Effect of acidity, alkalinity and neutrality of the medium on the growth of plants

Biology has always been one of my favorite subject as it provides me an opportunity to explore the real life observations and aspires me in the medical field. Having a curiosity to interlink subjects and establish a coherence between them to predict, analyze and explain the facts we observe around us, I have always been more fascinated in studying the science from nature than doing it from empirical equations or theoretical discussions in a text book. In my chemistry classes, I learnt about the importance of controlling the pH of the soil for the adequate growth of certain plants. The discussion made in the class while studying this topic evoked me to explore more in this topic and study in details how the growth of plants are affected by the nature of the medium.

Is there any correlation between the nature of medium (acidic, alkaline and neutral) for the growth of pea plant (Pisum sativum) and its growth (vertical height, dry mass), determined using graphical analysis and one variable statistics?

The growth of a plant can be measured in various ways –

• Recording the time in terms of number of days from planting the seed to the emergence of the first cotyledon.
• Calculating the percentage of seeds which germinates under the specified conditions.
• Measuring the number of leaves appearing in the plant.
• Measuring the surface area of the leaves.
• Measuring the vertical height of the plant
• Measuring the dry mass of the plant.
• Measuring the ratio of the height of root and shoot.

The current investigation deals with monitoring the growth of plants in terms of measuring vertical height and dry mass of the plant.

Plants require six critical conditions to optimize the growth –

• Temperature
• Moisture
• Carbon dioxide
• Water
• Sunlight
• Nutrients

Although the nature of the soil medium used (acidic or alkaline) or the pH of the medium to be precise does not control the growth of plants directly, it interferes with the availability of nutrients in the soil and the process through which it is absorbed by the root hairs.

The acidity or alkalinity of the soil do plays a major role in the growth of the plants. Although a pH range of 6-7 is an optimum range for the plants to grow yet there are certain plants which might grow well in a more acidic (pH lower than 6) or more basic (pH more than 7) soils. The value of pH of the soils do interfere with the availability of the nutrients in the soil which are essential for the growth of the plant. If a soil is found to be too acidic, liming reagents like CaCO3, MgCO3 are used to increase the acidity of the soil. Plant grown in too acidic region or too basic region suffers from iron chlorosis with a symptoms of yellowing in the leaves and wilting.

• The major reasons to justify the choice of pea plant for the investigation are-
• These are easily available and reproducible.
• They have a short life cycle.
• They can propagate easily and have several observable traits like seed color, height, flower position, stem height and many more.
• As the flowers in pea plants are not of a regular shape and the top part of the corolla is much larger than the lower part, they can undergo self pollination.

Most plants grows better in an optimum pH range of 6-7. Pea plant favors the growth in acidic medium as reported in literature. Any medium which is too basic or too acidic is not at all favorable for the growth of the plants. Hence pH =6 (6 moldm-3 HCl) , pH =8 (6 moldm-3 KOH) and pH=7(tap water as control medium) are used in this investigation.

There is no correlation between the nature of the medium and the growth of the plants.

There is a correlation between the growth of plants and the nature of the medium used.

• Protective clothing was used.
• No chemicals were ingested or exposed to skin.
• The workstation was always kept organized and clean.
• Apparatus were handled carefully.

Investigations does not give rise to any ethical issues in context to designing and conduction.

Environment was not harmed in any way. Growth of plants is a carbon negative process and has thus contributes towards reduction of CO₂ level in the atmosphere.

Concentration of stock solution (C₁) = 11 moldm^-3

Concentration of final solution (C₂) = 6 moldm^-3

Volume of final solution to be prepared (V₂) = 500 cm³

C₁V₁ =C₂V₂

(11*V₁) = (100*6)

V₁ = \(\frac{(100X6)}{11}\) = 54.5 

A clean and dry 100 cm³ of volumetric flask was taken and 30 cm³ of distilled water was transferred to it using a graduated measuring cylinder, 54.5 cm³ of concentrated HCl (11 moldm^-3 ) was transferred to the same flask using a 10 cm3 graduated pipette. Distilled water was added till the mark, the lid of the flask was closed and shaken to make it a homogenous solution. The pH of the solution was checked using a pH meter and found to be 6.

Molar concentration = 6 moldm^-3

Volume = 100 cm³ = ( 100/1000) dm³ = 0.1 dm³

Mass of KOH = number of moles X Molar mass = (Concentration X volume ) X Molar mass = (6 X 0.1 ) X 56.10 = 33.660 g

33.660 g of solid KOH pellets was weighed on a watch glass using a spatula on a digital mass balance. The weighed solid was transferred to a clean and dry 100 cm³ volumetric flask using a funnel and distilled water was added till the mark. The lid of the flask was closed and shaken upside down to dissolve the solid.

9 seeds of pea plants were taken and soaked in 100 cm3 of normal and clean water taken in a 250 cm3 beaker. The seeds were left overnight for germination. 9 plastic cups of equal size were taken and filled up with the soil procured from the local nursery. The germinated seeds were planted within the soil in the pots. The pots were kept near the window so that they are exposed to enough amount of sunlight. The soil was watered using normal and clean water everyday twice for seven consecutive days. The investigation was started from the eigth day.

After the plant has been allowed to grow for seven days, the data recording has been started.

A string was taken and aligned from the border of the container to the tip of the stem to measure the vertical height of the plant. The string was placed against a ruler to record the vertical height of the plant. The recording was noted down in a tabular form. This action was continued on every alternate days. The 9 pots were divided into three sets – each set having three pots. The pots were labeled as A-1,A-2 and A-3 (in acidic set), B-1,B-2 and B-3 in basic set and N-1,N-2 and N-3 in neutral set. The plants in set 1 was (acidic set) was watered with 6 moldm^-3 HCl, the plants in basic set was watered with 6 moldm^-3 KOH and the plants in neutral set was watered with normal tap water. The same apparatus ( a 100 cm³ glass beaker ) was used to water the plants. The volume of water added was kept same. 20 cm³ of water was added to each of the pots everyday. The volume of water added was controlled using a graduated measuring cylinder. The whole process was continued for 21 days.

At the last day of the investigation, the soil was removed off carefully around the roots without tampering the roots in any way. The plant was uplifted from the pot. The clump of soil adhered to the roots was washed off carefully in gentle stream of tap water and the dirt from the leaves and branches were also washed off in the same way. The plant was kept overnight in the hot air oven (at 600C) to loss the moisture content of it and allow it to dry. The dried plant was weighed on a digital mass balance and the mass was noted down.

• The seed coats were cracked and growth of radicles were noted during germination of the seeds.
• With the progress of time, growth of branches and leaves were noted indicating the lateral growth of the plant.
• The plants potted in basic medium turned yellow and stopped growing after 2 weeks.

For sample- 1

Initial height = Height of the plant on Day-1 = 2.3 cm ± 0.05

Final height = Height of the plant on Day-21 = 3.5 cm ± 0.05

Difference in height = Final height – Initial height = ( 3.5 – 2.3 ) cm ± (0.05 + 0.05) = 1.2 cm ± 0.1

Mean difference of three samples-

Mean = \(\frac{(1.2+1.3+1.3)}{3}\) = 1.3 cm

The maximum achieved by the plant has been considered as the final height as the plants did not survive till 21 days.

The bar graph plotted above clearly depicts the variation in the growth of the plant in terms of vertical height in various medium. A clear difference has been observed between the growth of plants in treated and untreated soil. It is observed that the growth of plants is more in acidic medium than in neutral medium and less in basic medium it is less.

In acidic medium, the change in vertical height of the plant is 1.3 cm for 21 days while in basic medium it is 0.1 cm for 21 days. The growth in acidic medium is around 13 times more than that in basic medium. The growth in basic medium is around 9 fold less in basic medium than in neutral medium. This set of data collected shows that the growth of plants is most favored in acidic medium and least favored in basic medium while it is moderate in the neutral medium.

To express the relationship or correlation between the type of medium and the vertical height of the plants, the pH of the medium is reffered to. The pH of the acidic medium is considered to 6 as 6 moldm^-3 of HCl has been used while the pH of the basic medium is considered to be 8, as 6 moldm^-3 of KOH is used. Kindly refer to the appendix for the pH calculations. The pH of the neutral medium (tap water) is considered to be 7. Thus to express qualitatively, we may say that the growth is most favored at pH=6 and least favored in pH =8. Thus as the pH of the medium decreases from 7 (tap water, control ) to pH=6 (acidic medium), the change in vertical height increases from 0.9 cm to 1.3 cm. So, a decrease in pH by one fold increases the growth by four fold. Again as the pH changes from 7 (tap water, control) to pH =8 (basic medium) the change in vertical height decreases from 0.9 cm to 0.1 cm; it means that increase in basicity of the medium by one fold decreases the height of the plant by 9 folds.

Thus it is also conclusively shown that the growth of the plant is more affected and significantly correlated to the alkalinity or the basicity of the medium and less significantly affected by the acidity of the medium as the increase in growth of plant due to change of the medium from neutral to acidic is higher than the decrease in growth of plants due to change in medium from neutral to basic.

The above graph clearly shows the effect of nature of the medium on the dry mass of the plant. The trends or pattern noted has similarity the one in observed while comparing the vertical height of the plant. The increase of dry mass is most in acidic medium and least in the basic medium. The dry mass is acidic medium is 3.237 g while that in neutral medium (control) is 2.841 g. So the dry mass of the plant increases by (3.237-2.841) = 0.396 g while the medium is changed from neutral to acidic. The dry mass of the plant in basic medium is 2.204 g while that in neutral medium is 2.841 g. Thus the dry mass of the plant is decreased by (2.841 – 2.204) = 0.637 g when the medium is changed from neutral to basic.

In similarity to the analysis of the effect of acidity or basicity on the vertical growth of the plants, it is observed that alkalinity of the medium has a major impact on the dry mass in comparison to acidity.

Null hypotheses:

There is no correlation between nature of the medium and the increase in vertical height of the medium.

Alternate hypotheses:
There is a correlation between nature of the medium (expressed in terms of pH) and the change in vertical height of the plant.

Reason for choosing chi square test:
As the samples were chosen following a simple random sampling and the data were presented in a categorical table, the chi square test of independence is an appropriate one variable statistical approach to this case. The chi square test was conducted at a significance level of 95%, α = 0.05

Degrees of freedom (DF) = (r-1 ) * (c-1 ) = (2-1)*(3-1) = 1*2 = 2

n = (3.8 + 0.4) = 4.2

E_{i ,} j = \(\frac{n_in_j}{n}\)= \(\frac{3.8×1.4}{4.2}\) = \(\frac{5.32}{4.2}\) = 1.27

With two degrees of freedom, at α=0.05 , the critical value of chi square statistic = 0.103 As the calculated value of chi square statistic (0.055263) is less than the critical value of chi square statistic (0.103), the null hypotheses cannot be rejected and it thus concludes that we cannot confirm the correlation between the nature of medium and the vertical height of the plant.

The basic aim of the investigation was to address the following research question-

Is there any correlation between the nature of medium (acidic, alkaline and neutral) for the growth of pea plant (Pisum sativum) and its growth (vertical height, dry mass), determined using graphical analysis and one variable statistics?

• As the data collected and coherently processed depicts that there is a correlation between the nature of the medium in which the plant (Pisum sativum) grow and its growth in terms of vertical height and dry mass.
   
• The vertical height of the plant increases to maximum extent ( by 1.3 units) in acidic medium and by minimum extent ( by 0.1 unit) in basic medium. Data analysis reveals that the effect of basicity on the growth of plants is more pronounced in comparison to acidity. Increase of acidity by one fold (changing pH value from 7 in control to pH =6 in acidic medium) increases the vertical height of the plant by four times while increase of basicity of the medium by one fold (changing pH from 7 in the control to pH =8 in basic medium) decreases the vertical height of the plant by 9 times.
   
• The effect of nature of medium on the dry mass of the plant also follows the similar trend. The dry mass of the plant in acidic medium is 3.237 g while that in neutral (control) medium is 2.841 g. The dry mass of the plant in basic medium is 2.204 g and thus much less than the value observed in neutral (control) medium. Likewise the vertical height study, this trend also shows that changing the medium from neutral to basic has a major impact in the negative direction while changing medium from neutral to acidic has a relatively lesser impact in the positive direction.
   
• Both in terms of vertical height and dry mass, we may confirm that the growth of the plant is most favored in acidic medium than in basic or neutral medium.
   
• Chi square test of independence was conducted in one variable statistical approach to validate the correlation and it was found that the null hypotheses (no correlation between type of medium and growth) cannot be rejected which also does not confirm that the alternate can be accepted.
   
• The qualitative observations made are in support of the results concluded. The plants grown in basic medium did not survive for the entire tenure of the investigation. It started turning yellow and wilting was noted after a week of investigation and died completely after around 14-15 days.
   
• As a scientific justification of the results concluded it may be claimed that as the medium becomes acidic, it is easier for the roots of the plant to absorb nutrients like Mg and others from soil which enhance the growth of the plant and favors the vertical growth as well as increase of dry mass.

• The growth of plants depends on various parameters apart from the acidity or alkalinity of the medium; though most of them have been controlled yet one factor was not monitored. This is the effect of microorganisms in the soil which might interfere with the data collected and the results concluded.
• Although it is claimed that all the plants were exposed to the same amount of sunlight but that also depends on where exactly they were placed and moreover no experimental evidences has been provided to show that all the plants were exposed to same amount of sunlight.
• The ideal temperature or the optimum value for the growth of pea plant is around 400C to 500C whereas the investigation was conducted in winter season and the usual temperature ranged between 200C to maximum 300C. Thus we may say that the test was conducted with a fair comparative idea but not in the optimum condition to promote the growth of plant.
• Although chi square test has been used to predict if there is any correlation or not but as the data values recorded are less than 5, the appropriateness of chi square test is not justified.

• Sufficient amount of data has been collected and processed coherently.
• To reduce or optimize the random error and systematic errors in the investigation, data was collected in triplets in the investigation and average values were used.
• All interfering variables were controlled significantly.
•  Data collected has been processed in one variable statistics to validate the correlation.

The investigation conducted can be extended further to study how the correlation varies with other species of plants. We can also vary other conditions like temperature, nature of fertilizers used apart from nature of medium to have a detail idea about the optimized conditions for the growth of the plant.

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