Growth Promotion By a Microbial Biostimulant Depends on Application Frequency in Semi-protected Fall-to-Spring Lettuce Production in Ohio

Microbial biostimulants (MBSs) contain one or more types of selected bacteria or fungi and other components and they are applied to enhance crop growth and/or tolerance of abiotic stress. Two sets of experience spur interest in MBSs among researchers, growers, and industry: 1) numerous scientific experiments demonstrating that pure cultures of individual strains can alter plant vegetative and fruiting parameters in controlled settings, and 2) MBS manufacturer and grower claims that MBSs can enhance farm productivity. Experience also indicates that MBS performance is context-dependent and that conditions under which they perform most consistently are not well understood. MBS effects under dynamic temperature, moisture, and other conditions common to fall- and springtime leafy vegetable production in semi-controlled settings in Ohio appear to be untested. The longevity of inoculant-derived microbes under these conditions and hence, optimal MBS application frequency, is also unclear. Therefore, we set out to determine whether fall-time MBS application enhances lettuce productivity fall and spring, whether these effects depend on application frequency, and whether soil microbial populations in inoculated and control plots differ long after treatment. A randomized complete block design with five replications was used. Organic ‘Parris Island’ lettuce seeds were sown on 5 Nov. 2015 at the OARDC at a rate of 9700 seeds∙m^-2 into five, 9.1 m x 1.2 m, wood-framed, un-heated raised beds covered with medium weight (34 gram∙m^-2) floating row covers and 6-mil embossed greenhouse film overtop gothic-shaped frames. Certifiable-organic rooting medium in the 30 cm-high beds consisted of 30% peat moss, 20% parboiled rice hulls, 25% field soil, and 25% compost by volume. “Biogenesis I NP Soil Amendment”, containing 12 common beneficial rhizobacterial species, was applied as a drench up to four times: 1) none (control), 2) at seeding, 3) at seeding and two weeks thereafter, and 4) at seeding and at weeks two and four after seeding. Plants and rooting medium were collected on 16 Dec. 2015 and 25 Jan. 2016 to measure leaf fresh and dry weight and ºBrix and to estimate rhizospheric microbial populations. Soil and air temperatures within the tunnels were monitored with ‘HOBO Pro V2 Temp/Ext-temp’ dataloggers. At Harvest 1, leaf fresh and dry weight and estimated microbial populations, but not ºBrix, significantly increased with application frequency. At Harvest 2, however, only plots receiving four applications registered significantly greater fresh weight and ºBrix values and microbial populations were lower than at Harvest 1. Other parameters were unaffected.