You ever wondered whether it’s worth inoculating your legumes? Or what your soil rhizobia levels might have been? Inoculating legumes plays a vital role in fixing nitrogen, but up until now, we’ve been only using educated guesswork to determine if it was necessary for certain paddocks. Now, there’s a new way to find out if it’s required or not.
Rhizobia bacteria is essential as it has a symbiotic relationship with legumes, where it is involved in the ‘fixing’ of nitrogen from the atmosphere. To allow this process to happen, you need an adequate number of rhizobia in the soil. Ideally, somewhere between 100 and 1000 rhizobia per gram of soil is required for rapid nodulation. However, even where you have had legume history and suitable rhizobia levels in the past, numbers can quickly fall below these required levels – this can be influenced by soil type, pH levels, management practices and temperature.
When rhizobia levels are low, it is recommended that the pulse seeds are inoculated with rhizobia at sowing. The more rhizobia that survive on the seed and remain in the soil until plant germination, the better the chances are of good nodulation on the developing roots and higher nitrogen fixation. Higher nitrogen fixation means better and higher-yielding legumes and more nitrogen built up for the following year.
Rhizobia numbers will vary throughout the year in the soil, hitting the lowest levels over the summer. Even on favourable soil types, when a host legume isn’t grown, it is normal for rhizobia numbers to decline.
SARDI has recently developed a PREDICTA RNod test that can measure the number of rhizobia in the soil capable of nodulating field peas, faba beans, lentils, vetch, lupin, serradella, and chickpeas. This will give a rapid and accurate assessment of your rhizobia levels in the paddock and whether you need to apply an inoculant. Previously, to determine whether a paddock is worth inoculating or not, we would have to look at; paddock history, when the last legume was grown, soil type, and soil pH.
Double-rate inoculation is another consideration that might be required to see benefits. This is particularly the case when sowing into soils with a pH below 5.5, dry sowing or when there hasn’t been a recent history of legumes in the paddock. This can be seen in the table below (table from to GRDC).
If you are interested in having one of these PREDICTA RNod tests done in a paddock going into any of the crops mentioned above this year, get in touch with one of our agronomists and we can get one organised.
Tip: Remember, legumes are very sensitive to acidic soils, and their rhizobia won’t survive below a pH of 5.5, so stay on top of your soil testing and lime where appropriate.