I present here the results of a small trial I did last summer that combined a couple of my interests at the time. I had noticed the presence of large numbers of symphylans, small centipede-like invertebrates, in the garden. I was also having more trouble than usual with germination of seeds – spinach, carrots, beets. Symphylans travel large distances through the soil – venturing more than a meter vertically during the year depending upon soil conditions. They also tend to form “hot spots” where they do considerable damage, yet only a few feet away there is no problem. Looking from the point of view of the symphylan, one wonders how they navigate through the soil, find each other, find food, etc. My assumption is that these behaviors are all mediated by pheromones and other chemical signals. Hence, I wondered if biochar, with lots of absorptive carbon sites – like a charcoal filter, could disrupt some of these processes and cause the symphylans to preferentially avoid biochar treated soil.
The experiment is designed to see if biochar added to soil can reduce symphylan damage to seedlings. The experimental set up uses two large plastic pots, 17” tall by 14” diameter at the top for the growing media.
Each pot is filled to within 8 1/2” of the top with plain garden soil, collected in a manner to be sure no symphylans were present. The soil used was either flooded with water for an extended period of time, or was taken from the hot dry surface soil in the garden where no symphylans are found.
A soil / compost mixture was made to fill the remainder of the pots. Compost and soil was checked to be sure very few symphylans were inadvertently included in the mix. A divider board was placed in the center of the pot and one side was filled with the soil/compost mix.
The biochar source for the experiment was Safeway brand charcoal briquettes.
Approximately 1.5 gallons of briquettes were allowed to soak covered in water for about 3 hours. After soaking, the extra liquid was poured off the charcoal and the wet briquettes were pulverized with a wooden pole in the bottom of a plastic bucket. The pulverized char consisted mostly of fine particles along with some large pieces up to 1-2 cm across.
The pulverized char was added to the remaining half of the compost/soil mixture and well mixed. The soil / compost / char mixture was placed in the pots on the other side of the divider. Hence on one side of the divider was plain soil / compost mix – on the other side was the mix with the added char. The divider was then pulled out so that the two mixtures were in direct contact with one another.
To one of the pots, about 100 symphylans were placed on the surface and allowed to bury into the soil. These included about 20% adult animals and 80% smaller, presumably juvenile creatures. The pots were watered regularly, the soil structure was allowed to develop, and weeds to germinate over the next two weeks.
The photos in Figure 5 show that more weeds are germinating in the soil without the biochar added. The pot with the symphylans also seems to have more weed germination, perhaps an artifact of the soil residue on the surface of the soil when the symphylans were added.
The two pots were weeded and the top ½ inch of soil was cultivated, using care not to mix soil across the char boundary. An additional 10 symphylans were added to the test pot.
Five batches of the largest spinach seed from stock on hand were selected. Each batch contained 30 seeds. One batch was test germinated in a paper towel. More than half the seed germinated in three days and 29 of the 30 seeds eventually germinated after 5 days in the paper towel.
Each of the remaining four batches of 30 seeds were planted in the pots using a drill jig with 1 inch spacing and 5/8” depth.
The seeds were watered and allowed to germinate. The next observation was on day 8 after planting.
Figures 6 and 7 show the pots on day eight. Germination is spotty and there is evidence of damping off problems. Germination is most consistent in the char section of the control pot where 19 of the 30 seeds had successfully emerged and were growing. On the non-char section of the control pot, 13 of 30 had emerged, but 2 were observed to be dead from damp-off. In the symphylan pot 14 of 30 had emerged on the non-char section of which 6 were dead from damp-off. On the char side, 9 of 30 had emerged of which 3 were dead or dying from what appeared to be damping-off. The table below shows the number of plants as they emerged from Day-8 for the next week. The weather was quite hot during the germination period, exceeding 100° F on some days.
|Number of Spinach Plants|
|Normal Soil||Soil + Char||Normal Soil||Soil + Char|
|Day 8||13 (2 dead)||19 (1 emerging)||14 (6 dead)||9 (3 dead)|
Once the period of emergence ended, so did the damping off episode. By day 14 weeds were becoming quite plentiful, so the pots were weeded. The germination rates for the two variables compared to the controls are shown below.
|No Symphylans (control)||Symphylans||No Char (control)||Char|
|(19 + 8) / 60||(6 + 4) / 60||(8 + 6) / 60||(19 + 4) / 60|
|45% ±6%||17% ±5%||23% ±5%||38% ±6%|
The plants were allowed to grow for another eight weeks, at which point the pots were becoming crowded.
The plants were carefully pulled out from the pots. The number of plants varied widely because of the initial germination problems. The plants from each of the four groups are shown in the photos below.
A careful look at the roots from the symphylan infested pot shows more bifurcation of the tap roots than with the control plants. Five of the nine plants (55%) in the symphylan pot had root bifurcation, compared to two of fifteen plants (13%) in the control pot. Damage to the roots of the young plants by the symphylans would be consistent with these observations.
We can summarize the results of this experiment in a few statements.
- Damping-off and germination problems dominated the germination phase of the experiment.
- The sections with char had the best germination rate – significantly better in the control pot.
- Plants growing in the soil with char grew more slowly.
- The symphylans may have reduced the germination of the seeds, but the poor germination rate in the control sample makes a firm conclusion impossible.
It’s hard to draw clear conclusions from the results of this. The poor germination and damping off problems were much larger than any effect of the symphylans.