Maximising the nitrogen (N) benefits of rhizobial inoculation

Maarten Ryder¹, Matt Denton¹ and Ross Ballard²

¹School of Agriculture, Food and Wine, The University of Adelaide, ²SARDI, Waite Campus, Urrbrae SA

Take home messages

• Inoculation of legumes with rhizobia can deliver substantial N inputs to southern farming systems even when the impact on legume yield is small.
• Targeted, strategic use of inoculants, using a risk/benefit approach is the best and most cost effective way to maximise N inputs from legumes.
• To maximise the chances of getting a positive response to inoculation, follow the guidelines that are set out in recent GRDC publications.
• Care needs to be taken in situations where the survival of rhizobia is compromised, such as dry sowing, acid soils, mixing with fertilisers and pesticides; follow the guidelines.

Introduction

Inoculation of legumes with rhizobia is a standard practice. However, we can optimise legume nodulation and improve nitrogen inputs by following a few basic rules of thumb and fine-tuning inoculation practices.

A recent national survey of legume growers has yielded useful information about current farmer knowledge and practice in relation to rhizobial inoculation.  The results of the survey are being used to guide and refine key messages going out to growers.

Inoculation can greatly increase the amount of biologically fixed N from legumes where they are sown for the first time or where soils are not conducive to rhizobial survival. For example, inoculation of faba bean in south western Victoria boosted fixed N from 32 to 196 kg N/ha, as well as increasing dry matter production and increasing yield by 1 tonne per ha compared with an uninoculated crop (Denton et al. 2013).  However, it is also common for growers to get fixed N benefits from inoculation even when the inoculation only leads to a small yield increase.

You have probably heard the phrases 'if in doubt, inoculate' and 'inoculation is cheap insurance' as well as the message to 'inoculate every year'.  These messages are sometimes appropriate, but may lead to unnecessary inoculation in some instances, or alternatively cause growers to become cynical about the need for inoculation which can result in the sub-optimal use of inoculant.  It is possible to adopt a more targeted and strategic approach to inoculation and N management by using some basic rules of thumb as guides about when and where it is best to inoculate.

A risk/benefit framework can be used with respect to the likelihood of obtaining a positive response to inoculation to assist in decision-making, through consideration of soil type, legume species and inoculation history.

After making the decision to inoculate, it is worth maximising the chances of success, as inoculation failure is generally difficult and expensive to remedy.  Again, following some general guidelines will be helpful to ensure successful legume nodulation, noting that there is a range of inoculant products available, with different application methods.

Changing practices on farm, such as the trend towards early (dry) sowing in some regions, is taking us into new territory with respect to recommendations about rhizobial inoculation.  Another important and common practical issue is the degree of compatibility between rhizobial inoculant and fertilisers and seed-applied pesticides and additives. Although it would be useful to know the compatibility of each rhizobial strain with all of the common chemical formulations, this information is currently not available.

The recent national survey of legumes growers has highlighted the need for common-sense, practical guidelines so that inoculation can be practised successfully in the context of a grower’s preferred operations at sowing.  Several recent GRDC publications give useful information about optimising inoculation and nitrogen inputs from N fixation.  These publications are available online or from the GRDC, and are listed at the end of the paper.

Nitrogen fixation benefits

Legumes (crop and pasture combined) are estimated to fix almost 3 million tonnes of nitrogen each year in Australia, which is worth around $4 billion.  This amount of fixed N makes a substantial (around 50 per cent) contribution to the estimated 6 million tonnes of nitrogen that are required annually for grain and animal production on Australian farms.

The contributions made by legumes vary considerably with the species (Table 1) and with the situation (soil type, seasonal rainfall and crop management).  Crop legumes fix about 110 kg of N per hectare annually, on average (Table 1).  However the range is large, varying in individual paddocks from close to zero to more than 400 kg N/ha.

Nitrogen fixation generally increases with increased crop biomass, therefore good agronomic management leading to good legume growth will favour higher N inputs from fixed N.  There are also significant contributions of fixed N from legume roots (Table 1). In the southern Australian environment, legume growth is strongly influenced by the amount of water that the crop or pasture can access from the combination of stored soil moisture and growing season rainfall.  Management practices that optimise water use efficiency, and also keep soil nitrate levels low, will favour legume growth and N fixation. The fixed N is used by the legume itself for growth, but any root and shoot residues remaining after grain harvest or pasture grazing (for pastures legumes) will contribute to soil nitrate which can provide N to subsequent crops.

Nitrogen fixation is greater when soil nitrate is below 50 kg/ha and virtually ceases at nitrate levels above 200 kg/ha (Figure 1).   Nitrogen fixation by chickpea (Figure 1) and field pea is more sensitive to high soil nitrate than for faba bean.

Table 1. Estimates of the amounts of N fixed annually by crop legumes in Australia

¹Total N fixed = Percent N fixed x Total crop N; data sourced primarily from Unkovich et al. (2010).

Figure 1. Impact of soil nitrate on chickpea nitrogen fixation in northern NSW. Source: unpublished data of WL Felton, H Marcellos, DF Herridge, GD Schwenke and MB Peoples.

In addition to providing an N benefit, legumes can provide a disease break benefit to increase the productivity of following cereal and oilseed crops by reducing the inoculum levels of key soil-borne pests such as nematodes and also fungal diseases. Cereals grown after legumes generally out-yield cereals grown after non-leguminous crops, partly due to the N benefit and partly due to pest and disease control by the legume break crop.

When, where and how to inoculate?

There is a low likelihood of response to inoculating grain legume crops or pastures where there has been a recent history of inoculation with the appropriate rhizobia (i.e. the correct inoculant group); the soil pH is above 6 (in CaCl₂); and recent nodulation, grain yields and pasture production have been good.  In these situations, inoculation every four years or so will be adequate because soil rhizobial populations will generally be maintained at above 1,000 per gram, which is considered adequate for good nodulation.  After four years there is increased likelihood of a response to inoculation because the rhizobia that persist in the soil can lose some of their capacity to fix nitrogen, so a top-up with the potent inoculation strain may be beneficial.  If the legume species (or another that uses the same rhizobia) has not been grown in the last four years, or soil conditions are hostile, then the probability of a response to inoculation is greater.

Such is the case where acid sensitive legumes (e.g. peas and beans) are sown into acid soils (pH 5.5 or less in CaCl₂).  In these situations it will be prudent to inoculate every time a crop is sown because rhizobial populations tend to diminish quickly under these soil conditions (refer to Table 2).  The exception to this acid soil rule is lupin, because both lupin and its rhizobial strain are well-adapted to acid soils.

Table 2. Sensitivity of key rhizobia to pH (red is sensitive, dark green is optimal)

Where a crop such as chickpea, which has a very specific rhizobia requirement, is grown for the first time, inoculation is essential as there will be no background of suitable rhizobia present.  A double rate of inoculant is often used in these situations, to enhance the likelihood of good nodulation.

In the recent GRDC publications about rhizobial inoculation, ‘good nodulation’ and ‘well-nodulated crops’ are frequently referred to, and guidelines are given about adequate numbers of nodules per plant.  How do we go about checking this?  We strongly encourage growers and/or consultants to look below the soil surface, dig up several plants about 2-3 months after sowing, wash out the root systems gently and look at the level of nodulation on the roots.  This is important, as it will help a grower to decide on the need for inoculation in future years.  A guide to assessing nodulation in pulse crops is provided at www.agwine.adelaide.edu.au/research/farming/legumes-nitrogen/legume-inoculation/.

A visual check of root systems is worthwhile to establish if a reasonable number of nodules is present and well distributed across the root system or whether there has been a nodulation delay or failure. Carefully breaking open nodules to determine if there is a pink or reddish colour in the nodules will show that the nodules are active.  Neither of these visual assessments, however, will give an indication of the actual level of N fixation being achieved; sophisticated scientific techniques are required to measure this.

Common inoculation issues faced by growers

Can I sow inoculated seed into dry soil?

Growers in some regions want to sow legumes early into dry soil.  Sowing inoculated seed into dry soil is not recommended where a legume crop is sown for the first time.  On the other hand, where a legume has been used frequently and the soil is not particularly hostile to rhizobia, the risk of nodulation failure resulting from dry sowing is much reduced.  Rhizobial formulations which are applied in furrow, such as granules or peat suspended in liquid, are placed deeper in the soil and will have a better chance of survival as the soil conditions will be less extreme at greater depth.  There is also some evidence from field trials that placing the inoculum deeper in the soil is beneficial in a dry sowing, but it should be noted that there has not been a great deal of definitive research on this topic to date.

Can I mix inoculated seed with fertiliser, including trace elements?

Some growers claim success in mixing rhizobial inoculant with fertiliser and/or trace elements.  Rhizobium biologists recommend against mixing inoculant with fertilisers (particularly superphosphate and others that are very acidic) or other, novel plant nutrition treatments.  However we recognise that farming operations need to be pragmatic for practical and economic reasons.  Small scale testing is highly recommended where mixing inoculum with fertilisers and micro-nutrients is contemplated.  Tanks should be cleaned well before they are used for rhizobial inoculum.  Placement of the fertiliser or trace elements away from the rhizobial inoculum (e.g. in furrow below the seed) is highly recommended.  It is worth noting that the detrimental effects of mixing inoculants and fertilisers etc. are often overlooked because legumes are often sown in paddocks not responsive to inoculation.  It is only when a nodulation problem suddenly appears in a paddock that is responsive to inoculation, that the harmful effect of mixing rhizobia with other products is considered.

If molybdenum is required as a seed treatment (Mo is sometimes needed for optimum nodulation, especially in acid soils), then molybdenum trioxide or ammonium molybdate should be used, NOT sodium molybdate (toxic to rhizobia!).

Can I mix rhizobial inoculant with seed pickles and pesticides?

Some combinations of rhizobia with some pickles and pesticides appear to perform satisfactorily, whereas others are very effective at destroying rhizobia.  The booklet Inoculating Legumes: a practical guide (see further readings) contains a table on page 40 that lists the compatibility of different rhizobia groups with seed-applied fungicides, and also discusses specific compatibility issues between rhizobia and certain insecticides and herbicides.  Pickled seed can be coated with rhizobia (except soybean and peanut), but the time interval between inoculation and sowing should be kept to a minimum, usually less than six hours.  The use of granular inoculants or liquid inoculantion into furrows can reduce this impact by separating the pickled seed from the inoculant.

The following mixtures are NOT compatible with peat, liquid and freeze-dried inoculants:

• chemicals containing high levels of zinc, copper or mercury;
• fertilisers and seed dressings containing sodium molybdate, zinc and manganese;
• fungicides such as Sumisclex® or Rovral®
• herbicides such as MCPA, 2,4-D and Dinoseb; and
• insecticides containing endosulfan, dimethoate, omethoate, or carbofuran.

National survey of legume growers

The survey, conducted in 2013, comprised 18 questions that explored grower knowledge and practice in relation to rhizobial inoculation.  It was completed by 405 growers, representing a farmed area of just over 1 million hectares, across all GRDC regions.

Results are still being analysed in detail, but initial indications are available.  Growers generally had a good level of knowledge about rhizobia and their use, though ten per cent did not know that rhizobia fall into different groups that are specific to certain crop and pasture legumes.  Virtually all growers know that rhizobia are living organisms, but 22 per cent stated that it was fine to mix rhizobia with fertiliser and eight per cent thought it was acceptable to mix rhizobia with pesticides.  As discussed above, combinations and mixtures can work in some circumstances, but care must be taken to avoid incompatibility and the risk of inoculation failure.

Ninety percent of survey respondents reported that they used inoculants.  Of the ten per cent that did not inoculate, over half specified that inconvenience was a reason and also that the benefit was not clear.

Peat formulation was by far the most common method of application (used by 82 per cent of respondents).  Other formulations were also important however, including granules (19 per cent) and freeze-dried formulations (14 per cent).  A substantial proportion of growers used more than one type of formulation.

Further reading

Inoculating Legumes: a practical guide (GRDC 2012) Free, online at www.grdc.com.au/GRDC-Booklet-InoculatingLegumes

Inoculating Legumes: The Back Pocket Guide (GRDC 2013) Free, online at www.grdc.com.au/Resources/Publications/2013/09/Inoculating-legumes-back-pocket-guide

Fact Sheet: Rhizobial inoculants (GRDC 2013) Free, online at www.grdc.com.au//~/media/Documents/Resources/Publications/Fact-sheets/Rhizobial-Inoculants-Fact-Sheet.pdf

References

Denton MD, Pearce DJ, Peoples MB (2013) Nitrogen contributions from faba bean (Vicia faba L.) reliant on soil rhizobia or inoculation. Plant and Soil 365, 363-374.

Unkovich MJ, Baldock J, Peoples MB (2010) Prospects and problems of simple linear models for estimating symbiotic N₂ fixation by crop and pasture legumes. Plant and Soil 329, 75-89.

Contact details

Dr Maarten Ryder

PMB 1 Glen Osmond SA 5064

0409 696 360

maarten.ryder@adelaide.edu.au