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 makes in the overall growth of plants is determined (D. Bower, D. Morgan, K. Phillips, B. Roeth, 2005).
The pH value of water is one of a number of environmental conditions that affects the quality of plant growth. The water pH value directly affects nutrient availability. Every plant has an optimum pH value and when a plant has a pH value above or below the certain value; it may result in less vigorous growth and nutrient deficiencies. Nutrients for healthy plant growth are divided into three categories: primary, secondary and micronutrients. Nitrogen (N), phosphorus (P) and potassium (K) are primary nutrients which are needed in fairly large quantities compared to the other plant nutrients. Calcium (Ca), Magnesium (Mg) and Sulfur (S) are secondary nutrients which are required by the plant in lesser quantities but are no less essential for adequate plant growth than the primary nutrients. Zinc (Zn) and Manganese (Mn) are micronutrients, which are required by the plant in very small amounts. Most secondary and micronutrient deficiencies are easily corrected by keeping the soil at the optimum pH value. The major impact that extremes in pH have on plant growth is related to the availability of plant nutrients or the soil concentration of plant-toxic minerals. In highly acid soils, aluminium and manganese can become more available and more toxic to the plant. Also at low pH values, calcium, phosphorus and magnesium are less available to the plant. At pH values of 6.5 and above, phosphorus and most of the micronutrients become less available. Therefore it is of high importance that the pH remains stable and constant of 6.5-7 for it to receive sufficient nutrients.
- Materials
- Potting Soil Mix
- Distilled Water
- 100mL Graduated Cylinder
- 18 Plastic Cups
- Aluminium Trays
- Vermiculite For Plants
- pH Buffer Solutions of 3, 5, 7, 9, 11
- Rule
- Permanent Marker
- Measuring Scales
- Beaker
- Method
- All necessary equipment from the materials section was assembled to conduct the experiment.
- Holes were poked in 18 plastic cups using a compass so that drainage could occur.
- 1 Broad-bean plant was placed in each plastic cup.
- An ice-cream container was filled with potting mix and was mixed in with a handful of vermiculite.
- The mixture was used to carefully fill each plastic cup containing a seedling.
- Each plastic cup was labelled with a pH number of 3, 5, 7 or 9 using a permanent marker. There were 4 plants of each pH level and two plants used as a constant.
- The plastic cups containing the seedlings were placed on aluminium trays.
- 5mL of the pH number labelled on each plastic cup was measured using a measuring cylinder and was poured into the soil surrounding the seedling. This was repeated twice a week.
- Plants were carefully measured using a ruler and the height of each seedling was recorded, this occurred twice a week. (The seedling was measured from the soil to the top of the stem, not to the tip of the top leaves.) The plants were weighed using scales twice a week also.
- After 14 days, the growth was calculated by subtracting the measurement at day 1 from the measurement at day 14.
- The average amount of growth at each pH level was found (e.g. add the heights of P1—pH 3, P2—pH 3, and P3—pH 3 and then divide by 3).
- Results were recorded on the table “Mean Growth pH Levels.”
- A line graph showing the relationship between pH and growth was recorded. The pH levels were put on the horizontal axis and growth in centimetres (cm) on the vertical axis.
Experimental Set Up 15th July 2011
Figure 3: Experimental Set Up Day 1
Safety
- Laboratory apron, safety glasses and gloves were worn while conducting experiment.
- Eating and drinking were banned in laboratory.
- Caution was taken when handling pH buffer solution. It was not digested or and did not come into contact with skin or eyes. If this had occurred, the affected area was to be washed straight away and the teacher was alerted immediately.
- This was outlined in the Risk Assessment (Appendices)
Variables
Independent – the pH buffer solution assigned to each plant
Dependent – the rate of growth of the plant including height, weight, leaves etc.
Controls – potting mix, type of plant, plastic cups the plants were planted in.
- Results
The following information was recorded:
- Weight
- Height
- Leaf Number
- Distance between Nodes
- Stem Height Compared to Total Height of Plant
Table 1: Day 1 Data Collection
Table 2: Day 4 Data Collection
Table 3: Day 6 Data Collection
Table 4: Day 8 Data Collection
Table 5: Day 12 Data Collection
Table 6: Day 15 Data Collection
Table 7: Day 18 Data Collection
Table 8: Day 20 Data Collection
Table 9: Day 22 Data Collection
Table 10: Averages of Measurements
Table 11: Total Growth of Plants
Figure 4: pH 3 Plants Final Day
Figure 5: pH 5 Plants Final Day
Figure 6: pH 7 Plants Final Day
Figure 7: pH 9 Plants Final Day
Graph 1: Height of Plants Day 1
Graph 2: Height of Plants Day 4
Graph 3: Height of Plants Day 6
Graph 4: Height of Plants Day 8
Graph 5: Height of Plants Day 12
Graph 6: Height of Plants Day 15
Graph 7: Height of Plants Day 18
Graph 8: Height of Plants Day 20
Graph 9: Height of Plants Day 22
Graph 10: Average Height of Plants
Graph 11: Average Weight of Plants
- Discussion
This investigation was aimed at answering what effect different pH levels have on the rate of growth of plants. The independent variable in this investigation is the pH level (3, 5, 7, 9) and the dependent variable is therefore the rate of growth of the plant which is measured by height, number of leaves, distance between nodes, weight and stem height compared to overall height.
It is clearly evident in tables 1 to 9 and graphs 1-9, that the level of pH given to each plant did have an effect on the growth rate of the plant. From the results of the experiment it was observed that the plants which were watered with a higher pH of 7 (neutral) or 9 (basic) were the pants which showed more growth. Whereas, the plants watered with a lower pH of 3 (acidic) or 5 (acidic) showed less growth. The plants that were watered with the pH 7, (Group 3) had the highest average total growth over the three week period which was 8.1cm. Group 4 watered with pH 9, was the only other group which showed any consistent growth, and in which all the plants survived. These plants had an average growth of 7.847cm, following closely behind group 3. Group 3, which was watered with 5mL of pH 7, had an average total growth of 8.1cm, only 0.264cm more than the average of Group 4 which was 7.847cm. This clearly demonstrates that plants that are watered with a higher pH neutral or basic do grow higher in a shorter time, as a result of the optimum pH range was met allowing photosynthesis to occur and the plants are able to acquire the nutrients they need to grow sufficiently. Evidently these findings supported the theory that when the plant receives the most favourable pH, the enzyme for chemical reaction that enables photosynthesis to occur is most active. The findings also support that the water pH value directly affects nutrient availability, hence why when the optimum pH is met; nutrient availability to plants is high. Thus why when plants are watered the optimum pH level, in this case 7-9, it results in maximum growth rate.
It can also be seen in tables 1-9, graphs 1-9 but in particular table 10, that the plants in group 1 and 2 which were watered with a lower pH of 3 and 5 (acidic) either died or showed little growth. By watering these plants with a lower acidic pH of either 3 or 5 the enzyme activity decreased which prevented the plants from photosynthesising as well as the availability of nutrients were significantly decreased. Thus resulting in a much lower growth rate compared to plants in group 3 and group 4. Although plant growth did not cease completely in groups 1 and 2, on average plants that were watered with a pH of 3 had a growth of3.35cm and plants watered with a pH of 4 had a growth of 4.025cm. It can be seen that the further away the pH is from the optimum pH is directly proportional to the rate of photosynthesis and availability of nutrients to the plant. Thus results indicate if the optimum pH for plant growth is met (7-9), the taller the plant and higher the growth rate.
As can be seen in tables 1-9, the leaf count and stem length for each plant was also recorded. However, as the experiment was only conducted over three weeks, the leaves and stems on the plants did not have adequate amount of time for these to grow substantially. Therefore no real noticeable amount of extra leaves or stems sprouting. Nodes formed, however, did not have sufficient time to develop into a stem or leaf. However the results that were obtained from the leaf count were still able to indicate what has already been established. Both Group 1, watered with pH 3 and group 2, watered with pH 5 had the lowest average leaf count of 1.83. Whereas group 3 watered with pH 7 had a much higher average leaf count of 3.16 than group 4 watered with pH 9 which had an average leaf count of 2.5.
In graph 1-9, there is a clear trend between the plants and their different pH levels. It is evident that the growth rate of plants is consistent throughout each graph. In graphs 1-9, pH 3 plants show a decrease in growth and then a slow increase, whereas pH 5 plants show a steady slow increase of growth. Also in graphs 1-9, pH 7 plants have a slight decrease in growth, however, they then have a large increase in growth immediately after. This is due to the plants starting at different heights, not due to pH 7 having a negative effect on plant growth. pH 9 plants also had a slow and steady increase (graphs 1-9). It can be seen in graph 10 the average growth of plants, beginning at pH 3 and ending at pH 9, the height is increasing steadily from pH 3 to pH 5 and then maximum height is reached at pH 7. The height then begins decreasing slowly after pH 7 to pH 9. This once again supports the research, in which plants grow best at a pH between 7 and 9. Due to the optimum pH for photosynthesis being 8, thus the optimum pH for photosynthesis met and enables maximum growth of the plants.
It can be seen in table 11, that total growth of all 18 plants. This was calculated by subtracting the initial height from the end height, thus giving the total growth over the three week period. It is clearly evident that pH 7 plants had the highest total growth as the four plants watered with pH 7 had the following total growth; 4.3, 3.1, 3.3, 5.2. The total growth of plants that received pH 7 by far had the most growth. pH 9 plants had the second highest total growth, followed by pH 5 and then pH 3.
In summary, from the results it is clearly evident that the broad-bean plants that were watered with a pH 7 solution had the largest rate of growth. Over the three week period in which the experiment was conducted, the average height of the pH 7 plants was 8.11cm. It is apparent from the data in tables one to nine that the pH 7 plants grew more abundantly and steadily. Graph one also clearly illustrates that the rate of growth reaches its peak at pH 7 and begins slowly declining afterwards; indicating that pH 7 was the optimum pH for these plants to grow. However the broad-bean plants that were watered with pH 9 solution also had a large growth rate following closely behind pH7. The average height of the plants watered with pH 9 was 7.84cm, only 0.27cm less than the pH 7 height. However, because pH 9 is at end of the optimum pH range for these plants to grow, the plants did not grow as high as pH 7. This is for the reason that plants grow best between pH 7 and pH 9. The broad-bean plants that were watered with pH 7 and pH 9 had a large growth rate because they were able to acquire the nutrients the plant needed. More importantly because these particular plants were watered with a pH 7 or 9 which is the optimal pH for these plants, the enzymes involved in photosynthesis was also receiving it’s optimum pH and could therefore carry out the chemical reactions in the cell, including photosynthesis. It is also evident that pH 3 and pH 5 dramatically reduced plant growth, as on average plants watered with pH 3 had a height of 6.16 and plants watered with pH 5 had an average height of 6.50 lower growth rate. This is because pH 3 and pH 5 are well below the optimum pH needed for these plants to grow adequately. Therefore the plants do not receive the nutrients they need to grow nor do the enzymes that catalyse the chemical reaction for photosynthesis to occur, thus growth is stunted. Thus hypothesis one stating 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 is supported by the results.
The experiment findings did provide the expected results; however, the accuracy of the data collected is of a low standard. To obtain more accurate results in the future, the plants should be the same height as each other at the start of the experiment or as be as close as possible. As the heights at the start of the experiment were so different (they ranged from 1.5-12.7), the results are not as accurate as they could have been, if the range of heights were not so different. Another fault which depletes the accuracy of the results of the experiment is that inaccurate measuring tools such as plastic rulers were used to measure the plants heights. The ruler used was not very accurate, the zero did not line up with the base of the plant. Also, where the plant ended was very subjective as the stem of the plants may have been lower in the soil, however, if plant was dug around to obtain a more accurate reading this would disrupt the plant thus affecting the results. A more accurate measuring system would be needed in future in order to achieve more precise results.
The data gathered from this experiment is very relevant as the results obtained show the degree of usefulness of pH for maximum plant growth. For gardeners and farmers when growing plants and produce it is important to know what level of pH will enable them to have maximum plant growth. This experiment is extremely relevant as it shows that pH 7-9 results in larger and steadier plant growth. As these results show a very substantial difference in the growth rate of plants watered with pH 3 and 5 and pH 7 and 9 in such a short period of time, this experiment could benefit gardeners and farmers immensely as they have the knowledge to maximise their plant growth. It is therefore an extremely relevant experiment and should be repeated over a longer period of time to obtain more accurate results.
After acquiring information from this investigation, it is possible for further investigations to be conducted continuing on from the above findings. Firstly, an investigation into the use of different amounts of pH 7 to determine if pH 7 could have a negative effect on plant growth if received in small or large amounts. Secondly, investigate how
- Conclusion
In conclusion, in completing this experiment it was found that the results did in fact support the formulated hypotheses. Firstly, 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. Secondly, if the pH is acidic (pH 1-6) then the growth of the plant will decrease because of availability of primary nutrients required for plant growth will be less. Lastly, if the pH is altered then the rate of photosynthesis will also alter. If the optimum pH is not met, then the plant will obtain sufficient food or nutrients, thus the plant will not grow sufficiently. Therefore the experiment was successful in supporting the research and hypotheses.