Vegetable Farming: Seed and Soil Inoculation:  Is it Worth the Investment?

BY DEREK CHRISTIANSON
If you have thumbed through a trade publication recently (Acres USA, Country Folks, The Natural Farmer) odds are you’ve seen the increased marketing effort for biological inoculants.  As we re-launch a monthly column on vegetable farming in the SEMAP Vine, I decided to take a bit of time sharing my thoughts on the potential beneficial impacts of seed and soil inoculation ahead of propagation season.  Most growers are familiar with the use of Rhizobia bacteria inoculants for legumes to help populate nitrogen fixing nodules in the roots, beyond Rhizobia though the understanding and use of inoculants is more variable. Are you considering using mycorhizzal inoculants, trichoderma harzianum, bacillus subtillus, and/or azotobacter this year?  In this first installment (second part coming in March) we’ll start to review the why, when, and how of inoculating seed and soil.

Depending on the history of your fields, crop rotation, and tillage practices you may or may not have a healthy and viable population of soil microbes ready for action each growing season.  Inoculation can help to repopulate microbes after a fallow rotation; akin to boosting yogurt and fermented foods in your diet after a round of antibiotics.  Host specific microbes which work together with a specific variety of plant, like the synergistic relationship between legumes and rhizobia bacteria, are among the most commonly used inoculants in vegetable production, and are typically seed applied.  You’ll want to ensure you’re selecting the right type of rhizobia based on the legume you are planting.  I was most acutely reminded of this many years ago, when I planted edible soybeans on a field which had been in silage corn for a decade and was rotating out of a vetch cover crop.  Amidst the bustle of spring planting, I completely omitted the inoculation of the seed and sure enough come mid-summer I had a very nitrogen starved and yellow tinged soybean crop (the rhizobia which proliferated in the vetch stand, weren’t matched up with soybeans).  Back in the day, before commercial inoculants were readily available, farmers would often use soil to inoculate new fields; for example transferring and broadcasting soil from an existing alfalfa field (with it’s resident microbial population) onto a new seeding of alfalfa.  Once commercial inoculant sources became inexpensive and widely available, the practice of importing soil fell out of practice.  As a rule, I think its worthwhile to spend the time and money to purchase fresh rhizobia inoculant for all of my legumes each season; an especially important practice if you rely on 4 or 5 year rotations for your fields.  We slightly moisten the seed (using a very light touch will avoid making a mess of your seeder) and then sprinkle inoculant and mix to coat the seed.  It’s important to remember to store your inoculant properly if you are only using a portion of the package, follow label instructions and common sense to ensure your microbes are viable.  I like to inspect the roots during the growing season to assess our microbial population, it usually takes 2 or 3 weeks after germination for nodules to appear.  If properly inoculating legumes, inspection throughout the course of the season may alert you to other limiting factors (for example rhizobia require molybdenum to fix nitrogen, acidic soils critically short of molybdenum might limit nodulation even with proper inoculation).

Over the last decade a wide ranging variety of commercial inoculants have moved beyond rhizobia, to include a focus on mycorrhizal fungi and other beneficial soil microbes to advance crop health.  Over the years, I’ve reviewed both marketing literature and scientific studies in assessing how helpful other inoculants may be for profitable crop production.  One of the commercial inoculants which stands out as meriting attention is Trichoderma harzarium; widely available in commercial inoculants – including the popular Rootshield line from BioWorks.  Sweet peppers are among the crops which have shown positive response to the use of Trichoderma, which is a fungus which colonizes root systems and may prevent pathogenic fungi from reducing crop yields.  It’s sold and listed as a biological fungicide, but there is also research examining potential nutrient use efficiency of Trichoderma.

There are a wide array of scientific studies examining the impact of inoculants (including Trichoderma) on yields; a quick review or literature search will likely provide varying results.  I think its important to keep in mind that many research projects are carried out in controlled environments (i.e. greenhouses) and results may or may not correlate with field performance.  To that end, I think if you are considering the use of  new biological inoculants on your farm, its worth considering carrying out a controlled trial or two to help judge the impact of your actions.  We’ve done this to varying effect over the years and it seems some years crop response is more favorable than others.  A method I might suggest, which is by no means scientific is to trial inoculation of a crop by seeding 50% prior to inoculating the remaining 50% (for those interested in more rigorous research, SARE just updated their free publication “How to Conduct Research on Your Farm or Ranch”).

At Brix Bounty such a trial might look like this… seed 1×200 of a selected sweet pepper variety, then sprinkle Rootshield on the remaining seed and seed a 2nd 200 cell flat (you could then sprinkle additional root shield on the seed before covering – I often like to “over inoculate” if I’m setting up a trial.  Label each flat with Inoculant or No-Inoculant.  When potting up a few weeks later (we use 38’s or 50’s for peppers depending on the time of year), make sure to label each finished flat accordingly.  When transplanting track where each variable is set out in the field.  If you are very studious you could set out replicated plots (i.e. inoculated, non inoculated, inoculated, non inoculated, tec).  Monitor yields or plant health using observation through the season and assess the ultimate question “is it worth it”?  I like to carry out such trials over at least two seasons, before drawing any significant conclusions, to avoid attributing a positive or negative outcome to seasonal factors.

It’s important to keep in mind, that Rootshield is recommended for soil application in temperatures above 50 degrees and soil temperatures in the field or greenhouse may impact how the microbes colonize the root system.  Ordering fresh inoculant and following storage conditions is recommended by the manufacturer; we’ve ordered directly online, from seed companies (especially for rhizobia) and used Progressive Grower in West Wareham to source inoculants in the past.

In March we’ll discuss the potential benefits of mycorrhizal fungi, broad spectrum and indigenous microbe inoculation and some of the crops which have shown positive response to inoculation over the years.

The views expressed in this article are contributed by the author Derek Christianson, farmer at Brix Bounty Farm in Dartmouth.  Trade names are used to provide real world examples, but do not constitute an endorsement.

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