This project seeks to investigate how application frequency of indigenous microbial inoculant, also known as indigenous microorganism (IMO), may impact the size and diversity of the soil microbial community, soil nutrient availability, and yield, and leaf tissue nutrient density in lettuce production. If this method improves soil and plant nutrition by increasing soil microbial diversity, it will provide much needed evidence about how IMO works, give farmers additional information on how often to apply IMO, and allow farmers to make informed decisions about using IMO compared to other microbial amendments.
In light of research suggesting overall nutrient density of vegetables has decreased in the US since the 1960s, many vegetable farmers have become interested in the effect of soil health on plant leaf nutrient density. With research suggesting tillage and soil drying negatively impact soil microbial populations, and commercial microbial inoculants commonly used to encourage beneficial fungal associations in large-scale soybean production, the concept of feeding the soil first has become a driving mindset for many vegetable farmers, who spend time and money trying out different methods to improve soil and plant health.
One inexpensive inoculant commonly used by small farmers is known as Indigenous Microorganism (IMO). IMO is distinguished from other microbial amendments in that, in theory, forest soils are used to culture a more diverse set of indigenous microbes. Some farmers report anecdotal benefits of IMO and related Korean Natural Farming practices, and they make broad-ranging claims including eradication of diseases and pest pressure on their farms, no longer a need to add nitrogen fertilizers to their crops, and improved crop quality and yield. Microbial diversity may well provide these benefits, but available research is limited on the impact of IMO application on microbial diversity, nutrient availability, and leaf nutrition.
While microbial inoculants are used by many vegetable farmers interested in improving the soil microbiome, limited research exists on IMO. Much of the existing research has focused on indirect effects of IMO, such as plant yield and size. With existing research showing both positive correlations and no correlations with plant yield and size, we aim to look at more direct impacts of IMO, including soil microbial diversity, soil nutrient availability, and plant nutrient density. This will allow us to better understand how IMO may work from non-anecdotal data, and contribute to standardized evidence to help farmers evaluate whether these practices might benefit them. Also, through testing both a single and repeated application of the microbial inoculant, we will also gain more knowledge of whether repeated applications are effective, unnecessary, or costly for the farmer.
Indigenous microorganisms (IMO) may provide an opportunity for farmers to inexpensively increase the microbial diversity of soils, with the possibility of many benefits following increased diversity, such as nutrient availability, increased plant size and nutrient quality, plant raw probiotic quality, reduced disease pressure, and nitrogen fixation by legume and non-legume denitrifying bacteria. This research may also give more information on how to increase nutrient density of plant leaf tissue.
IMO can be used on a farm of any size in any area of the world, with little cost to the farmer. Whether or not it is effective, or if it might take multiple applications or years to reach efficacy, is to be seen.
The Bionutrient Food Association in Massachusetts, dedicated to improving soil and plant nutrient quality, are influential among the up-and-coming young farmer community. Talks by farmers employing these microbial amendment practices are very popular at NOFA conferences, and farmers frequently discuss these topics on social media.
Endless Farm is a 2-acre organic no-till farm in Johnston, Rhode Island, producing greens and herbs since 2018.
Treatments were arranged as a randomized complete block design. Five 50′ x 30″ rows were established, with eight 8.2′ x 2.5″ plots established within each row. Plots were planted with three rows of lettuce (Lactuca sativa var. Cardinale) spaced 9” apart.
An indigenous microorganism (IMO) inoculant of aerobically fermented rice, cultured in a nearby forest soil, was prepared according to the method of Llamelo et al (2016). Inoculant was applied at a concentration of 200 mL IMO2 to 10 L water (Llamelo et al., 2016).
Plots were treated with no IMO, treated with IMO one time, or treated with IMO four times during the study. Control plots received an application of well water, and well water was also applied to plots not receiving IMO that week, to correct for the positive effect of a moist environment on microbial biomass. The treatments were as follows: 1) inoculant applied once after transplanting, and well water applied for the following three applications until harvest 2) inoculant applied approximately weekly after transplanting until harvest, and 3) well water applied approximately weekly after transplanting until harvest.
The study was conducted at Endless Farm LLC in Johnston, Rhode Island (41°84’N, 71°54’W). The site was established in the spring of 2019 within a 1.4 acre field. The soil at the site is an Enfield silt loam (coarse-silty over sandy or sandy-skeletal, mixed, active, mesic Typic Dystrudepts) (http://websoilsurvey.nrcs.usda.gov/app/). The site has a 0-3% slope from northwest to southeast. There is no record of tile-drainage on the site. Before the initiation of the experiment in the spring of 2019, the site had been planted with leafy greens and not been treated with microbial inoculants, and in 2017 been planted with tulips. In 2016 and earlier, the site was fallow.
Soil samples were collected on March 31st for standard fertility analysis (pH, Modified Morgan extractable nutrients (P, K, Ca, Mg, Fe, Mn, Zn, Cu, B), lead, and aluminum, cation exchange capacity, and percent base saturation). Plots were weeded during the week prior to planting. On July 1st, feathermeal was added to each plot before planting at a rate of 75# N/acre, and calcitic limestone was added to amend the pH according to the recommendations for each test plot.
Lettuce (Lactuca sativa var. Cardinale) was seeded on May 15th and thinned to one germinated seed per cell. Plants of the same visual size were transplanted on July 2nd and were not root-bound. Crops were watered daily with an overhead rotating impact sprinkler system. Weeds were pulled on July 27-30th. Lettuce heads were harvested and weighed from August 16th through 18th. Each head in the plot was weighed and counted.
The indigenous microbial inoculant was prepared by cooking rice at a 1:1 ratio of rice to water by weight. Cooked rice was placed in on a forested hillside at Snake Den State Park in a permeable basket under the duff. Molded rice, with a majority of white mold, was collected and mixed with equal weight of brown sugar to make IMO2. IMO2 was mixed with well water at a ratio of 5:1, and the slurry was strained and applied with a sprayer in the evening to avoid strong sun. Attempts in July to inoculate moldy rice failed, and applications after July 30th were from a batch of IMO2 donated by a farmer from Orleans, Massachusetts, and prepared as described above. The microbial inoculant was applied on July 8th, 15th, 31st, and August 8th.
Soil samples were collected on March 31st and August 19th for standard fertility analysis (pH, Modified Morgan extractable nutrients (P, K, Ca, Mg, Fe, Mn, Zn, Cu, B), lead, and aluminum, cation exchange capacity, and percent base saturation). Six 2.4-cm diameter soil cores were sampled to 15 cm, composited, and air-dried before mailing for analysis.
Phospholipid Fatty Acid Analysis
Soil for PLFA was sampled to a depth of 15 cm. Five samples were taken per plot with a soil probe, composited, and subsampled to get 5-10 grams of soil per plot. The soil probe was cleaned with hydrogen peroxide between plots. Samples were placed in a freezer overnight at -10F until being shipped on ice. PLFA samples were taken after planting and prior to first microbial inoculant application on July 3rd, and on August 8th before harvest.
Leaf Tissue Analysis
Twenty mature lettuce leaves were collected at random from heads from each plot during harvest, and air-dried for leaf tissue analysis.
Some lettuce bolting from heat wave during 7/22 week