Rough Draft of Soil Sample Study

ISAT 302

February 4, 2013

Honor Pledge:___________________________________________________

Abstract

The purpose of this experiment was to determine five fundamental soil properties: percent moisture, soil pH, soil texture, bulk density, and percent porosity.  The percent moisture was found by first measuring and weighing each soil sample on the day they were retrieved.  Next the samples were placed in an oven to dry and after a week they were reweighed.  Dividing the initial weight of the sample by the weight of the dried sample with provide the percent soil moisture.  The soil pH was found by adding CaCl₂ to a small sample of soil and mixing it on a centrifuge.  Once mixed, a pH electrode is inserted into the sample, determining the pH.  The soil’s texture was found by placing a piece of sample into a set of sieves which are shaken on a mechanical shaker.  Each sieve was individually weighed and the weight of each soil fraction was tabulated as a percentage of the total soil mass.  The bulk density was found by measuring the mass of a dry soil sample and dividing it by its volume. The percent porosity was determined by finding the volume of the voids in the unpacked soil sample and dividing it by the volume of the packed down soil sample.  For the grass hill the soil moisture was 21.7%, the bulk density was 0.939 grams/cm³, the pH was 5.7, and the porosity was 50.1%.  For the forest area the soil moisture was 23.1%, the bulk density was 0.775 grams/cm³, the pH was 3.1, and the porosity was 57.7%.

Introduction

                Soil, a mixture of organisms, weathered material, vegetation decay, and gases and water, is one of the most important substances on earth because it drives and supplies nutrients to the entire flora in this planet; therefore, it is extremely important to correctly manage and maintain soil at its healthiest levels.  In this experiment, we hope to gather and analyze various soil samples from the arboretum and surrounding grass to answer the following question:  Do soil characteristics in the Arboretum differ based on the land cover, specifically are characteristics different in managed grass areas than forested areas?  We will gather a total of six samples of soil from two different areas, three in the arboretum and three in a grassy area next to the arboretum.  To do this we will use a set of instruments to mark our gathering position and to actually gather a core sample of soil to be analyzed.  Statistical sampling will be used throughout the process in order to ensure that sampling is representative and unbiased in order to get the most accurate data for the whole area of study.  There are many methods to devise an experiment using statistical sampling but we have chosen to use Stratified random sampling in which we choose three random spots for the area of study.  After data gathering, we will take the samples to the lab and apply series of tests to answer our questions.  This lab will help us acquire a better knowledge on field collection and analysis when dealing with soils.  ADD RESULTS INTO THE INTRO

Materials and Methods

Our purpose of the study is to gather soil samples from both a forested area in the arboretum and a grassy area, take them back to the lab and conduct a series of experiments, combine the results and conclude if soil characteristics in the arboretum differ based on land cover, with an emphasis on differences between managed grass areas and forested areas.

Materials used:

Part One: Gathering data from Arboretum

·        Flags

·        Tape Measures

·        GPS Units-to gather the coordinates of each point

·        Random Number Tables

·        Lab Notebook-for recording results

·        Hand Auger-to dig up soil core samples

·        Plastic Bags-to insert soil sample and record sample location, date, time, and our initials

·        Six Drying Cans to insert soil samples and conduct the soil moisture experiment

·        Digital Scale- to weigh out 100g of the selected soil sample

Part Two: Analyzing Data in Laboratory

·        Tin Cans for handling soil samples

·        Digital Scale to conduct mass calculations

·        Labeled Centrifuge Tubes for PH testing

·        0.01 M CaCl₂ solution for PH test

·        PH Electrode for PH testing

·        Sieves for separating the particle sizes in our soil samples

·        Mechanical Shaker to let soil fall into right sieves

·        100-mL graduated cylinder for Bulk Density Testing

·        Munsell Soil Charts

Methods Part One: Data and Soil gathering

1.      We first discussed our sampling design in order to get samples of soil that are representative and unbiased.  We headed out to the arboretum and used stratified random sampling which is random sampling within pre-established groups.  Stratified random sampling is used when there is a known environmental gradient or condition that may impact the parameter being measured.  In this case, the environment is divided into “strata” or groups based on the known gradient or condition, and samples are randomly collected within each stratum (Figure 3).  Using the earlier example of sampling critters on a beach, a stratified random sampling approach could be used by dividing the beach into three strata (below low tide mark, between low tide and high tide marks, above high tide mark).  Within each of these groups, samples would be selected randomly.  The advantage of stratified random sampling is that it ensures that each group in the population is evenly represented.

2.      We selected three spots on the woods on Stratum 2 displayed on the attached map and recorded the information on our lab notebooks, we also drew a map of the area that indicates signigicant terrestrial features such as ditches, hills, structures, drainage ways, etc.

3.      We then laid out our sampling strategy in the designated sampling areas and determined the locations where the 6 samples will be collected.  Flags, tape measures, GPS units, and random number tables where available for our use in establishing and implementing our sampling design.     

4.      We headed out to our first location, but could not get GPS data due to poor reception, we did locate on a map where we dug our six samples.  We used a hand auger to collects a sample of about six inches of soil, placed it in a bag and marked the bag with the sample location, date, time, and your initials.  Mixed the soil thoroughly by inverting the bag several times, crushing large soil clumps as necessary.  We also gathered visual observations of each location and made sure that the hole was filled up with other soil.

5.      We repeated the same procedure for all six samples, returned all equipment and headed out to the lab to prepare a sub-sample for moisture content test.

6.       To do this, we marked a clean and empty drying can with your sample location, section number, and initials.  Recorded the mass of the empty can and lid.  Transfered 50-100 grams of soil to the can and recorded the mass of the can, lid, and soil.  Placed it in a 100 ^oC oven to dry until the following week.  Leaved the rest of your sample, in its sealed plastic bag, in the designated container.

Methods Part II: Lab Soil Analysis

1.      Removed the soil samples you collected in the previous lab session from the oven, and weighed the can, lid, and sample together.

2.      Using this weight and data recorded in the previous lab session, we calculated the soil moisture for our samples:

Soil moisture (%) =                    

Soil pH Procedure

1.       We Labeled a centrifuge tube, using the appropriate label to identify the sample.  Weighed 10 grams of field-moist soil (from plastic bag) into the centrifuge tube, and prepare one sample from each sampling location.

2.       Added 0.01 M CaCl₂ solution to each centrifuge tube, up to the 45 mL mark.  (The calcium ions in this dilute solution neutralize the negative surface charges on clay particles.) Stirred the soil/CaCl₂ mixture using a spatula for approximately 15 seconds or until thoroughly mixed, and then secured the cap.  Centrifuge the sample at 1700 rpm for 5 minutes.

3.       We then proceeded to dip the pH electrode into the supernatant while it was still in the centrifuge tube. Stirred the pH electrode gently and recorded the pH of the solution when either the reading had equilibrated, or after 15 seconds.  After determining the soil pH, rinsed the electrodes thoroughly using distilled water, rinsed out the centrifuge tubes, and placed the tubes in the designated container.

Soil Texture Procedure

We obtained a set of sieves for separating the particle sizes in your soil.  The set of soil sieves provided were stacked in order (from finest to coarsest screen size) on top of the closed bottom pan. [We will be using the #5, #10, #60, and #230 mesh sieves, along with a bottom pan.]  Placed 100 grams of the air-dried soil in the uppermost sieve and cover it with the lid.  Placed the sieves on a mechanical shaker and allow them to shake at intensity setting #3 for about 3 minutes.  Removed the sieves from the shaker and observed the soil fractions from each sieve. Determined whether the particles are solid or aggregated. Refered to Table 1 for some typical characteristics of each size fraction. Described the appearance of each particle size fraction in your notebook. 

Weighed each sieve and pan with the soil in it. Disposed of the soil into the appropriate soil waste container by wrapping and brushing each sieve and pan, and then weighed the sieves and pan to obtain their tare weights. Tabulated the weight of each soil fraction and expressed it as a percentage of the total soil mass. 

Bulk Density Procedure  

The bulk density of a soil (D_b) is defined as the mass per unit volume of dry soil.  The unit volume includes both the solids and the pores, the spaces between soil particles.

                        D_b = M_s /V_T                 where   M_s = mass of dry soil (grams)

V_T = total volume of soil (cm³)

Normally, the bulk density of soil ranges from 1.2 – 1.80 g/cm³.  Higher values of bulk density up to 2.0 g/cm³ are possible, but these soils have been compacted by heavy machinery and essentially have no pore spaces between the soil particles.  These are the steps we took:

1.      Determined the weight of a CLEAN, DRY 100-mL graduated cylinder.

2.      Filled the 100-mL graduated cylinder to the 40-mL mark with unpacked soil.

3.      Packed this soil by tapping the cylinder on the counter 100 times, or until settling had ceased, to simulate the degree of compaction in the field.

4.      Weighed the cylinder with the packed soil and determine the weight of the soil (M_s).

5.      Recorded the volume of the packed soil (V_T).

6.      Saved this packed soil in the cylinder for the next part of this experiment.

7.      Calculated the bulk density of the soil.

Percent Porosity Procedure

8.      Filled a second graduated cylinder with tap water to the 50-ml mark (V_w).

9.      Slowly poured the soil from the first graduated cylinder into the 50-ml of water (in the graduated cylinder), being careful not to trap significant amounts of air in the soil.

10.  Recorded the new volume of the soil-water mixture (V_sw).

11.  Using the volume of the soil we determined in the previous section, we determined the percent porosity for the soil (Equations 1-3).

Calculation of the percent porosity for the sample:

                        V_s = V_sw – V_w                                      (Equation 1)

                        V_v = V_T – V_s                                        (Equation 2)

                        % Porosity = (V_v/V_T) x 100                 (Equation 3)

where   V_s = volume of solids

V_sw = volume of solids and water

V_w = volume of water

V_v = volume of voids (pore spaces)

Finally after all of these procedures we entered the group sample ID, sample location, color, % moisture, pH, bulk density, and % porosity for our soil samples into the spreadsheet that is on the computer in the laboratory. 

Results and Discussion

Properties of Soil

Table 1 shows the properties for each groups individual soil samples taken from the grass hill.  The properties measured were bulk density, moisture content, pH, and percent porosity.

Table 2 shows the properties for each groups individual soil samples taken from the grass hill.  The properties measured were bulk density, moisture content, pH, and percent porosity.

Comparison of grass and forest samples

Table 3 shows the comparison of grass samples, including range, mean, standard deviation (STD), and coefficient of variation (COV) for the moisture content, pH, bulk density, and porosity data.  The provided data is based off of all the collected data taken from all ISAT 302 students.

Table 4 shows the comparison of forest samples, including range, mean, standard deviation (STD), and coefficient of variation (COV) for the moisture content, pH, bulk density, and porosity data.  The provided data is based off of all the collected data taken from all ISAT 302.

The bulk density of the grass hill shown in Table 3 is greater than that of the forest area shown in Table 4.  The reason for this is most likely due to the vast root system in the forest.  The roots from the trees  break up the soil which lowers its bulk density.  In the fall trees lose their leaves and fall to the ground as organic matter.  This organic matter is broken down in the soil, but since its dead leaves it is not very dense.  Therefore since the forest soil has more organic matter the bulk density would be expected to be lower.  The amount of organic matter also relates to the percent porosity which is the amount of space in the soil.  Roots and organic matter cause spaces in soil which increases its porosity. 

The moisture content of the forest area is slightly greater than the moisture content of the grass hill.  This is surprising because we would expect trees to transpire water out of the soil therefore causing the soil moisture in the forest to be less, but it wasn’t.  This can be explained by the extreme slope of the grass hill we took our samples from.  Since the hill was so steep most of the water at the top runs down to the bottom and then is transported away by a ditch.  The moisture content can also be related to the percent porosity.  In the grass area the percent porosity was lower than in the forest area.  A soil with more space has a better carrying capacity for water which explains the higher moisture content in the forest.

The pH in the grass hill is higher than that in the forest.  This is a surprise because the amount of organic matter in the forest is much higher than that of the grass hill and the more organic matter the higher we would expect the pH to be.

Soil Fractions

A total of 68 grams of sample 6 from the grass hill and 99 grams of sample 1 from the forest were taken and separately added into the top of a stack of 5 separate sectioned sieves and shaken for 3 minutes.  After three minutes each of the sections were disassembled and the mass of particles left in each sieve were weighed.  The first section of the stack had a #5 mesh sieve which keeps coarse gravel from reaching section 2 of the stack.  The size of the particles in the #5 sieve are greater than 10 mm in diameter and the number of particles are less than 1 per gram mass.  The second section of the stack had a #10 mesh sieve which keeps fine gravel from reaching section 3 of the stack. The sizes of the particles in the #10 sieve are between 10 – 2 mm in diameter and the number of particles are 25 per gram mass.   The third section of the stack had a #60 mesh sieve which keeps very coarse sand from reaching section 4 of the stack. The sizes of the particles in the #60 sieve are between 2 – 1 mm in diameter and the number of particles are 90 per gram mass. The fourth section of the stack had a #230 mesh sieve which keeps fine sand from reaching section 5 of the stack. The sizes of the particles in the #230 sieve are between 0.25 – 0.05 mm in diameter and the number of particles are 46,000 per gram mass.  The fifth section of the stack is the bottom pan which catches any silt or clay particles that the #230 mesh sieve lets through. The sizes of the particles in the bottom pan are less than 0.05 mm in diameter and the numbers of particles are greater than 722,000 per gram mass.  

Table 5 provides data of sample 6’s soil fraction which was taken from the grass hill.  The mass of each fraction and the percentage of the total are found in the table.  Below the table provides a description for each sections fraction.

Table 6 provides data of sample 1’s soil fraction which was taken from the forest.  The mass of each fraction and the percentage of the total are found in the table.  Below the table provides a description for each sections fraction.

Table 5 shows the fractions for sample 6, taken from the grass hill.  The data shows that the majority of the particles were found in sections 1-3 of the stack of sieves.  This suggests that the majority of the soil is made up of very coarse sand, fine gravel, and coarse gravel.  Section 3 held the most particles out of all the other sieves therefore the sample is mostly very coarse sand. 

Table 6 shows the fractions for sample 1, taken from the forest.  The data shows that the majority of the particles were found in sections 1 and 3 of the stack of sieves suggesting that the sample was made up of mostly coarse gravel and sand.  We also found a fair amount of mass in the bottom pan of the stack which indicates that the sample was also partially made up of silt and clay.

Moisture vs Porosity

Figure 1 shows the graph of the percent moisture versus the percent porosity from soil samples taken from the grass hill. A trend line was added with the equation y = 0.4506x + 40.654, and R-squared value of 0.0084 which indicates that .84 percent of variation in the data was accounted for.

Figure 2 shows the graph of the percent moisture versus the percent porosity from soil samples taken from the forest area.  A trend line was added with the equation y = 0.2735x + 49.419, and R-squared value of 0.543 which indicates that 54.3 percent of variation in the data was accounted for.

According to Figure 1 there are no trends in the percent moisture versus the percent porosity in the soil samples taken from the grass hill.  Realistically we would expect to see an increasing trend between the two parameters.  We would expect soils that are highly porous to have high soil moisture contents because porous soils contain spaces in the soil where water can be stored. 

According to Figure 2 there is a slight trend in the percent moisture versus the percent porosity in the soil samples taken from the forest area that show that high porous soils samples contain more moisture than lower porous soil samples.  This trend makes sense because porous soils contain spaces in the soil where water can be stored. 

Additional Sampling

It would be wise to take additional samples for both areas of the arboretum.  An interesting study would be to section off areas in the arboretum and test several samples per section.  Then using a map of just the pre-determined section, plot each individual parameter within that section to show its variability.  After that using a map of the whole collective are, plot the each section’s averaged parameter to show the variability throughout the whole arboretum

Sources of Error

Some areas where groups took their samples from all had different results for parameters.  For example if one group took their three samples from the top of the grass hill and another group took theirs from the bottom most likely the soil moisture content of the two groups will be different which will affect the accuracy of the overall results.

Conclusion