Compost continues to push its way into mainstream farming, and vermicompost is hanging out in the background, ready to become a part of the conversation.
And with good reason. Some worms can break down and process waste products rather quickly — faster than traditional, non-worm composting practices.
Davis, California-based BioFiltro uses worms to treat wastewater, which leaves the company with a significant amount of vermicompost byproduct. But could that byproduct come out of the filter and into the field?
“We wanted to see if it could be used in walnuts, and then see if it could be expanded to other perennials or specialty crops,” said Veronica Suarez Romero, University of California (UC), Davis, PhD candidate and project co-lead.
The project looked at fields with crop rotations, as well as a walnut orchard in Northern California.
Cutting fertilizers
The idea of the project was to reduce commercial, synthetic fertilizers with vermicompost. When the project began, fertilizer prices were quite high, influenced by the coronavirus pandemic and the Russia-Ukraine war. But it also sought to look at providing other nutrients while decreasing greenhouse gas emissions.
Nitrogen (N) fertilizers were replaced with different quantities of vermicompost — up to 25% in the study — that were produced from different sources, such as dairy waste and residues from a pepper processing plant.
Vermicompost was applied for two consecutive years at the orchard at five different rates: the control (no vermicompost), 7%, 10%, 14%, and 25% replacement of synthetic fertilizer. Soil health data and greenhouse gas data were collected from the orchard, including CO2, nitrous oxide, and methane.
The biggest challenge the team encountered was that since they were trying to provide N via vermicompost, an organic source, it took more time for that nitrogen to become available, and timing of the application was key. Even then, the results were favorable.
Suarez Romero measured the carbon and nitrogen in the soil, as well as yield. Leaf samples were taken to measure the same nutrients.
The biggest takeaway: Despite cutting fertilizer, the trees sustained yields in all of the treatments and were not affected by the two-year reduction of fertilizer inputs.
These results initially brought skepticism and were thought to have been due to the trees’ resiliency and their nutrient stores. But leaf samples revealed that there was enough nitrogen, and even though the vermicompost was enough to supply the extra N, the leaf samples had more than the recommended N concentration.
“Basically, the trees were living in luxury,” Suarez Romero said. “With these results, I wish we had substituted higher percentages, because I think it’s possible to replace even more.”
Carbon, phosphorus, and tree homogeneity
In the first year, soil carbon increased in the highest vermicompost treatment. In the second year, all of the vermicompost treatments had caught up to one another and were the same.
But was that increased carbon stored in the ground?
Extra analysis determined the quality of the carbon. Using carbon fractionation, it was discovered that applying vermicompost increased the total carbon in the soil and increased the fraction that is very stable, making it potentially usable for sequestering carbon, and opening up opportunities for further research.
Due to the nature of the study and its funding, carbon quality data was taken only at the end of the two-year project. It’s unknown if the same results would occur after just one year of application.
Other soil nutrients also increased, particularly phosphorus (P). Available soil P increased in the highest rate of vermicompost application, allowing another opportunity for fertilizer reduction.
Another significant discovery was the homogeneity of the trees. The tests from all three rate applications showed that the nutrient levels were very similar compared to one another.
“Vermicompost is kind of easing the management of the orchard; the nutrient supply was evened out for the whole field,” said Suarez Romero. “It was something we weren’t expecting.”
The trend was the same for most of the nutrients, though there were differences in some, such as iron and copper.
Even though the control didn’t receive any vermicompost, it still saw a minor increase in nutrients due to residues being left in the field.
“It’s very important to know that all of that organic matter needs to be put back into the soil in order to replenish the nutrients that are taken out during harvest,” said Romero.
As of this writing, Suarez Romero is still awaiting the results of the microbial analysis.
The project was made possible with the help of William Horwath, Sabina Dore, BioFiltro, HydroFocus, the Turkovich Family, and with funding from the California Climate Investments Program and the California Dept. of Food and Agriculture Healthy Soils Program.
Because the project went so well, BioFiltro is funding additional studies on different crops, at different rates, and in different soil types.
“All of these sustainable practices, they are not one size fits all,” Suarez Romero said. “They work in some areas, and don’t in others. It depends on your conditions — in the soil, the climate, the management — and all of that makes a difference.”
Main image: Vermicompost spread between tree rows. Rates were controlled by passing the manure spreader several times around designated plots.
Photos and graphs: Veronica Suarez Romero
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