The Impact of Organic Soil on Growth Rate and Average Yield of Wisconsin Fast Plants
Sheena Forsberg and Christopher Thompson
Department of Biological Science
Saddleback College
Mission Viejo, CA 92692
Rapid Cycling Brassica rapa (AKA, Wisconsin Fast Plant) is a cruciferous plant, which is
known for its resilience and ability to adapt to various types of growth mediums as well as its
ability to complete its entire life cycle in a relatively short period of time. RC B.rapa is well
acclimated to grow in a laboratory environment in which it receives constant fluorescent light
and constant access to water. As such it is an excellent candidate to test the effectiveness of
various growth mediums. The hypothesis being tested is that employing the use of organic soil
as a growth medium instead of conventional soil will result in a higher rate of germination, a
higher average flower yield, overall height, and cotyledon width. In this experiment all
measurements were obtained after the plants had grown for a period of two months in their
respective growth mediums: OSH potting soil mix and Edna’s Best Organic Potting Soil. The
conventional soil resulted in a germination rate of 97.2% with average height of 8.57cm, the
average flower yield being 4, and the mean cotyledon width as 1.72 cm. The organic soil had a
rate of 100% germination with the following results: average height 7.87cm, average flower
yield 3 cm and the mean cotyledon width 1.11cm. Results concluded that overall there was no
statistical difference between the two mediums with the exception of cotyledon width in the
conventional soil. According to the t-tests there was a significant difference between the
conventional soil and the organic soil with respect to cotyledon width (p= 3.26313E-20) as well
as number of flowers (p= 0.00045796).
Introduction
Rapid Cycling Brassica rapa is a member of the cruciferous family of plants and is therefore well
adapted to grow under many environmental conditions. One key growth factor is the RCBr’s ability to grow
under constant fluorescent light as opposed to sunlight which allows for greater flexibility in laboratory
studies. In addition to this RCBr has a rapid life-cycle which makes them an ideal test subject, as their lifecycle
completes in approximately 45 days after sowing (Kelly 2004). Due to its rapid lifecycle the potential
applications of RCBr to experimental plant biology are diverse (Kelly 2004; Musgrave 2000). RCBr was used
in this experiment to test which type of soil is best suited to produce the most positive results of various plant
growth factors of this member of the crucifer family (Stephens and Kostewicz 2009). According to Duke
RCBr requires relatively low nitrogen levels (1978), and a high turnover margin requires relatively low quality
fertilizers as suggested by Wu (1972). Therefore it was believed that the organic soil would prevail in
producing the most favorable results. However according to a 21 year study conducted in central Europe it was
found that “nutrient input (N, P, K) in the organic systems to be 34 to 51% lower than in the conventional
systems, whereas mean crop yield was only 20% lower over a period of 21 years” (Maeder, et al. 2002).
Additionally it was found that soil pH was slightly higher in the organic systems (Maeder, et al. 2002), which
is a factor believed to benefit RCBr (Carolinalabs.com 2009).
This experiment was performed in order to determine whether there is a significant difference when growing
plants, in this case Rapid Cycle Brassica rapa (RCBr), in organic soil as opposed to conventional soil
containing chemical fertilizers. It is predicted that the RCBr grown in the organic soil will have more total
germinators, taller plants, wider cotyledon widths and more flowers per plant than those in the conventional
soil group.
Materials and Methods
Two bags of soil were purchased at Orchard Supply Hardware, a conventional potting soil (OSH
Fertilized Potting Soil) and one fully organic potting soil (E.B. Stone Edna’s Best Potting Soil). Seeds of
Brassica rapa were purchased online through Carolina Labs. Two growing trays and fluorescent lighting were
also obtained. 12 wells of each tray were lightly packed with damp soil, one tray with conventional soil, the
other with organic soil. Each well of each tray was then implanted with three B. rapa seeds at a depth of .6 cm
(Day 0, September 26th, 2009). The two wells were then placed under constant fluorescent lighting, indoors,
with constant temperature and humidity, and constant water in each tray. Observational were then taken each
day until each group flowered. Upon flowering, (Day 22, October 17th, 2009) measurements were taken using
a centimeter ruler of; height from soil to top of plant (n=35, n=39 (conventional, organic)), the average of two
cotyledon widths per plant (n=35, n=37), and the number of flowers on each plant (n=35, n=39). The average
height, average cotyledon width, and average number of flowers per plant per cell were then analyzed
statistically through an unpaired, one-tailed t-test.
Results
The mean height for RCBr in conventional soil is 8.574  .345 cm ( S.E.M, n=35), while the mean
height for RCBr in organic soil is 7.87  .317 cm (n=39, Figure 1); running an unpaired, one-tailed t-Test for
these data yields no significant difference between the two groups (p= .067662). The mean cotyledon width for
RCBr in conventional soil is 1.72  .0371 cm (n=35), and the mean cotyledon width for RCBr in organic soil is
1.11  .0270 cm (n=37, Figure 2); Running an unpaired, one-tailed t-Test one these data obtains a significant
difference between the two groups, cotyledon width in conventional soil being greater than that of those in
organic soil (p= 3.26313E-20). The mean number of flowers for RCBr in conventional soil is 4.08  .423
flowers (n=35), while the mean number of flowers of RCBr in organic soil is 2.51  .115 flowers (n=39, Figure
3). Running an unpaired, one-tailed t-Test on these data obtains a significant difference between the two
groups, the number of flowers on plants grown in conventional soil greater than that of those grown in organic
soil (p= .00045796).
Figure 1. Mean height of RCBr grown in conventional soil (8.574  .345 cm ( S.E.M, n=35) and organic soil
(7.87  .317 cm (n=39). (p= .067662).
Figure 2. Mean cotyledon width of RCBr grown in conventional soil (1.72  .0371 cm (n=35) and organic soil
(1.11  .0270 cm (n=37). (p= 3.26313E-20).
Figure 3. Mean number of flowers of RCBr grown in conventional soil (4.08  .423 flowers (n=35) and
organic soil (2.51  .115 flowers (n=39). (p= .00045796).
Discussion
The data and results show that there is a difference in all categories tested between the two groups,
organic soil germinating in 100% of seeds planted, while conventional only 97.2%, and conventional soil
yielding plants with greater averages in the other three categories. However, regarding height and ability to
germinate, there was no significant, statistical difference between the two groups, therefore, in the two
categories with statistical differences, conventional soil proves to be the better growth medium. The RCBr
grown in conventional soil had cotyledons which excessively outsized those grown in organic soil, and the
number of flowers in the conventional soil group also outnumbered those in the organic soil group. In the case
of this experiment, the hypothesis being tested was incorrect, and proved wrong. The conventional soil is made
specifically for this reason, to be a better substitute for organic soil. Even in B. rapa, which requires low
nutrient levels and is easily sustainable, the conventional soil allowed for better growth. Other than the growth
medium, all variables in this experiment were kept equal except for one; the pH content of each soil. Because
B. rapa is assumed to grow better in a higher pH environment (Carolinalabs.com 2009), the greater pH of the
organic soil did not affect the hypothesis. If this experiment were to be expanded on, however, soil pH should
be equal and the null hypothesis assumed.
Literature Cited
CarolinaLabs.com. “Wisconsin Fast Plants.” Carolina Biological Supply Company. 2009.
Duke, J.A. 1979. “Ecosystematic data on economic plants.” Quart. J. Crude Drug Res. 17(3-4):91-110
Kelly, Martin G. “Education Resources Information Center.” Demonstrated Ways to Use Rapid Cycling
“Brassica rapa” in Ecology Instruction and Research. 11 2004.
Stephens, J.M., and S.R. Kostewicz. Producing Garden Vegetables with Organic Soil Amendments. July 2009.
Wu Leung, Woot-Tseun, Butrum, R.R. and Chang, F.H. Part I. Proximate Composition Mineral and Vitamin
Contents of East Asian Foods. Oxford University Press 1972.