STAGE 6: The Methods

Microbial inoculants are typically applied to row crops at planting either by seed treatment or in-furrow delivery.  Seed treatment is most widely used on a commercial scale because it is suited to management practices and requires less material than in-furrow delivery.  Foliar application to coincide with certain plant development stages is also increasing in practice. 

Addition of the microbial inoculant to the seeds can occur either as an overtreatment or at the same time as pesticide seed treatment.  Pre-inoculation of seeds weeks (or months) in advance is desired by farmers given the tight planting window. 
The potential for long-term viability of cells on inoculated seeds depends on the microorganism, cell protectant in the formulation, and pesticide compatibility. With pre-inoculation even several weeks in advance of planting, considerable cell loss on-seed can translate to more variable and potentially poor performance in the field.  Therefore, seed treatment of biological N-fixing products should occur as close to planting as possible.  

In-furrow delivery provides some advantages to the microorganisms in avoidance of harsh conditions during drying in a seed treater but requires larger quantities of inoculant compared to on-seed application.  Typically, inoculants are added directly to the in-furrow tank, which may include a mixture of starter fertilizer, fungicide, and/or insecticide.  The in-furrow liquid stream places the microbes in the general area around the seed, but also wastes considerable inoculant applied between seeds. 

Foliar application is receiving increasing interest in biological N-fixation, particularly for endophytic bacteria for their ability to colonize intercellular spaces in plant tissue for N-fixation.  Compared to more precision applications with seed treatment and in-furrow delivery, a downside to foliar application is the greater amount of material needed to spray leaf surfaces.   In addition, exposure to UV radiation and desiccation during drying on the leaf surface may cause microbe losses before the cells can colonize the plant tissue.  

In conclusion, the application methods for N-fixing bacteria, including in-furrow, foliar, and seed treatment, offer diverse options for harnessing the power of these beneficial microorganisms. Each method presents unique advantages and considerations.