nodulation in legumes

By David Jones, Broad Acre Agronomist, Msc. Agriculture

Legume nodulation. If there is one topic that we receive more advice on than any other, it is probably how to improve nodulation in legumes, and ultimately the nitrogen fixation, of our pulse crops. And yet in many areas of the country, despite addressing these factors, nodulation in legumes is still extremely hit and miss.

So let’s go back a step and remind ourselves of the factors that influence nodulation in legumes:

  1. Low soil pH (acidic)
  2. Low Molybdenum, Cobalt
  3. High soil Nitrogen
  4. A source of quality rhizobium inoculant
  5. Herbicides
  6. Seed dressings
  7. Soil structure and moisture

Rhizobium inoculants

pH is very interesting, not least because it may be affecting legumes in several ways. Firstly, the availability of other nutrients further down the list (low pH tends to reduce Molybdenum availability for example). Secondly, it also affects the viability of rhizobium bacteria itself. There is currently a lot of work going into acid tolerant rhizobium strains to improve nodulation on soils below the low pH 6’s. And Thirdly, legumes tend not to thrive in acid soils.  For now, should we be correcting pH with lime?

Largely it depends on the soil and the actual pH. I have some soils at 6.2 which is probably not limiting. However on lower organic matter soils (below 4% OM) I have seen good increases in nodulation where the soil has been limed. Interestingly however this seems to be far less reliable on higher Organic Matter soils.

I did a series of experiments earlier this year in Timau where there was NO nodulation where 5 t/ha of calcitic lime was applied on a 6% OM soil, either incorporated or left on the surface. And this was inoculated seed!!! In which case, does the pH need to be corrected several months ahead of planting?

Effect of nitrogen on nodulation

In fact, the most recent trial we have harvested last week had 20 l/ha of liquid lime in furrow at planting, and showed no difference in yield. Calcium availability was increased but pH was not altered when measured at harvest 19 weeks after application.

Check Nutrients With A Soil Test

Low levels of some micronutrients can result in poor nodulation, so if Cobalt and Molybdenum are low for example a seed dressing will help. Check with a soil test to make sure.

Another very interesting question is whether high soil nitrogen is inhibiting nodulation. There comes a point in high fertility soils where there is no point in the crop fixing nitrogen for itself if there is an abundant supply. After all, why would you expend energy cooking dinner for yourself if the restaurant next door is offering free meals?

To test this theory I applied a high dose of nitrapyrin (a nitrification inhibitor) to a 5% OM soil last year… but again, no effect! The idea was to lock up nitrate in the soil and trick the plant into thinking it needed to get on and fix some of its own.

Another really important point is that even if pH is not limiting your nodulation, calcium supply to the plant often is and I see this very frequently in leaf analyses.

effect of soil pH on nodulation

Quality Rhizobium

If you are limited by the absence of the correct strain of rhizobium bacteria in soils, you will absolutely have to inoculate. Perhaps our greatest limitation here is knowing whether the rhizobium have been stored correctly and not over heated, which will kill the bacteria.

Inoculant vs seed dressing

And finally, do not dismiss the effect that some herbicides can have on nodulation and plant growth…

Effect of herbicide on nodulation

Summary

  1. Use a quality inoculant from a reputable supply. Ask how it has been stored.
  2. Soil test and correct any micronutrient deficiencies
  3. Seed borne Ascochyta in peas is a bigger risk here than the lack of nodulation, so treat your seed whatever.
  4. Get your soil structure and drainage right.
  5. If you suspect herbicides of a certain seed treatment are negatively impacting nodulation, leave a small untreated area to find out.
  6. Try liming to correct pH, but be patient. It is better to do it well in advance than immediately before planting.

What Do Trials Mean? – Part 1

Trials is a subject that I have been wanting to tackle for a while in Think Agronomy, and not just for the benefit of those salespeople trying to push their products onto farm.

Having been involved in field trials since 2003, I want to let you into some of the secrets used in the trade and what questions you can ask as a farmer in order to see through the sales pitch.

barley wheat crop trials Kenya

I am going to be slightly controversial from the outset and make my view very clear that the quality of trials that I have seen in Kenya is very poor, and is seriously limiting our yields. If we cannot measure and evaluate new techniques, we cannot even begin to make progress.

The first point to understand is variation. It occurs across all fields, as this yield map shows us.

yield map

Yield maps are an extremely useful tool for observing large scale changes in management, but they are notoriously poor at showing small yield differences. When I tell you that this field all had the same treatment, you can see that a ‘tramline trial’ going up and down could give some very misleading conclusions because of natural variation and weighing error on the combine.

This is beautifully illustrated by our variety trial last year. If we had just looked at one plot of each variety – the first replicate – new variety RGT Planet did marginally better than Cocktail. Averaging all four replicates however shows a much greater difference.

RGT Planet variety

So this week’s lesson when you are shown yield data in the farm office by a salesman; ask “was the replicated?”!!!

Farming for the future requires a change of approach. Monoculture, soil degradation and climate change are threats to the future of how we feed the planet.

Agventure Ltd set up the Center of Excellence for Crop Rotation to help farmers diversify cropping systems and introduce techniques which have a long-term outlook to improve soil health.

The Center of Excellence for Crop Rotation works extensively with Crop Nutrition Laboratory Services Ltd (CROPNUTS).

Till next time,

Happy farming!

David

About David

 

David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.