The Effects of Fish Emulsion Fertilizer on Duckweed Growth

Phillip Bullington

Introduction:

            In 2001, my wife Leslie and I had a 20’x50’ garden at our home in the Jemez Mountains.  We grew lettuce, kale, spinach, butternut squash, zucchini, tomato, honeydew, corn, chili, and snow peas.  The crop was so successful that we had a constant supply of fresh produce for ourselves and all our friends as well.  I used organic mushroom compost to enrich the soil and for the first time regularly used Alaska Fish Fertilizer to nourish the plants.  I believe the fish emulsion was the key to the successful crop.  The gardens I had tended in the past had never been as abundant. 

            For this experiment, I tested the hypothesis that using Alaska Fish Fertilizer would greatly increase the growth rate and population of duckweed.

Methods:

            I filled four Styrofoam bowls with a mixture of ordinary tap water and 1 milliliter of fish emulsion.  The fertilizer is completely derived from marine fish and consists of 5 percent water soluble nitrogen, 4 percent chlorine, 1 percent phosphoric acid, and 1 percent potash (potassium oxide).  These bowls were labeled A thru D.  Another set of four bowls were to act as a control group and contained only tap water, these were labeled E thru H. 

            Sixteen duckweed plants were placed in each of the eight bowls.  The bowls were then placed in the greenhouse for the duration of the experiment.  The bowls were checked every two or three days to maintain the water level and to count the duckweed population in each bowl.  The experiment lasted 26 days.  Data was compiled and charted.

Results:

            The duckweed grown in the fish emulsion mixture did have a higher population when compared to the duckweed grown only in tap water.  By the end of the experiment, the lowest count in the fertilized A-D group was 44 duckweed and the highest was 60.  In the end, the control group had a low population of 4 and a high of 20. 

Discussion:

            The results of the experiment did support my hypothesis, but not as greatly as I had predicted.  At seven days, the population of duckweed in all but one bowl had dropped.  After the second week, however, it could be seen that the fertilizer had a direct effect on the duckweed’s growth rate.   The population in the fertilized bowls steadily increased by approximately 4 duckweeds per day.  Of the duckweed in the control group, only one bowl grew beyond the starting population.  The trend line for Cup B, which had the highest population, suggests 137 duckweed at 40 days (y = 1.9248x + 3.4854).

            I believe the main reason that the population of the fertilized duckweed did not increase as greatly as I had anticipated has much to do the algae material that grew in each of the bowls.  The algae material (Figure 1) appeared in the fertilized bowls at day 16.  The algae coated the bowl and the water’s surface and also covered much of the duckweed.   I believe the algae compete with the duckweed for nutrients in the water.  Had the algae material not been present, I believe that the duckweed’s growth rate would have sharply increased.   The most likely explanation for the presence of the algae is that it was in the fish emulsion, and the water and light enabled its growth.

            If I were to do this experiment again, I would add a mild algaecide that would not damage the duckweed, at the first sign of any algae growth.  I also would have another 2 groups, one mixed with 2mL and the other with .5mL of emulsion to see if the duckweed needed more or less fertilizer to meet growth expectations.  I would have also continued the experiment for a longer period of time to better see the effects of the fertilizer.

While the results of this experiment do suggest that fish emulsion does promote plant growth, it does pose some questions that would require additional research.  Would emulsion made from fresh water fish have more or less effect on plants than sea going fish?   Can a plant take in the nutrients from fish emulsion better in soil or water?   What part or parts of the fish do most of the nutrients come from?  These are questions that I believe are worth the time and research.