Greening Under the Evergreens: A Biochar Soil Amendment Study


Soil is collected under the evergreens.

The area under the evergreen trees in my backyard looks barren because nothing seems to grow there. I wanted to test if the fertility of this soil could be improved with the addition of biochar. Biochar is the product of pyrolysis of organic matter. Compared to other soil amendments, it has a high surface area and porosity that enable it to adsorb, or retain, nutrients and water and provides a habitat for beneficial microorganisms to flourish, which can enhance plant growth.

In this study, mung beans were planted indoors in soil samples from under the evergreens, and this soil was amended with 1%, 5% and 10% biochar by volume. Four biochars of different initial pH values, from different organic sources, were added at the three levels and compared to control soil. Their seed germination and plant growth were monitored.

My hypothesis was that the addition of biochar will make the soil more fertile. Based on existing studies, a 5% biochar addition, by volume, of the most alkaline (highest pH) biochar would have the greatest positive effect on seedling growth, because it is known that pine needles can turn soil acidic, which hinders plant growth. However, results showed that there was no significant effect from the addition of biochar on seed germination, plant height, or even soil pH, and no improvement in plant growth was seen with the biochar amendments tested here.



Biochar is the carbon-rich product obtained when organic biomass, such as wood, leaves, or grasses, is heated to the point of thermal decomposition under a limited or zero supply of oxygen (Howard, 2014). This process of heating without oxygen is called pyrolysis. Biochar is thought to aid in plant growth and soil development by providing housing for microorganisms and fostering their growth while assisting in water and nutrient absorption and retention (Howard, 2014). There is evidence to suggest that biochar was used for soil amendment over 2,000 years ago in the Amazon Basin, in the unusually fertile soils known as terra preta (black soils) (Hunt et al., 2010). More recent studies in both tropical and temperate climates have demonstrated biochar’s ability to increase plant growth, reduce leaching of nutrients, increase water retention, and increase microbial activity (Spokas & Nooker, 2014).

In this study, the impact of biochar on the soil under evergreen pine trees was investigated. It is known that evergreens shed pine needles that can change soil chemistry and also turn the soil acidic, thereby inhibiting any competitive plant growth in the vicinity. This phenomenon of plants protecting their space through chemical inhibitors is referred to as allelopathy (Ferguson, Rathinasabapathi & Chase, 2013). Mung bean seeds were planted indoors in control soil from under the evergreen trees and soil amended with 1%, 5% and 10% biochar by volume. Four biochars of different pH from different organic sources were added at the three different levels and compared to control soil. Plant height was measured and recorded for a period of 30 days. MinitabTM software was used to analyze the data for statistical significance using an ANOVA test. The results showed that there was no significant effect from the addition of biochar on seed germination, plant height, or soil pH, and the microbial biomass was lowered.


Research Objective and Hypothesis

Question: Will the addition of biochar to soil under the evergreen trees make it more fertile for plant growth?

Hypothesis: I hypothesize that the addition of biochar will help in making the soil more suitable for plant growth. I also hypothesize that a 5% addition, by volume, of the most alkaline biochar (highest pH) will have the highest impact on mung-bean plant growth. I think that 1% might be too little to impact pH while 10% might be too much and make the soil too alkaline for good plant growth.


Materials and Methods


  • Soil from under evergreen trees in my backyard
  • Biochars shown in Table 1 supplied by Dr. Kurt Spokas (USDA-ARS & University of Minnesota)
  • Mung beans seeds (Swad brand)
  • Wingscape plantcam
  • 36-well seed starting trays
  • 5-by-3-foot table for setting up trays
  • Gardening gloves
  • Particulate face mask
  • Hammer for crushing biochar chunks
  • Containers for mixing
  • Cup measure and graduated liter cylinder
  • Quart freezer bags (Ziploc)
  • Colored plant markers for labeling
  • Permanent marker for labeling, notebook and pencils
  • Yellow & green 301 masking tape
  • Measuring tape and level
  • 40-watt plant grow light
  • 3-mL vial for watering, tap water


Soil Preparation:

A table was set up in my basement for this experiment, and a grow light was installed above the table. A timer was set to turn on the grow light at 6 a.m. and turn it off at 6 p.m., providing 12 hours of light exposure. A time-lapse camera was set up to take a picture from the same position every three hours from 90 degrees above the table. Three 36-well (4x9) seed trays were placed on the table. The individual well positions were identified by row and column in the rectangular matrix of the tray. 

To prepare the amended soil, hardwood, macadamia nut shell, and pine biochars were first pulverized, using a hammer to break up any chunks. The wood pellet biochar was already a powder. One liter of soil from under the evergreens in my backyard was measured and biochar by 10%, 5% or 1% volume was added. A sub-sample of each soil treatment was bagged and set in the freezer for later soil analysis by a commercial soil-testing lab (A&L Laboratories in Memphis, TN).


Soil is gathered from under the evergreens (top left). The biochar is crushed into a powder (top middle). The biochar is measured (top right). The biochar is mixed with the soil (bottom left). 36 well seed starting trays are filled with soil mixture (bottom middle). The seeds are planted, watered, and placed under a grow light (bottom right).

The control and the 12 amended soils (four biochars at three levels each) were added to the wells, with each soil treatment replicated six times in a random order generated by the MinitabTM experiment designer software. Each well was labeled per its soil lot information, with the appropriate marker placed in the control and amended soil wells. The soil was saturated with water and allowed to free-drain for 24 hours.

Table 1: Key Parameters of the Biochars Used in this Study

Biochar Identification Parent Material Pyrolysis Temp (°C) Biochar Density (g/mL) Biochar pH
MacNut Macadamia nut shells 550 0.24 6.20
Cowboy Hardwood mixed 500 0.21 6.55
ChipE Wood pellets harwood 400-600 0.37 10.18
Pine Pine chips softwood 650 0.18 9.54


Seed Planting

Mung beans were pre-soaked prior to planting for 24 hours. Three presoaked mung beans were added to each well and pushed with a marked pencil so each one was buried 2.54 centimeters into the soil. With three mung beans per well and six wells per lot, the total possible seeds that could germinate was 18. A week after germination, each well was thinned and only one plant, the tallest one, was left in each. During the course of the experiment, 3mL of water was added to each well every day. Plant height was measured every other day and any plant growth comments recorded.

At the outset the grow light was at a height of 96.5 cm from the trays (Position A). The experiment was repeated in the same soil with the grow light lowered to a height of 30 cm (Position B), and the trays were placed diagonally to make sure the light was as uniformly received as possible.

Data was gathered and analyzed for both the experiments. The first experiment is referred to as Experiment A (grow light at 96.5 cm) and the second experiment in the same soil is referred to as Experiment B (grow light lowered to 30 cm above the trays).


Safety Precautions:

Gloves were used while handling biochar and soil. A dust mask was worn to prevent inhalation of biochar during handling and pulverizing.


Results and Discussion

The mung beans started germinating three days after being planted. The data in Figure 1 shows the total number of seeds that germinated out of the 18 for each of the soil lots by Day 7 (the day of thinning, when all but the tallest in each were pulled out). On average, approximately 62% of the seeds germinated. The data for plant height is shown in Figure 2. It is evident that the plants were not growing much as the days progressed.

Figure 1: Percentage of Seeds that Germinated by Day 7

MSP Naturalist Figure 1

Total number of seeds out of 18 that germinated 7 days after planting for the control  and 4 biochars, labeled as ChipE, Cowboy, MacNut and Pine for 1%, 5%, and 10% by volume.


Figure 2: Average Plant Height after 10, 15, and 25 days

MSP Naturalist Figure 2

Average plant height at 10, 15 and 25 days (end) for the control and the 12 biochar treatments.

Moreover, as shown by the data in Figure 3, a significant number started dying. The plants appeared stringy, possibly due to lack of adequate light (based on conversations with Dr. Spokas).


Figure 3: Percentage of Plants Surviving After 10, 15 and 25 days

MSP Naturalist Figure 3

Number of plants alive after 10, 15 and 25 days for the control and the 12 biochar treatments.  

As a result, the light was lowered from 96.5 centimeters to 30 centimeters above the trays, and the experiment was repeated. The plants from the first experiment were pulled out, and new mung beans were planted in the same wells with the existing soil. See the configuration in Figure 4 below.


These photos show the grow light and tray configuration for Experiment A (left) and Experiment B (right). In Experiment B the light was lowered and the trays were placed diagonally to ensure even light.

The data for the second experiment with the lowered light, Experiment B, is shown below in Figure 5. Surprisingly, a significantly smaller number of seeds germinated in the second experiment. On average, only 17% of the seeds germinated.


Figure 5: Percentage of Seeds Germinating by Day 7 for Experiment B (Lowered Light)

MSP Naturalist Figure 6

Number of seed germinated on day 7 for Experiment B (lowered light)

The increase in plant height with time was significantly better compared to Experiment A, as shown in Figure 6. However, a large number of plants still died in the repeated experiment, shown in Figure 7. 


Figure 6: Percentage of Plants Surviving at 10, 15 and 20 days (end) in Experiment B  

MSP Naturalist Figure 7

Average plant height, for Experiment B, 10, 15 and end (20 days) for the control and the 12 biochar treatments.


Figure 7: Percentage of Plants Surviving After 10, 15 and 20 days (end)  

MSP Naturalist Figure 8

Number of plants alive after 10, 15 and 20 days (end) for the control and for 12 biochar treatments.

Although a significant number of data points were lost in this second experiment, due to lack of germination and plant death, a statistical analysis was still conducted by pooling the data for Experiments A and B and analyzing it in MinitabTM for statistical significance using an ANOVA test. The analysis in Figure 8 shows that the biochar type or biochar concentration was not statistically significant for germination (p values well over 0.05). The analysis was also conducted comparing Trays 1, 2 and 3 to ensure that tray placement did not play a role; as expected, it was not statistically significant. The fact that the number of seeds germinated was much higher in Experiment A and lower in Experiment B was statistically significant (p value < 0.05).


Figure 8: MinitabTM Main Effects Plot for Seed Germination at Day 7

MSP Naturalist Figure 10_minitab

MinitabTM Main effects plot for seed germination at day 7

The plant height data was also analyzed (Figure 9), and the results show that none of the variables are statistically significant (p values higher than 0.05). Also, there was no impact of biochar type or concentration on plant growth for either experiment.


Figure 9: MinitabTM Main Effects Plot for Plant Height at Day 10

MSP Naturalist Figure 11_minitab

MinitabTM Main effects plot for plant height at day 10

Soil pH data generated by A&L Laboratories on the mixed soil and the soil collected from the wells after Experiment B, is shown in Figure 10.


Figure 10: MintabTM Main Effects Plot for Soil Buffer pH

MSP Naturalist Figure 12_minitab

MintabTM Main effects plot for Soil Buffer pH

The buffer pH was essentially the same, which suggests a lack of a long-term alteration in the pH with addition of biochar (the control soil had a pH of 6.92, or very mildly acidic).



Mung bean seeds were planted in soil from under the evergreen trees in my backyard (the control) and in biochar-amended soil. Seed germination and plant growth were monitored. The results indicate that my hypothesis was not supported. The biochar amendments examined here did not impact plant growth in the soil from under the evergreens. However, it should be noted that the analysis was conducted on a very small dataset as a result of low levels of germination and subsequent high levels of plant death.


The soil pH was largely unaltered with the addition of biochar. I had thought the soil would be more acidic due to the pine needles. However, this wasn’t the case, and the addition of biochar did not have a significant effect on the pH. The addition of biochar could have impacted the nutrient and water available for seed germination. However, further experimentation is needed to eliminate other potential factors, such as the fungal growth in soil seen during this experiment. The fungus could have played a role in hindering mung bean germination.


Sources of Error

I chose mung beans for this study. It is possible that the seeds were contaminated or they were not the best choice for growing in the experimental environment chosen. It is also possible that the lack of light in the first experiment caused a high level of moisture in the seed trays and allowed the fungus to take hold, which hindered growth for the second experiment as well. The experiment was conducted in seed trays. The root growth could have been inhibited by the lack of space to grow.


Recommendations for future work

I would recommend that the experiment be conducted outdoors, since the artificial grow light distance does make a difference in seedling development and further experimentation would be needed to optimize the light level based on the soil and seeds used. If conducting the experiment indoors, it would be beneficial to conduct some baseline studies to identify optimal light exposure, humidity, water levels, etc. Transplanting the seeds to larger pots could be considered to allow for root growth once germinated. Also, plant germination could be conducted in potting soil and the seedlings could then be grown in control and amended soils. I would also consider conducting the experiment with other seeds in addition to mung beans.



I would like to thank my mom for being my advisor and guiding me through the whole process. She taught me how to formulate the problem, search the internet for information, think critically, and helped me write the results like a scientist. I would also like to thank my dad for helping me in planning and conducting the soil experiments and analyzing the data with MinitabTM and my brother for being a great STEM role model.

Finally, I would like to acknowledge Dr. Kurt Spokas, a soil scientist with the USDA-Agricultural Research Service and adjunct faculty in the Department of Soil, Water and Climate at the University of Minnesota, for his help and guidance throughout the project.



Ferguson, James, Bala Rathinasabapathi, and Carlene A. Chase. “Allelopathy: How Plants Suppress Other Plants.” University of Florida Institute of Food and Agricultural Sciences Extension, July 2003. Web. 1 June 2014.

Howard, Teresa. “The Effect of Biochar on the Root Development of Corn and Soybeans in Minnesota Soil and Sand.” International Biochar Initiative. Web. 9   May 2014.

Hunt, Josiah, Michael DuPonte, Dwight Sato, and Andrew Kawabata. “The Basics of Biochar: A Natural Soil Amendment.” University of Hawaii, College of Tropical Agriculture and Human Resources. Soil and Crop Management (Dec. 2010): 1-6.

Spokas, Kurt, and Erick Nooker. “Impact of Biochar on Specialty Crops.” 2012 U.S. Biochar Conference. Web. 1 June 2014.