The Effects of Used Spent Coffee Grounds on the Growth of Brassica rapa

Blerina Iljas, Daniel Bell, Eric Nguyen and Sarah Tran

Organismal Biology 140-03

November 14, 2019

Abstract:

There is conflicting information about the benefits or harms of using spent coffee grounds (SCG) in place of fertilizer. If spent coffee grounds improve plant growth, this substitution could provide a possible alternative and sustainable source of soil enrichment. In this experiment, spent coffee grounds were added to soil in three different ratios (17%, 33% and 50% coffee) to test the effects of those additions on the growth of Brassica rapa. It is hypothesized that when coffee grounds are incorporated at low levels (17%), Brassica rapa will grow at an increased rate. If coffee grounds are added at high levels (50%), plant growth will be impeded due to phytotoxicity, and at an intermediate concentration of coffee (33%), it will result in a relatively consistent growth rate. Three groups with varying proportions of coffee grounds were compared to a control group grown in soil with no amendments. However, all four groups contained approximately the same amount of soil. Measurements of plant height were recorded weekly for five weeks. At the end of the five weeks, the total mass of freshly harvested plant growth was measured for each of the four groups. The results were highly varied and inconclusive. It is recommended that future experiments should be completed with improved research methods. Introduction:

In recent years, evidence is growing about the negative environmental and health impacts of the use of inorganic chemical fertilizers. Concerns for the environment have also sparked interest in the reduction of waste, with attempts to recycle or repurpose waste products whenever possible. These recent shifts in opinion, combined with the fact that the world produces SCG in large amounts (Campos-Vega et al., 2015) have led to research into the use of spent coffee grounds as an alternative to inorganic fertilizers. Many nutrients are required for plant growth; Casierra-Posada and others (2017) describe that iron is one of these important nutrients. Morikawa and Saigusa (2008) provided evidence that SCG improve the availability of iron to the plants. This shows that incorporating SCG may have a positive effect on plant growth. However, further studies show differing results. In another experiment conducted by Hardgrove and Livesley (2016), they found that at low volumes (2.5%), SCG prevented the growth of certain vegetables and flowering plants. This conclusion contradicts our hypothesis that low concentration of spent coffee grounds promote an increase in plant growth. Similarly, findings in a study conducted by Turek, Freitas and Armindo emphasized that at high concentrations of SCG, the development of plants was hindered (2019). This reinforces the idea that SCG can have a negative effect on plant growth, but only at high concentrations. Furthermore, at high enough levels, it may result in a noticeable decrease in the total mass. This supports our hypothesis that high concentrations of SCG impede plant growth. One explanation for this phenomena is it could

be caused by the nature of coffee grounds; coffee grounds may cause nutrient toxicity to occur, and as a result, may interfere with plant growth (Cervera-Mata et al., 2019). A second explanation for this phenomena would be there was an excess of iron built up which stunned the growth of the plants (Casierra-Posada et al., 2017). Overall both extremes have a negative effect on plant growth.This raises a new question of how much coffee grounds should be added such that it will still have a positive effect, but not impede plant growth. The present experiment tests this underlying question. In this experiment, if SCG are found to improve growth of Brassica rapa, this would suggest that SCG may be used as a soil amendment for other plants including food crops. The purpose of this experiment was to determine the effects that various levels of additions of spent coffee grounds to potting soil would have on the growth of Brassica rapa. Spent coffee grounds were added to the soil in three ratios to test the potential impacts of caffeine phytotoxicity at the higher ratios of coffee grounds to soil, with the expectation that lower ratios of spent coffee grounds may provide growth benefits while higher ratios would impede growth. Methods and Materials:

In preparation for the experiment, three pots of Starbucks Pikes Peak coffee were brewed (which turned out to be too much for this experiment; the grounds from one 12 cup pot would have sufficed). The wet, spent coffee grounds were spread out in a thin layer on paper towels to dry over the course of two days. The objective was to measure the dry weight of the grounds in order to determine the needed ratio of soil to coffee grounds. This may also be a practical consideration; if spent coffee grounds were to be packaged for use as a soil amendment, it may be necessary to dry it first to prevent mold growth. Eight styrofoam quads (with 32 compartments in total) were used as the plant containers, and they were able to hold about one gram of soil in each compartment. A diamond wick was inserted into the hole in the bottom of each compartment with half of the wick sticking out from the hole. A digital scale was used to measure out the quantities of soil and coffee grounds, and four groups of eight compartments each were created. The eight compartments in the first (or control) group consisted of 0.90 grams of potting soil. Each of the eight compartments in the second group (A) consisted of a mix of 0.15 grams of SCG and 0.75 grams of potting soil (17% coffee). Each of the eight compartments in the third group (B) consisted of a mix of 0.30 grams of SCG and 0.50 grams of potting soil (33% coffee). Each of the 8 compartments in the fourth group (C) consisted of a mix of 0.45 grams of SCG and 0.45 grams of potting soil (50% coffee). In all cases, the soil and coffee grounds were thoroughly mixed before placing them in the compartments. A single seed was placed in each of the 32 compartments, below about 1/3 of the total depth of soil. A plastic container was filled about 2/3 full of water and one algaecide tablet was added to the water to prevent algae growth. A felt wick mat was pre-wetted and was laid completely across a lid that was placed over the plastic container and the remaining portion of the felt wick mat was in the

water inside the container. The styrofoam quads were placed on top of the wick mat and lid. The entire apparatus was placed under a four foot wide lighting strip. Measurements of plant height were taken at one-week intervals using a metric ruler. Pictures were also taken at each measurement period to document plant growth. At the end of the experiment, plants from each group were harvested separately by cutting the plant at soil level. The total mass of all wet plant matter harvested from each group was measured with the digital scale. Results:

Figure 1 Initial setup of Brassica rapa growing experiment

Table 1 Height (in cm) of each plants, one week after the start date

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

Control Group A Group B Group C

Figure 2 Growth of Brassica rapa after one week In the week after the experiment setup, it was found that the felt wick mat was not pulling up enough water from the plastic container to the end furthest from the water which surrounded Group C (the high concentration of coffee grounds-77%). None of the seeds had sprouted yet (Figure 2). Table 2 Height (in cm) of each plants, two weeks after the start date

0 1.3 2.0 2.1 1.2 1.8 1.1 0

0 0 0 2.1 0 0 0 2.2

1.0 2.0 0 2.0 1.8 0 0 1.4

0 1.5 2.2 1.5 1.9 1.3 1.5 1.5

Control Group A Group B Group C

Figure 3 Growth of Brassica rapa after two weeks Roughly 63% of the seeds sprouted (four from the control, six from group A, five from group B, five from group C) by the second week of the experiment. All of the sprouted plants were green (Figure 3).

Table 3 Height (in cm) of each plants, three weeks after the start date

0 4.0 2.7 3.2 0.5 1.4 1.5 0

0 0 0 2.1 0 0 0 1.2

4.8 6.1 0 3.7 1.5 0 0 1.9

0 1.3 2.4 2.2 2.4 0.5 1.6 1.5

Control Group A Group B Group C

Figure 4 Growth of Brassica rapa after three weeks

The majority of the plants eventually took on reddish-purple hues. Some plants quickly shriveled up and died by week three (Figure 4).

Table 4 Height (in cm) of each plants, four weeks after the start date

0 8.0 2.8 3.5 0.8 1.6 1.5 0

0 0 0 2.2 0 0 0 1.4

0 6.5 0 4.5 1.5 0 0 2.5

0 1.9 1.8 2.3 2.2 0.5 2.2 1.2

Control Group A Group B Group C

Figure 5 Growth of Brassica rapa after four weeks By week four, some of the plants that were shorter than the control group’s flowering plants already had some flowers blooming. The color of all of the flowers seemed to be the same yellow shade, no matter the reddish-purple or green color of the rest of the plant (Figure 5).

Table 5 Height (in cm) of each plants, five weeks after the start date

0 8.2 2.8 2.6 0.8 1.2 1.5 0

0 0 0 1.7 0 0 1.5 1.4

0 6.8 0 4.0 0.5 0 0 2.2

0 2.0 1.8 2.7 2.3 0.5 2.4 1.3

Control Group A Group B Group C

Figure 6 Growth of Brassica rapa after five weeks

Most of the plants were withering by the fifth week. However, there was one newly sprouted plant in group C which sprouted green. The leaves of the reddish-purple plant in the back of group C were pinched and it was observed that they were well hydrated (Figure 6).

Table 6 Average height (in cm) of each group every week

Week Control Group Group A Group B Group C

1 0 0 0 0

2 0.7 1.5 1 1

3 2 2 0.8 1

4 2 2.1 0.8 1.1

5 2.1 2 0.7 1.3

Figure 7 Average height in Brassica rapa over the course of 5 weeks

Table 7 Measured final masses of each group (with bowl) after 5 weeks

Measurements

Object being measured Mass (g)

Mass of bowl used to weigh the SCG 4.698g

Mass of bowl + SCG of Control 4.788g

Mass of bowl + SCG of Group A 4.781g

Mass of bowl + SCG of Group B 4.733g

Mass of bowl + SCG of Group C 4.779g

Calculations:

otal mass of each group (mass of bowl SCG of each group) (mass of bowl)T = + −

Table 8 Calculated final masses of each plant

Group Total Mass of Each Group (g)

Control 0.090

Group A 0.083

Group B 0.035

Group C 0.081

Figure 8 Final mass comparison across all 4 groups

There were no consistent growth patterns detected, except that the control group yielded slightly more growth than groups A, B, and C. The control group achieved the greatest heights by far with its 8.2mm and 6.8mm tall plants (Table 5). Group B (33% coffee grounds) yielded a total mass roughly 40% less than the other groups (Table 8). Roughly 38% of the plants did not sprout at all during the course of the experiment (Figure 5).

Discussion:

The results of the experiment showed that there was no consistent growth over time. Plant growth was spontaneous. During some weeks, the plants showed a lot of growth whereas others showed little to none. The data shows that there was slightly more growth in the control group than any of the other three groups. In particular, Group B showed the least growth. It yielded a total mass that was roughly 40% lower than the other groups. On average, the mass of the control group, group A and group C were within 0.008g of each other. The outlier, group C, had an average mass difference of 0.050g when compared to the other three groups. The results found by Chilosi and others (2020) coincide with these uneven results since they found that even though SCG contribute necessary nutrients and are not phytotoxic at low levels, adding SCG can still cause decreased plant growth. Another study that our results aligned with was the study conducted by Turek and others (2019). Our results agreed with their conclusion that at high enough levels, there would be a noticeable decrease in plant growth, as shown in our experiment by the height. After five weeks, in all three groups with the coffee grounds, the average height of those plants were less than the average height of the control group plants.

Initially, some seeds sprouted quickly. When the plants first sprouted, they were all green. Over time, some of the leaves turned reddish-purple, and some plants quickly dried up and died. Some of the plants grew slowly throughout the entire experiment or never sprouted at all. There are a number of variables that could have contributed to these results, such as bad seeds, variations in soil density and volume, and an ineffective watering system. The felt wicking mat did not draw enough water toward the end furthest from the water which could have resulted in the diamond wicks bringing scarce amounts of water to the soil. Thus, setting back the growth of that group. Also, the warped and uneven plastic lid may have prevented the diamond wicks from making sufficient contact with the felt mat. Additionally, the soil was not pressed into the compartments, so the light airy nature of the soil may have prohibited sufficient contact with the diamond wicks. The watering system should be tested first to confirm that it is functional if this experiment were to be repeated. Large particles of wood or other debris should be filtered from the soil if small compartments are used. The soil should be lightly pressed into place to ensure soil contact with diamond wicks and seed. The process of recording the total mass of each group of plants was a way to track the development and growth of Brassica rapa. The results of this experiment show that adding coffee grounds does have an effect on regular soil. Another way to further research this topic may be to try adding coffee grounds to polluted soil to see if the results would differ. Overall, for the majority of the experiment, plant growth was erratic. The plant heights of groups A, B, and C were similar to each other. The results of the experiment were highly irregular and inconclusive.

Conclusion:

This experiment describes that different concentrations of SCG cannot be the deciding factor of plant growth. Other properties of coffee such as density and the nutrients it provides and absorbs also play a role in the growth of plants. Plants require an adequate amount of space to grow and water uptake relies on that space. Therefore, we cannot pinpoint whether it is the effect of SCG or the space within the styrofoam compartments that caused the inconclusive results. This is a prime example of why repetition is a vital aspect of the scientific method. Before going forward with the idea to replace traditional fertilizer with SCG, more tests and studies need to be done that produce more consistent and replicable results.

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