loamy and sandy soil.

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