Our first look at inoculation of peas left us asking more questions. We got a high level of inoculation on one paddock but very little in another.
Everything pretty much the same: Both were the same pea variety planted at the same rate on the same day with the same equipment using the same inoculator.
- Why did we get different effects?
- What encourages or discourages inoculation?
- How many nodules is enough?
GRDC recently published an excellent guide to inoculating legumes. Their Back Pocket Guide is quick to review and very informative.
Legume inoculation involves encouraging specific strains of rhizobia bacteria to colonise plant roots. They form a symbiotic relationship, fixing atmospheric nitrogen and getting resources from the plant. Each type of legume is nodulated by a specific type of rhizobia, and different legume – rhizobia associations will tolerate different soil conditions.
The GRDC Pocket Guide notes pea rhizobia prefer a neutral to alkaine pH of 6.0 – 7.5. It suggests peas nodulate with Inolculant Group E, but also with Group F. The likelihood of crop response to inoculation is said to be high where peas or vetch have not previously been grown. Where there has been a recent well nodulated host crop, the likelihoood of crop response if said to be low.
The GRDC Pocket Guide that after 8 weeks, 100 pink nodules per plant on heavier textured soils and 20 nodules per plant on lighter soils are satisfactory.
What did we do?
We planted Ashton peas VA 1301 on 12 November at 200kg seed per hectare.
Soil testing in June had showed the two Paddocks had almost identical nutrient status including nitrogen. Both were very low in sulphur. Paddock 2 had slightly more organic matter.
Becker Underwood Nodulator supplied by BASF Crop Protection was applied at slightly different rates in the two paddocks.
The seed was drilled by Parick Nicolle using his modified Great Plains drill. The Nodulator was in a separate box to keep it away from fertiliser and avoid separating out. New tubing directed the inoculator into the disc coulter so it was applied with the seed in the seed furrow.
2: Nodulator/seed tube junction above coulter
3: Inside view of double disc coulter showing final delivery spout with combined seed and Nodulator
What did we see?
Paddock 2 had the highest pea yield but least evidence of nodulation.
In Paddock 6 we saw significant development of nodules on the pea roots as shown in the photo above. We noted the nodules were much bigger than we are used to seeing, and appeared almost as a series of nodules fused together.
In Paddock 2 we found very little nodulation. Not even as much as we might usually see in crops in this region.
Why the differences?
The obvious background differences are irrigation and the paddocks’ histories.
Paddock 2 is our buried-drip irrigation block and the pea crop received significant irrigation which kept it above moisture stress point. Paddock 6 had no irrigation, and the crop did go into moisture stress. We suggest this is the primary cause of the yield difference.
Paddock 6 was planted out of long term pasture which had received little fertiliser in the last ten years. Paddock 2 was planted out of an over-winter cover crop of broad beans, a legume with potentially similar inoculant relationships to peas. The ground had been cultivated prior to bean planting, and the beans were chopped in to prepare the Paddock for pea planting.
So some possible mechanisms spring to mind:
- The rhizobium niches in Paddock 2 might have been occupied by “wild” strains. As the GRDC Pocket Guide indicates, where a recent bean crop has been grown, a crop response to inoculation is less likely. Even if the wild strains did not inoculate our peas, and we saw little evidence of nodules, they may have reduced the ability of Nodulator to do so.
- Possible higher soil nitrogen after the bean cover crop may have provided sufficient for the crop without a need for nodulation. In retrospect, some more soil nutrient testing at the end of the crop may have helped clarify the potential role of different nutrition.
Many thanks to:
Nicolle Contracting, True Earth Organics, Drumpeel Farms, BASF Crop Protection, McCain Foods, Ballance AgriNutrients, Fruitfed Supplies, Haydn Greville Groundspraying and Agronica, ThinkWater and Netafim and the Centre for Land and Water for supporting this preliminary investigation.