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Sample Internal Assessment
Sample Internal Assessment

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Table of content
Defining the problem
Research question
Background research

What is the effect of temperature on mineral content and ph in alfisols?

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Table of content

Defining the problem

My interest in the effects of fire on ecosystems was piqued by the recent spate of devastating bush fires in Australia. In the bush, fires frequently occur as part of the ecosystem. Minerals from the burned vegetation are restored to the soil with the ash. What about the organic material in the soil itself, though? Does that burn as well, or is it only the vegetation? In wildfires, temperatures can reach over 250°C at the soil's surface and as high as 114°C in the top 2.5 cm of the ground. But at 7.5 cm, the temperature drops to 59–67°C. What impact would this have on the soil's mineral properties? I made the decision to look into how heating local soil samples within this temperature range affected the pH and a few minerals for which there were appropriate testing.

Research question

What impact does temperature have on the chemical properties of pH and the mineral properties of phosphorus, nitrogen, and potassium content of alfisol soil (70, 80, 90, 100, and 110°C)?


The increased temperature will cause microorganisms and organic matter to burn, increasing the amount of minerals in the burned soil. These microbes and organic matter control the bio-geochemicals of minerals like phosphate, nitrate, and potassium in soil for absorption by plants. The same organic matter burning will make the burned soil more acidic.

Background research

Background investigation: Alifisols are soils that have had part of their minerals washed away. Despite their frequent lack of phosphate and nitrogen in Australia, they have a reasonably high reproductive rate. These soils primarily developed beneath hardwood forests and have a clay subsurface.


Proteins, vitamins, and chlorophyll contain nitrogen. Nitrate (N03-), which comes from the mineralization of organic materials and the use of fertilisers, is absorbed by plants and is necessary for the growth of vegetative activity. Additionally, water nitrates irrigate the soil. Nitrogen single test by Hanna Technology.


In natural waterways, phosphorus is present as phosphate (P04 3-) ions. Waterways and irrigation systems from dams are to blame for the introduction of phosphate into the soil. By way of runoff pollution from industry, phosphorus may be accessible to soil. As a plant, phosphorus plays a critical role in the development of roots and buds. (Hanna Technology - Analysis of phosphate and phosphorus in water) However, eutrophication, or the abnormal and excessive growth of plants, is mostly caused by an overabundance of phosphorus.


Potassium is present in tissues responsible for the growth of plants (primary and secondary meristems) (Hanna Technology - Potassium in Agricultural examination). Potassium regulates how much water is absorbed by the roots and in the control of cellular activity. Potassium helps safeguard plants to diseases. The problem of lack of potassium is frequent in alfisol soils.


Complex carbohydrates are produced by soil-dwelling microorganisms and are utilised by plants as a source of energy. For plant absorption, plant enzymes break down big nutrient molecules like fulvic and humic acids into smaller ones. Mycorrhizae, a helpful fungus in the soil, function as a significant secondary root system and assist in delivering these synthesised materials to the plant roots. The ideal pH for these biological ingredients is 6.5 (according to Guide to Organics and Hydroponics in Gardening).

Figure 1 - Table On Variables
Figure 1 - Table On Variables

Control experiment

The control experiment, which is a part of the design, involves taking soil straight from the earth and testing it for pH, phosphate, nitrogen, and potassium without adding the independent variable of heat. By using this control, it will be possible to compare the normal soil composition with the subsequent mineral content after increased heat exposure.


Industrial Oven (110, 100, 90, 80, and 70 degrees C)
Electronic Scales +/- 0.001

Hanna Technology - HI 3895 Agriculture Test Kit (Chemicals listed in table below) Colorimetric reference cards, transfer pipette, volumetric reference card, 4 test tubes.

Alfisol Soil
Large Crucibles

500cm3 beaker


1000cm3 beaker

Protective oven mitts
Sharpee© Labelling Marker
Spotting glass

5cm3 micropipette

Pipette pump
Figure 2 - Table On Apparatus Required For Experiment With Associated Quantity


Distilled Water
POTASSIUM test reagent - Tetrasodium salt dehydrate, lithium hydroxide monohydrate, sodium tetraphenyl borate
NITROGEN test reagent - Citric acid monohydrate, barium sulphate
PHOSPHORUS test reagent - Mixture of crystalline powders


Universal Indicator

 Barium Sulphate(s)

Figure 3 - Table On Chemicals Required For Experiment With Associated Quantity

Practical Risk Assessment and Safety The Risk Assessment strategy takes into account risk evaluation and practical safety measures. Please refer to the STUDENT ACTIVITY RISK ASSESSMENT and PRAC ORDER FORM in Appendix A.


Soil sample

The soil was taken from an uncontrolled woodland. The sample was taken in an area without any nearby agriculture in order to prevent any outside influences from changing the mineral content. It was also taken from the top 20 cm of the soil, at least 20 metres away from the track that provided access to the site. There was no recent history of fires at the location.


  • Set the industrial oven to an experimental temperature of 110 degrees Celsius (or 100, 90, 80, or 70°C) in the practical setup room.
  • Transfer the calculated soil mass to each crucible and record any qualitative findings.
  • Place the crucible in the oven only when it has heated to the proper temperature.
  • Permit the earth to burn for 24 hours.
  • Take the crucible out and write down any qualitative findings (such as porosity and granularity).


For each method, five soil samples were heated. As a control, three samples were left unheated.

pH measurement

  • Sample 4g of soil, place it in the proper test tube from the agriculture test kit, and then top off with distilled water to the lower graduation mark (2.5mL).
  • Fill the test tube from the test kit with the contents of the packet marked "HI 3895-pH reagent," which includes barium sulphate and calcium chloride dehydrate.
  • Put the test tube cap back on and give it a gentle shake for 30 seconds. Use the stopwatch to measure time precisely.
  • To assure the creation of the colour complex, give the sample in the test tube 5 minutes to stand.
  • Compare the sample colour to the pH color-card. the pH of the sample. For the best measurement, make sure the amount of light coming into the lab is maximised. Hold the colour card about 2 cm from the sample for comparison against a uniform white background.

Estimating N, P, K levels

  • Add 120g of the soil sample to the beaker after zeroing the balance.
  •  Apply a 120:960 reduction of the soil-to-distilled-water ratio (225:1800).
  • To achieve optimal efficiency, give soil 24 hours to separate into its component parts, such as organic matter, clay, silt, etc. separation left undisturbed (the better the results of the mineral test, the clearer the extract gets)
  • Transfer 2.5 mL of clear general soil extract to a test tube using a pipette. (avoid moving dirt, prevent stirring up sediment, and squeeze the pipette's bulb before dipping it into the solution used to extract soil)
  • Add the contents of the test-specific reagents (potassium, phosphorus, and nitrogen; HI 3895-N, HI 3895-P).
  • Put the test tube cap back on the device and tighten it. For 30 seconds, gently shake the test tube.
  • Permit the sample to stand in the test tube for 30 seconds to allow the colour complex to develop.
  • Using a uniformly white background and consistent lighting, compare the colour visible in the sample to the reference colour card.
  • Note detailed observations and pertinent information.

Modifications to the method

To increase the amount of soil in each trial, rocks were taken out of the measured mass in crucibles. Mineral transport is not aided by rocks.


Some samples received additional Universal Indicator in order to demonstrate the pH of the soil.


In the tests, shaking the test tubes was necessary to get the reagents and soil extract to react roughly. As reagents may not have connected with soil in these shaken more softly compared to those shaken more forcefully, this may have led to inconsistent readings.

Figure 4 - Table On Tabulated Raw Data To Show The Levels Of Ph, Nitrogen, Phosphorus And Potassium In Soil After Burning At Varying Temperatures - 70, 80, 90, 100, And 110°c
Figure 4 - Table On Tabulated Raw Data To Show The Levels Of Ph, Nitrogen, Phosphorus And Potassium In Soil After Burning At Varying Temperatures - 70, 80, 90, 100, And 110°c
Explanation/ Implication
Grey soil after burning
Discolouration caused by absence of organic matter regulating soil content - suggests increase in silt and clay levels which limit the oxygen and carbon dioxide emission and absorption by soil.
Colour change was evident immediately - did not have to wait
The colour complex developed immediately in most trials - except nitrogen tests.
Sunlight changed content

During sedimentation, the 1000cm3 beaker was placed in the sun for the  90°C trial. This may have influenced the process of mineral separation in  soil, as soil absorbs sunlight.

Different amounts of sediment
Vary levels of floating dead organic matter, and clay, silt etc on bottom of beaker. This may be the result of the source of soil - increased level of organic matter at levels of soil near the surface, however deeper down, the soil may contain more clay and rocks etc.
Some organic matter transferred in trial
The method was to leave organic matter undisturbed when transferring liquid soil extract, however, the micropipette had to penetrate the surface and matter adhered to/ transferred by the pipette to the test tube.
Figure 5 - Table On Observations Noted In The Duration Of The Experimentation Process With Associated Explanation And Implication On Results If Applicable.
Figure 6 - Table On The Averaged Results Of Ph And The Overall Results Of Mineral Tests At Varying Temperatures
Figure 6 - Table On The Averaged Results Of Ph And The Overall Results Of Mineral Tests At Varying Temperatures
Figure 7 - Relative Mineral Content Of Soils Where 1 = Trace, 2 = Low, 3 = Medium And 4 = High
Figure 7 - Relative Mineral Content Of Soils Where 1 = Trace, 2 = Low, 3 = Medium And 4 = High

The graph displays the averaged findings from the experiment testing for the minerals potassium, nitrogen, and phosphorus. The graph shows the mineral content of the soil under different temperatures as well as the control (unheated). Although there doesn't seem to be a clear pattern for the other examined minerals, nitrogen and phosphorus, there does seem to be a trend towards rising potassium levels when the temperature rises above 90°C. It seems that 80°C and 90°C are the temperatures that create the maximum levels of phosphorus and nitrogen, respectively.

Figure 8 - Average Pit Of Soil Samples Error Bars =  ± Smallest Increment Of Measurement
Figure 8 - Average Pit Of Soil Samples Error Bars = ± Smallest Increment Of Measurement

This graph compares the average pH values of the soil samples cooked to various degrees with the unheated control sample. Even while there is some difference between the unheated sample and the one heated to 70°C, there appears to be a general progressive reduction in pH values for the heated samples.

Figure 9
Figure 9

The graph displays the average findings from the experiment to measure the pH of burned soil. After the soil was subjected to rising temperatures, the pH level was measured and found to have decreased (becoming progressively acidic). The findings indicate that there is a relationship between rising temperature and soil acidity, with pH decreasing as temperature rises. This correlation is suggested by the linear trend line. The coefficient of determination (R2 value), which is equal to 0.8929, supports this correlation. This score is high and suggests a negative correlation as well as a good match with the trend line.


The information gained is consistent with the hypothesis that increasing warmth causes the pH of the soil to increase. According to the study, heating alfisol soils increases their mineral content.


The increase in temperature that is directly related to an increase in mineral content is shown in Figure 6, which averages the raw data. The highest potassium levels are found at 110 °C. However, not all mineral compounds exhibit this pattern. At 90°C, nitrate compounds are most abundant in burned soil. The concentration of phosphate molecules in soil peaks at 80 °C. This implies that at particular temperatures, the number and type of minerals present in soils might change. The combustion of microbes in the soil is thought to be the cause of the overall increase in mineral content. In addition to soil absorption by plants and other creatures, microbes and organic matter store and control minerals like nitrates and phosphates. Heat causes the organic matter in the soil to burn, releasing the minerals and raising their levels. This happens when the soil is subjected to heat. This does not, however, imply that plants can utilise these increased materials. Organic chemicals and minerals for plants can be produced by naturally decomposing creatures like worms. The plants would only have a limited supply of minerals without these decomposers to keep the cycle going. Additionally, despite the fact that there are more minerals available, plants still need other aspects of soil, such as porosity, which affects water retention. Extreme heat damages soil's capacity to retain water for plants and other organisms, which is why burned areas can't support plant development until it returns naturally. Burnt soil, however, may be combined with soil that lacks minerals in order to increase the mineral content of the soil for plants. This would improve the chemical characteristics of the soil and promote plant growth. Additionally, alfisol soils in Australian locations are often naturally lacking in minerals like potassium and nitrogen.


The third graph shows how the pH changes as temperature exposure increases. A strong relationship exists between falling pH and rising temperature. The high R2 value of 0.8929 coefficient of determination lends support to this. In the soil, nitrogen is found in the form of nitrates (N03). - ). The soil will become more acidic as a result of the temperature increase since it raises the amount of nitrates in the soil. Phosphates also exhibit this (Holleman, A. - Inorganic Chemistry, 2001). The constant linear trend line demonstrates that the pH will continue to get more acidic at temperatures over 110°C until all of the organic stuff is burned.


The pH of the soil affects the solubility of minerals in the soil. At a pH of 6 to 7, the majority of minerals are easily accessible. At a pH of 6.5, phosphorus is more soluble, whereas at a pH of 5, aluminium, iron, and manganese are more solubilized. Numerous plant species are poisoned by aluminium, which is also notably high in alfisols. Therefore, the pH drop that has been observed with heating may increase some valuable minerals' initial accessibility, but a significant pH drop may also reduce soil fertility.


Despite a few minor omissions, the data collected is trustworthy. Numerous physical characteristics of the alfisol soil family, which is the predominant soil family found in most parts of Australia, are well-known. Since the test soil may have included mineral levels different from those of an alfisol soil found in Western Australia, the results cannot be extrapolated to other soil types. Additionally, it was untrustworthy to the extent that the controls' pH tests varied. These outcomes may have been influenced by the controls' water content. The water evaporated from the heated samples.


It was not noted whether the original organic matter in the soil affected the mineral composition or how many plants were present near the source. This would be a sign of the universal character of the findings for regions with comparable levels of organic matter. Furthermore, the amount of sunshine that the places where the soil was collected received were uncontrolled. The pace at which plants synthesise oxygen depends on the brightness of the sun. The photosynthetic activity of plants would be lower than that of plants that receive sunshine continuously if the soil were in a particularly shaded place. Plant biomass would decrease as a result, as would the amount of organic matter in the soil. The mineral concentration of the test soil could be lower if photosynthesis is reduced.


The experiment included multiple trials for the samples, allowing the consistency of the results across trials to indicate the reliability of the data. The reference cards' inherent ambiguity makes it possible that the amount of minerals in the soil solution could be either half a grade greater or half a grade lower than what was previously determined. Additionally, human visual analysis error is important. A measuring tool like a colorimeter may be able to analyse the concentration more objectively than the reference cards because they don't require the experimenter to observe to a standard measurement.

Evaluation of the method

The conclusions are constrained by a variety of systematic and random errors related to the data's ambiguity.


The colorimetric indicators cards are a source of systematic mistake in the data. These instruments don't use numbers; instead, they compare the colour of the solution after adding the reagent to a standard TRACE, LOW, MEDIUM, or HIGH reference on the card. The data, with the exception of pH, cannot be turned into numerical data due to this design limitation. As a result, the degree of uncertainty surrounding the data is half of the lowest increment, such as the difference between HIGH and MEDIUM. This raises a lot of ambiguity because it implies that the concentration of the mineral in the solution may vary. The pH tests ran into the same issue. To colour the soil using this method's universal indicator and compare the resulting shade to a pH reference chart,


The method contained the random error that it instructed users to "Allow soil to separate into components; organic matter, clay, silt, mineral content, etc., for 24 hours to ensure maximum separation undisturbed (the clearer the extract becomes, the better the results of the mineral test)". Due to experimentation time constraints, each test was not given a full 24 hours, which led to a yellowish extract rather of a translucent one. This implies that the findings might not have provided the best indication of the mineral concentration of the burned soil. Additionally, the sedimentation process took a maximum of 24 hours. The range of time is 30 minutes to 24 hours because the soil utilised does not indicate how long the sedimentation process will need to take. It was believed that a comprehensive result might be obtained in 24 hours.


The reagents for the mineral indicators were prepared in sachets, which were opened and introduced to the aqueous extract of the soil. Due to the difficulties in releasing the chemicals, the complete amount was not added in each instance. The solution may turn lighter as a result, demonstrating a qualitative contrast that may not accurately reflect the amount of mineral in the sample.


Another chance mistake occurred when other students briefly opened the oven, which controls the temperature of the heating, causing the temperature to drop from 110 to 97°C. To verify correct findings were generated, the trials will need to be repeated because of the variable heating process. The experiment might have been impacted by the 13°C temperature change.

Modifications to experiment

A professional version of the HI 3895 test kit was used to test the mineral content of the soil in this experiment, which increased the accuracy of the results. The HI 3896 agriculture test kit employs a similar process to the HI 3895 agriculture test kit, but uses colour compactors and gives the colour comparison numerical values. The results will have a vastly more quantifiable base as a result.


A calibration curve made using standard solutions of diluted phosphate stock solution can also be used to test the unknown concentration using an ultraviolet-visible light spectrophotometer. In comparison to the reference card, the spectrophotometer will deliver a more precise measurement. Other minerals like potassium and nitrate compounds may also be affected by this.


The manufacturer of the agriculture test kit used to determine the pH and mineral content of the soil, Hanna Instruments, also offers a product known as a phosphate checker. The device employs a silicon photocell that functions similarly to an ultraviolet-visible spectrophotometer, however this test just looks for orthophosphates using the reagents, and there is no need for calibration to establish the concentration. The equipment measures between 0.00 and 2.50ppm with a 4.0% uncertainty. In addition to phosphates, the device is made for nitrates and potassium compounds.


The pH of the soil can be measured more precisely with a pH probe in relation to the pH tests. In order to use this device, a specified amount of dirt must be extracted, and then distilled water must be added. With the aid of a funnel and filter paper, this solution can be purified. The pH probe will precisely measure the solution's pH after calibrating in electrolytes and cleaning in distilled water.


A change to the experiment could involve changing the independent variable from temperature to time. The experiment has shown that a change in temperature will affect the mineral content of soil, however a derivation of the experiment might look at the impact of burning time. This would give examples of both uncontrolled and controlled bush fires that are more ecologically valid.


Since all organic stuff or the vast majority of it burns at higher temperatures, a further variant of the experiment might look at the impact of lower temperatures, which will only completely destroy some organic materials.

UV-Vis Spectrophotometer
Accurate measure the concentration of phosphate ions in soil extracts
Phosphate checker
Works in similar effect to UV-Vis apparatus, does not require calibration curve - measures phosphate concentration directly.
HI 3896
Precise agriculture test kit compared to HI 3895 - assigns numerical values
Lower temperatures
Maintain higher content of minerals with regulation by remaining organic matter.
Duration of heating
Change the independent to time -> better reflects the ecological factor of fire and soil.
pH probe
Used to measure the pH of the solution - filter soil extract and measure pH
Control sample water content
Air dry the control soil at room temperature
Figure 10 - Table On Summary Of Modifications To Experimental Design


Beadle N.C.W (1940) Soil temperatures during forest fires and their effects on the survival of vegetation Jof Ecol Vol 28 Issue 1 pp180-192


Holleman, A.(2001), Inorganic Chemistry, San Diego, Academic Press Hanna Instruments (2008) Phosphate analysis. United States 38050


Hanna Instruments (2009) Nitrate analysis, United States 38073


Hanna Instruments (2010) Potassium analysis, United States


Hanna Instruments (2008) pH analysis, United States 38058


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