14 Aug When Should You Inoculate Your Legumes?
When thinking about seeding your pulse or soybean crop, and the multitude of crop inputs available for them, you may be asking yourself: Do I need an inoculant this year? And if I choose to use one, when and how should I be applying it? In this week’s edition of Growing Possibilities, we will be discussing why an inoculant is an essential tool to pulse and soybean growers, and when & how you apply it is important.
Soybeans and Pulses have the ability to fix their required nitrogen (N) from the air. This is accomplished by forming a symbiosis with rhizobacteria that fix this N for them. Ensuring the bacterial species that forms a symbiosis and creates nodules with your specific crop is in your soil is the most essential step in getting your plants the N they need. Inoculating your pulse or soybean crop is an easy way and one of the least expensive ways to improve your crop yield. It only takes a 1/3 to ½ bushel per acre yield increase to pay for the costs of inoculation for soybeans, an easy task for most inoculants. It takes 4.8 lbs of N to grow one bushel of soybeans on average (1), so yearly inoculation most probably the cheapest form of insurance you can buy to make sure your crop is getting the N without investing in the N fertilizer. Rhizobia from previous inoculations can persist in soil and can sometimes nodulate a new crop, but most often these bacteria are not enough. Strong nodulation and optimal N fixation require fresh, healthy populations of bacteria that are selected & grown specifically for nodulating & fixing N for whichever crop you have chosen. Field flooding is another issue that will reduce the bacterial populations in your soil, as air-breathing rhizobia will drown in water logged soil. Supplying fresh, healthy bacteria to replace those lost during flooding will bring your soil’s nodulating power back up to standard.
If you are using an N fertilizer on your crop, this may reduce nodulation. If soil N is high, plants will use this N resource first before forming symbiotic relationships with N fixing bacteria. Generally, soil N levels above 35 lbs/acre will reduce nodule formation and N fixation by bacteria, so take this into account before deciding to inoculate (2).
Once you have decided that you are going to inoculate, the next step is to figure out how you are going to apply. Applying an inoculant on-seed prior to planting or in-furrow during planting are the two major ways of application. Both method works but we believe that seed inoculation may be more advantageous than in-furrow application. However, choose an application method that suits your operation. For example, a farmer who buys pre-treated seed may choose to have their seed dealer apply an inoculant with the rest of the seed treatments to save themselves the time and effort of applying it themselves.
Dual inoculation is recommended when planting on virgin ground. In general, dual inoculation is the application of both a liquid inoculant on-seed and a liquid or granular inoculant in-furrow. Dual inoculation increases the rhizobia population in virgin and new legume acres and is a recommended best practice for virgin legume acres. Dual inoculation has been shown to have a greater effect on soybean crops with the development of new varieties and increasing new acres in Western Canada (3).
Whichever method you choose, inoculation must be done either before or during seeding. Applying an inoculant later in the season is not an option. Noticing a lack of nodulation during the growing season and deciding to apply inoculant after the fact is not a solution, as bacteria need to be present when the seed is first put into the soil to form symbiosis for proper N fixation.
1) Cafaro La Menza N, Monzon JP, Specht JE, Grassini P. 2017 Is soybean yield limited by nitrogen supply? Field Crops Res. 213, 204-212.
3) Quarry Seed. 2014. Inoculant and seed treatment trial. 2014 Oakville, MB Summary Report. Quarry Seed, Stonewall MB.