The purpose of this investigation was to determine what effects different pH levels, more specifically pH 3, 5, 7 and 9 have on the rate of growth of plants.

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  1. Abstract

The purpose of this investigation was to determine what effects different pH levels, more specifically pH 3, 5, 7 and 9 have on the rate of growth of plants. It was hypothesised that if the pH is neutral, the growth of the plant will increase because the maximum availability of primary nutrients required for plant growth is greatest at a pH value between 7 and 9. Consequently if the pH is acidic then the growth of the plant will decrease because availability of primary nutrients required for plant growth will be less at a pH value between 1 and 4. An experiment was designed to determine what pH had the most positive effect on the rate of plant growth. It was found that pH 7 caused the largest rate of growth for the plants, with pH 9 following closely behind. Therefore the optimum pH for plants was in fact a pH of 7-9, however, in particular 7.  

  1.  Aim

To investigate the effect of varying levels of pH on the growth of broad bean plants, thus determining the differences in growth between broad bean plants with higher pH levels and lower pH levels.

  1. Hypothesis

If the pH is neutral, the growth of the plant will increase because the maximum availability of primary nutrients required for plant growth is greatest at a pH value between 7 and 9.

If the pH is acidic then the growth of the plant will decrease because availability of primary nutrients required for plant growth will be less at a pH value between 1 and 6.

If the pH is altered then the rate of photosynthesis will also alter. If the optimum pH for photosynthesis is not met, then the plant will not obtain sufficient food or nutrients, thus the plant will not grow sufficiently.

  1. Introduction

This investigation focuses on the effects that different pH levels may cause on the rate of growth of plants. This topic is of high relevance and interest in the ‘real word’, as plants are a source of oxygen and food which are the necessary for not only humans and animals to live but to maintain our planet Earth. Therefore by investigating what pH level increases a plants rate of growth, we can gain a greater knowledge and understanding which will thereby enable us to grow plants sufficiently thus maintaining our planet as well as adequate food and oxygen.

Growth in plants occurs chiefly at meristems where rapid  produces new cells. As these cells differentiate, they provide new  (J. Kimball, 2011). Plant growth is effected by environmental factors which can be defined as all external conditions and influences affecting the life and development of an organism. Environmental factors include: temperature, moisture supply, radiant energy, composition of the atmosphere, soil aeration and soil structure, soil reaction, biotic factors, supply of mineral nutrients and absence of growth-restricting substances (S.W. Broome, Unknown). Plant growth and distribution are limited by the environment factors. If any single environmental factor is less than ideal it will become a limiting factor in plant growth. Limiting factors are also responsible for the geography of plant distribution (University of Arizona, 1998).

The combination of two hydrogen atoms with one oxygen atom forms the most common substance on earth, water. The combination of these two atoms plays a major role in all the processes of physical life. A certain number of molecules of oxygen naturally dissociate into the hydrogen H+ ion and the hydroxide ion (-OH). The relationship between the concentrations of these two dissociated ions is expression through what is called the pH scale. The term pH stands for potential of hydrogen. The more hydrogen ions, the more acidic the solution is, thus the fewer hydrogen ions, the more alkaline the solution is. Just as a kilometre is a measure of distance, the pH unit measures the degree of acidity or basicity of a solution. pH is measured on a scale of zero to fourteen, with zero being the most acidic, fourteen being most alkaline and seven being neutral (Dr. Lemler, 2011). pH variation can have a number of effects on the growth of plants (G. Moore, 2004). The plants grow best between pH 7 and pH 9. The lower and higher pH levels, especially pH 3 and 11, dramatically reduce growth (L. A. Ventura, 2010).


Enzymes are proteins that are catalysts of chemical reactions. Catalysts are non-consumable substances that reduce the activation energy necessary for a chemical reaction to occur. Enzymes are highly specific to the reactions they catalyse. They are of vital importance for plants as chemical reactions of the cells and tissues are catalysed by enzymes. Without enzymatic action those reactions would not occur or would not happen in the required speed for the biological processes in which they participate (Biology Q&A, 2011). The activity of enzymes is strongly affected by changes in  (J. Kimball, 2011). The most favourable pH value is the point in which the enzyme is most active, also known as the optimum pH. Extremely high or low pH values generally result in complete loss of activity for most enzymes and the optimum pH value will vary greatly from one enzyme to another (T. Banas, 2011). pH is also a factor in the stability of enzymes, as with activity, for each enzyme there is also a region of pH optimal stability (T.P. Bennett, E. Frieden, 1969).


The process of converting light energy to chemical energy and storing it in the bonds of sugar occurs in plants and is commonly referred to as photosynthesis. The process of photosynthesis is where the green pigment in the plant's leaf (chlorophyll) absorbs energy from sunlight and, using this energy, water, and carbon dioxide, produces oxygen and simple sugars (E. B. Rayburn, 1993). The plant then uses these sugars to make more complex sugars and starches for storage as energy reserves, to make cellulose and hemicellulose for cell walls or with nitrogen, to make proteins. How the plant uses its energy depends on the developmental stage of the plant and on environmental conditions (Edward. B Rayburn, 1993). However the level of pH in the plant can have detrimental effects on photosynthesis. The pH levels effects the enzymes in the cells of the plant. At different pH levels the enzymes shut off and no longer assist carrying out the chemical reactions in the cell, including photosynthesis. Therefore as pH drifts in either direction away from the optimal pH, the normal rate of photosynthesis would decrease. Since plants consist of primarily water they normally have a neutral pH (7). In general both acid conditions 5 and below and basic conditions 9 and above are generally unhealthy for plants (Yahoo Answers, 2011). As stated previously, pH affects the activity of enzymes, thus affecting the enzymes involved in photosynthesis. The main enzyme involved in plant photosynthesis is RuBisCO, which stands for Ribulose-1, 5-bisphosphate, carboxylase and oxygenase. RuBisCO (Wikipedia, 2011). RuBisCO incorporates CO
2 into plants during photosynthesis and it functions optimally at a pH of 8, which is neutral (T. Banas, 2011). Raising or lowering the pH from 8 will negatively influence the rate of photosynthesis because RuBisCO will begin to work more slowly. When the pH reaches 6 on the low side and 10 on the high side, RuBisCO will cease working altogether (T. Banas, 2011).

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The overall chemical reaction involved in photosynthesis is: (J. Stein Carter, 2004)

Figure 2: Diagram Demonstrating Photosynthesis



In larger ecosystems, plants receive water via rainfall, groundwater or bodies of water. The pH of these sources can be affected by a variety of influences, such as acid rain and run-off pollution. The pH of water sources will affect the growth and health of the plants and ecosystems that rely on them. Thus, the health of an ecosystem is directly related to the quality of its water sources. Therefore it is essential that difference the pH of a water source actually ...

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