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How does PH level affect the plant growth?

How does PH level affect the plant growth?

Introduction: (Initial Observation)

It is well known that chemical properties of soil has a major effect in the plant growth. Now we can optimize the plant’s growth by adding specific fertilizers and minerals. Another property of soil that we can control and may have some affects on plants growth is the PH of soil. PH shows acidity or alkalinity level of soil. As a mater of fact we may even explore that different plants require different soil PH for their best growth. In the United States, soil pH ranges from four to ten”‘. Each species of plant has different needs, different plants may prefer different pH levels. This information promoted the investigation of the question, “What is soil pH and what effect does it have on plant growth?”. In this project we will study the effect of pH in plant growth.


In this project you will study the soil PH, not the PH of water used to irrigate the plant. Acidic water such as acid rain have different effects. I have included the information about acid rain project at the end of this project for your reference.

This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Before any experimentation, it was necessary to obtain more information on soil pH. This was accomplished with a pilgrimage to the local library and an excursion in the net. It was found that soil pH (potential of hydrogen), which measures the amounts of hydrogen and hydroxyl ions, is not important to the plant in itself but is important because it influences the supply of dissolved nutrients that plants need to absorb from the soil. The availability of different nutrients changes at different pH levels. Soil pH can also affect the growth of certain fungi and bacteria which, in turn, affect plant growth. Soil pH can be modified very easily. It can be raised by adding an alkaline solution (lime) and lowered by adding an acidic solution (acetic acid, sulfur). Following are samples of collected information.


Soil pH and Landscape Plants


Soil pH is the most commonly-used index of plant root-zone acidity or alkalinity. Soil pH is important to plants because (1) it influences the chemical form of many elements in the soil, and (2) it influences soil microbial processes. Some elements influenced by pH are essential nutrients for plants, so soil pH affects plant nutrition. Other elements are toxic when present in excessive amounts, and soil pH helps to determine how much is in solution at any one time.

While keeping in mind the importance of soil pH, it should be noted that concern about its impact in typical residential or commercial landscape situations is often exaggerated. The purpose of this publication is to help you put landscape soil pH into proper perspective and help you manage soil pH for better plant performance.


What Is the “Desirable pH Range” for My Plants?


There are plenty of charts and tables around that list the “desirable pH range” for just about any plant you might wish to grow. Sometimes the term “optimum pH range” is used. While these guides are helpful in a general sense, they present problems in many Florida situations. First, the ranges given are usually narrow. Landscape plants are more tolerant of pH than is implied in the “desirable pH range” commonly given. Second, “desirable pH ranges” are generally biased toward fine-textured mineral soils such as silt loams and clays. Because fine-textured soils have greater quantities of aluminum (Al) and micronutrients than coarse-textured soils, they have greater potential for Al toxicity and less potential for micronutrient deficiencies. There are far more coarse-textured soils (sands) in Florida than there are fine-textured ones.

See Table 1 for a listing of landscape plants which have documented pH tolerances or sensitivities. If the species you are interested in is not listed, you can probably presume that the species will do fine in Florida soils with pH in the 5.0 to 6.5 range.

Consider correcting soil pH only when it is appreciably higher or lower than the ideal for the kind of plants you are growing. You can determine this by having your soil tested by a responsible lab. If your soil pH is within 0.4 of a pH unit of the ideal range, adjusting the pH will probably not improve plant performance (Figure 1).


How Do I Raise My Soil pH?


First, have a lime requirement test run on your soil. Such a test measures your soil’s buffering capacity and tells how much lime you need to apply. Next, apply the prescribed amount of agricultural limestone. Always test before liming. Don’t just assume that lime is needed. Many soils already contain excess lime. Such soils will typically have pHs between 7.0 and 8.2.


How Do I Lower My Soil pH If It’s Too High for the Plants I Want to Grow?


When soil pH is high because of naturally- occurring lime (like limestone, marl, or sea shells), there is no practical way of permanently lowering the soil pH. There simply is too much lime present to neutralize. The same is often true near new masonry buildings where excessive waste concrete and mortar fell on the soil during construction. Under those circumstances, you should select plants which are tolerant of high pH conditions to avoid continuing plant nutritional problems.


Can’t I do Anything to Help My Acid-Loving Plants Grow in High-pH soil?


Sure, but be prepared for a never-ending, up-hill battle. Elemental sulfur added to soil will result in a lower soil pH. That’s because soil bacteria transform elemental sulfur to sulfuric acid. The acid in turn neutralizes any alkalinity with which it comes in contact. However, as soon as the sulfur is “used up”, soil pH will return to its original value. The cycle of pH dropping and then rising again to its original high level can be as short as a couple of weeks, depending on the rate and method of sulfur application. If you try to get around this cycle by putting on high rates of sulfur, or if you make the applications too frequently, you run the risk of damaging your plants.

Never apply more than 5 to 10 pounds of sulfur per 1,000 square feet per application. For specimen trees or shrubs, it is sometimes successful to acidify only a small zone of soil near the dripline of the plant. To do this, dig a small hole about a foot deep and 8 to 10 inches in diameter, mix 2 to 3 tablespoons of sulfur into the soil taken from the hole, and return the amended soil to the hole. Repeated annually, that volume of acidified soil is frequently sufficient to prevent micronutrient deficiencies commonly associated with high soil pH. Observe your plants’ performance carefully if you embark on any program of sulfur additions.

Please note that sulfate sulfur does not affect soil pH. There is much misunderstanding on this point because some sulfate compounds (e.g., ammonium sulfate, aluminum sulfate, iron sulfate) have soil- acidifying properties. However, there are many other sulfate compounds which do not acidify soil. Examples include calcium sulfate (gypsum), magnesium sulfate (Epsom salt), and potassium sulfate.

Heavy applications (e.g., 200 pounds per 100 square feet) of organic matter such as manure, composted leaves, and peat help some landscape plants overcome the adverse effects of alkaline soil pH. Since these materials decompose with time, annual or semi-annual applications are usually required.


Table 1. Landscape plants of notable soil pH preference and tolerance.

Prefer soil pH below 5.5

Tolerant of a wide range of soil pH

Prefer soil pH above 6.0
ivy, English & Algerian
lantana, weeping
oaks elm
holly, American
cherry laurel
hydrangea, pink
hydrangea, blue
red cedar
crape myrtle
  hawthorn St. Augustinegrass  

Soil pH is referred to as the “acidity” of the soil and is measured by the number of Hydrogen ions present in the soil solution.

When the soil pH is too “acid” (low pH) or “alkaline” (high pH), nutrients present in the soil become locked-up or unavailable. Correcting the pH has the same effect as applying fertilizer since it “unlocks” plant nutrients already present.

pH Description
< 5.5 Strongly acid
5.5 – 5.9 Medium acid
6.0 – 6.4 Slightly acid
6.5 – 6.9 Very slightly acid
7.0 Neutral
7.1 – 7.5 Very slightly alkaline
7.6 – 8.0 Slightly alkaline
8.1 – 8.5 Medium alkaline
> 8.5 Strongly alkaline

Most plants grow best within a pH of 6.5 to 7.2 (7 is neutral).

Question/ Purpose:

The purpose of this project is to see how does the pH level of soil affects the plant growth.

Identify Variables:

Our independent variable is soil PH. Dependent variable is the plant growth.


My hypothesis is that slightly acidic soil for example PH 5 must result the best plant growth. My hypothesis is based on my gathered information that more minerals will be water soluble in this pH and micro organisms will grow best. Micro organisms can decompose organic maters to simplest form useable by plants.

Experiment Design:

(You can modify this experiment and use other seeds or different number of test samples)
This experiment is designed to test the effect of pH on plant growth. The results of this experiment may provide useful information on growing plants. When soil pH levels at which a plant grows best are determined, plants can be grown much more effectively and efficiently.


Materials and Method:
In this experiment, sixty Kentucky Wonder bean seeds are planted in starter cups. They are arranged in six rows and ten columns. Each cup is labeled with a letter for its column and a number for its row (use PH as the row number). Each cup is filled with one forth cup of soil. A bean seed is planted in each.

Prepare 6 empty/ clean 2 liter soda bottles and fill them up with water. Keep one as a control and just test and record it’s PH. To other bottles add material that can increase or decrease PH and make bottles with PH of 5, 6, 7, 8 and 9. You can increase the PH by adding hydrated lime or ammonia. You can also decrease PH by adding acetic acid or sublimed sulfur.

Rows one through five are watered with solutions that produce soil pH 5 through pH 9 respectively. Row six is left as a control. It is watered with water only. The plants are watered with one eighth cup of solution or water every day.

Continue watering until all bottles are empty. This experiment will take two to 4 weeks to complete. Record the final results in a table like this:

Results table: Plant heights on final day.


0 cm


Then calculate the average plant height in each row (each PH) and record the results in a table like this:

Average final plant heights:

PH 5 6 7 8 9 6.11

Materials and Equipment:

Can be extracted from the experiment.

You will need an electronic pH meter or pH indicator papers to test and adjust pH.

Results of Experiment (Observation):

In addition to the completed tables from previous section, write which pH created the tallest plant? In which pH plant did not grow at all?


You will need to calculate the average of height for plants in each row.

Summary of Results:

(This is only a sample, don’t count on it!, do your own experiment)
In this experiment, the effects of soil pH on the growth and properties of Kentucky Wonder plants, a species of pole bean was investigated. Sixty Kentucky Wonder seeds were planted in sterilized starting mix. They were watered, (ten each), with solutions with pH of five, six, seven, eight, nine, and plain water for a control. It was observed that as plants were watered with solutions that produced increasingly higher pH levels, they grew taller in the same amount of time. None, however, growing as much as the control, watered with plain water to produce a soil pH level of 6. 11. It was concluded that the variety of bean plant tested, Kentucky Wonder, grows best in soil with a pH level around six.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

In our experiment we used water with certain pH and assumed that soil will not modify this PH. So we used the pH of water as the pH of soil. To be more accurate we need to make soil with certain PH before starting our experiment. We also need to monitor and readjust the soil PH during our experiment. This is a potential error and we may get different results if we use soil with adjusted PH.


Visit your local library and see some books about plants and conditions for plan growth. Use them as your references. Search the internet for keywords such “plants”, “PH”, “Growth” to find more information.

Following are some web based articles about the soil pH:


Why are acids low numbers on the pH scale if pH means potential of hydrogen and acids have more hydrogen ions than bases?


Since the concentration of Hydrogen ion (H+) is usually a very small number such as 0.00001, we usually write it as power of 10. For example 0.00001=10-5. The number -5 here is also known as the logarithm or log of 0.00001.
The definition of pH is: pH = – Log (H+) = -Log (0.00001) = – (-5) = 5

In other words the pH is 5 when the concentration of H+ is 0.00001.

In other words if the concentration of (H+) is 1/1000000, then the pH is 6 and if the concentration of (H+) is 1/100 then the pH is 2. As you see 1/100 (or 0.01) is much larger value than 1/1000000 (or 0.000001). So less pH means more hydrogen ion.

Acid Rain

Rain and wet weather don’t always mean good news for plants, especially in an area hit by acid rain. Acid rain is caused by the burning of fuels such as oil and coal. This burning releases sulfur dioxide and nitrogen oxide gases, which react with ozone in the atmosphere to form two destructive substances: sulfuric acid and nitric acid. Rain then washes these acids out of the atmosphere and down onto Earth, harming forests and lakes.

How exactly does acid rain affect plant growth? Be your own weather-person and find out! (A note of caution: Don’t try this at home! Do this experiment in school under the supervision of a science teacher.)

Material and Equipment:

    • seeds (bean seeds work well)
    • two plastic pots
    • potting soil
    • light source
    • marker
    • plastic wrap
    • distilled water
    • 2-liter plastic soda bottle
    • medicine dropper
    • nitric or sulfuric acid (ask your science teacher to help you get this)
    • pH paper (again, ask your science teacher to help you get this)
    • two spray bottles


  1. Plant the seeds in pots with moist potting soil, water them and place them in bright light.
  2. When the bean seedlings have emerged with their first pair of full-grown leaves, label one plant container “acid” and the other “control.”
  3. Use a separate piece of plastic wrap to cover each half of the soil surface in each pot. The stem should poke through between the two pieces of plastic.
  4. Pour one liter of distilled water into a clean 2-liter soda bottle.
  5. Ask your science teacher to help you add a drop of nitric acid or sulfuric acid to the distilled water. Swirl the water in the bottle to mix.
  6. Test the water pH using pH paper. If the pH is above 3, add more acid. If it is below 3, add more distilled water. Test the water pH until it is about 3. Then pour it into a spray bottle.
  7. Place the control plant into a sink and mist the leaves with a spray bottle full of pure distilled water. Let the leaves dry, then bring the control plant back to its growing location.
  8. Place the acid plant in a sink, and mist the leaves with the spray bottle filled with pH 3 solution. Let the leaves dry, then bring the acid plant back to its growing location.
  9. Observe any differences in growth and leaf color between the acid and control plant.


Experiment Results:
The plant sprayed with the pH3 solution will be badly damaged. Its leaves will turn brown or yellow.

Rain is normally somewhat acidic because carbon dioxide gas will dissolve in it to make carbonic acid. As a result, normal rainwater has a pH of 5.6. Fossil fuels like coal or gasoline change the pH, however. When these fuels are burned, they release sulfur dioxide and nitrogen oxide gases into the air. These gases react with sunlight, ozone, and water vapor to form nitric and sulfuric acids. Rain that is tainted by these acids has a pH that is much less than 5.6. When the pH is below 5.6, it is called acid rain, and this low pH can harm plants.

For a further investigation, find out if you have acid rain in your area. Place plastic containers outside to collect rainwater, then measure the pH of this water with your pH paper.