Hiuling Zoe Yu
December 10, 2011
Gardening Class
Growth response of Pinto Bean Phaseolus vulgaris to differing pH in soils
Introduction
Soil pH is one of the most important environmental factors in soil and can directly impact
the plant growth. If the soil is too acidic or alkaline, the bacteria that grow on the roots of the plants
would not be able to function properly to take up rich nutrients from the soil. For that reason, the
experienced gardeners and farmers are aware of the condition of their soil, such as pH level and
organic matters that are contained in their soil, because both would ultimately affect the quality of
the fruits and vegetables. In order to maximum the outcome from gardening, it would be beneficial
to acknowledge the suitable pH range for each plant. In this research, pinto bean Phaseolus vulgaris
will be the test subject to find out the growth response to differ soil pH ranging from 3.5 to 6.5.
Soil pH and nutrient for plants
pH is a measure that determines whether the soil is alkaline or acidic. The pH scale runs
from 0 to 14, with neutral being in the middle at 7. Less than 7 is acidic and more than 7 is alkaline.
Before pH scale was developed by a chemist whose name is S. P. L. Sorensen in 1909, the farmers
would taste their own soil to check for the pH level. Acidic soils taste “sour”, alkaline soils taste
“bitter”, and neutral soil taste “sweet.” Different vegetables and fruits prefer different pH levels
(Riddle , 2011)(Owen, 1996).
Soil pH is very important to plant growth because if the soil is too acidic or alkaline, plants
cannot take up or absorb nutrients, such as these two major ones: macronutrients and
micronutrients. Macronutrients are the ones that the plants need the most to survive. Those
nutrients are Nitrogen (N), phosphorus (P), and potassium (K). For micronutrient, the plants would
only need a small quantity of them. The micronutrients are boron (B), copper (Cu), iron (Fe),
chloride (Cl), manganese (Mn), molybdenum (Mo), and zinc (Zn). As the water drains through the
soil, the nutrients would be dissolved in the water then the plant’s roots would absorb them (Hosier
& Bradley, 1999) (Wright, Hanlon, Sui & Rice, 2009).
Over a long period of time, the soils can change its pH level depending on the mineral and
limestone deposits from the annual rainfalls. The chemical make-up of limestone is calcium
carbonate (CaCO3). When CaCO3 reacts with water, it will be broken down into calcium ions and
carbonates. Then the calcium ions would be absorbed by the soil while the carbonate would react
with hydrogen ions. This reaction will cause an increase of soil pH. To cause a decrease of soil pH,
aluminum sulfate (Al2(SO4)3) can be used to do so. When Al2(SO4)3 dissolve in water, it will be
broken down into sulfuric acid (2H2SO4) (Wright, Hanlon, Sui & Rice, 2009).
Methods
The experiment was performed with pinto beans Phaseolus vulgaris growing indoors under
several fluorescent light tubes grow light at Lesley University Biology classroom. There are total of 7
plastic pots with adjusted soil pH: 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, and 6.5. In order, the size of each pot:
Each pot has three pinto beans growing for the purpose of all three would be receiving the same
amount of water and nutrient. The soil is adjusted by adding aluminum sulfate (Al2(SO4)3) and
calcium carbonate (CaCO3). The air temperature of the room is 26⁰C  1⁰C and the relative humidity
is 42%  2%. The pot was placed under the fluorescent light tubes for total of 16 hours every day.
First, all the pinto beans would be wrapped in a moist paper towel for encouraging
sprouting. Next before the pinto beans were planted, their mass is recorded. Then to record the
numbers of days from planting the beans into the soil to having the first leaves, calls first cotyledon,
emerging to the surface of the soil. Then, the following measurements were recorded every three
days: plant height, number of leaves, surface area of the leaves, and leave coloration. To measure
the height of the plant accurately, it should be measured from the border of the pot to the top of
the stem because the soil may become compacted from watering over time. At last, the weight of
the plant will be recorded after carefully removing all the soil from the roots on the last day of the
experiment.
Data and Discussion
Gathering all the data in height (see figure 1) of all the samples from each pots except for
the samples from 3.5 and sample 5.0c were not included because 3.5 samples did not have any
growth at all during the entire time and 5.0c was damaged. The samples in 6.0 and 6.5 pots had a
rapid growth during the first six days while the beans 5.0a, 5.0b, 5.5a, 5.5b, and 5.5c began growing
almost a week later. Comparing those four sets of data, it seems 6.0 and 6.5 beans are much taller
than the other ones at the beginning. When the beans were taller at first, it does not indicate that
the plant is growing better, healthier and faster than others. They may be just wanting for more
sunlight, so taller might not be always the key feature to seek for if looking for a healthy plant
(DeWynter, 2011). However, in this case, the taller the plant would mean the more growth. If there
was tremendous growth, it would mean the plants is rapidly using nutrients in order to grow in
height. As you can see, the samples that were growing in the soil pH lower than 6.0 tend to have a
slower growth in height. Cause of the deferred early stage development, it affected the rate that
the beans were growing in the next few weeks. However, the growing rate stayed constant. The
overall height in pot 4.0 was below seven centimeters while the maximum height was 19
centimeters. The difference is significant because both 6.5 and 4.5 pots had the same
environmental control and setup, so for beans in 4.0 being really short is a problem. If the soil was
too acidic which considers between pH 4.0 – 5.0, it had high concentrations of soluble aluminum,
iron, and manganese. Those elements could be harmful to certain plants (Bickelhaupt, 2011)(Hipps,
Davies, Dodds & Buckley, 2004) (Wright, Hanlon, Sui & Rice, 2009). In this case, it was also harmful
for pinto bean, too. For an extreme example, the samples from 3.5 pot did not have any signs of
growth at all during the whole time.
In figure 2, pot 4.5, 4.0, and 3.5 are not included due to lack of leaves to measure the
surface area. Even though 6.5 had a rapid growth in height, the average leaf surface area was larger
than 6.0’s; however, about three weeks later, the leaves started to shrink. Even though some leaves
were shrinking, there were new leaves sprouting out. That was an interesting fact to see because
the fact that shrinking leaves usually indicate the plants did not have enough water to support the
new leaves so the older leave needed to die off first. On the other hand, 6.0, 5.5, and 5.0 samples
were growing constantly in leaf surface area.
There is a significant difference in seed weight and plant weight, only for those beans that
were grown in pH between 5.0 to 6.5 (see figure 3). The samples that were grown in pot 3.5, 4.0,
and 4.5 gained no weights or little weights in about three weeks. 5.5b had the heaviest in weight
and overall height is between 5 to 8 grams. These weights mean how these samples have taken
nutrients, light, water, and necessary chemicals then to transform them a physical form. Therefore,
the sample that has the most weight would mean it has more intakes of nutrients, water, and etc;
however, it does not necessarily mean it would be nutrient valuable since this research could not
provide that kind of information (Wright, Hanlon, Sui & Rice, 2009).
Conclusion
Pinto bean Phaseolus vulgaris has an obvious growth respond to different soil pH. It tends to
prefer soil pH between 5.5 to 6.0 based on the fact that the positive responses in weight, height,
and leaf surface area. If the soil pH is between 3.5 to 4.5, the pinto beans would tend to grow very
little or would not grow at all. Soil pH adjustment by aluminum sulfate and calcium carbonate could
only temporarily correct the pH of the soil. This treatment would be easier to adjust soil if planting
in small pots. This treatment would not be suitable for a large garden. However, there are other
methods to maintain soil pH, such as adding limestone or wood ashes in your soil if wanting to
increase the soil pH. When those materials are in the soils, they will slowly weather away when
water breaks them down. Putting compost and manures in the soil will decrease the soil pH in the
garden. This method is the easiest and cheapest but it takes months to change the pH value in soil.
Reference
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