Growers with sandy soils could improve the ability of their legume crops to fix nitrogen (N) in the soil by addressing some key factors unique to their soil type. Good nodulation is key to ensuring N fixation takes place, however, one South Australian agronomist and his team who are researching options to improve cropping outcomes on sandy soils, believe growers can make a few extra changes to enhance the N fixation process.
Consultant Sam Trengove works across the Yorke Peninsula and Mid North regions, where pulse production on loamy red-brown earth soils is common. Lighter-textured sands also occur across the region, often as smaller areas in large variable paddocks.
Crop and pasture legumes can provide an inexpensive source of nitrogen for cropping systems. Soils can lack suitable rhizobia if they have not been introduced through previous inoculation, or hostile soil conditions limit their survival. In these cases, the nodulation process can be improved by inoculating crops with their specific rhizobia. Mr Trengove says the challenge for growers is that rhizobia are sensitive to soil moisture, acidity and other conditions, and growers need to follow the correct procedures for inoculation to ensure survival of the rhizobia and good nodulation.
Mr Trengove and Trengove Consulting team members Stuart Sherriff and Jordan Bruce are among a group of researchers working on a GRDC investment aiming to improve nitrogen fixation in pulses. He believes growers can complement their inoculation strategy with some farm practice changes to boost nitrogen fixation results.
Their trial work spans deep ripping and soil amelioration, soil pH and liming, herbicide safety and tolerance, variety evaluation and crop nutrition on sandy soils where growers commonly include legumes in the rotation. This work has focused on lentils in particular.
Mr Trengove says there are a number of rotational benefits from growing pulses, although high prices and the possibility of generating good economic returns is a key motivation. However, he says growers should also be considering nitrogen fixation benefits, as this has the potential to reduce fertiliser budgets across the rotation.
“There is a great deal of value to growing legume crops within a rotation when you consider how nitrogen fixation can benefit other crops in the system,” he says. “But we often observe that legumes, in particular lentils, struggle on certain soil types. Lentils can provide so much value to a cropping system, including on sandy soils.
“Growers on the Yorke Peninsula and in the Mid North have been growing pulses for a long time, and many would be dealing with sandy soils in some parts of their paddocks. It has been common for lentils to lack vigour throughout the season and mature earlier than the rest of the crop on the constrained sandy patches within a paddock, while leaving soil moisture behind after harvest.”
Australian growers sow inoculated legumes on about 2.5 million hectares – about 50 per cent of the total area sown to legumes – each year. The amount of nitrogen fixed by legumes is estimated at 2.7 million tonnes annually, which equates to a value of about $4 billion.
Mr Trengove says pulse growers should make sure their inoculation strategy is doing a good job and consider practice changes to increase nitrogen fixation opportunities. “Including legumes in the farming system can produce significant benefits, but these may not be realised if the legumes are not well-nodulated.”
Where background rhizobia numbers are low and inoculation responses are likely, he says, making small adjustments to inoculation techniques could deliver improvements to nodulation and nitrogen fixation, particularly when sowing into dry soils.
“For example, growers who are dry sowing should consider the effect this could have on the rhizobia in the inoculant that requires moist conditions to survive, especially on sandy soils. Pulse rhizobia are also sensitive to soil acidity, which further restricts rhizobial survival.
“South Australian Research and Development Institute (SARDI) trials have shown doubling the rate of inoculant is a simple strategy that can assist with effective nodulation in these scenarios.”
The northern Yorke Peninsula landscape includes some typical dune-swale soils, where the sandy soils can suffer from multiple constraints including non-wetting, low nutrient supply, acidity and compaction. To address these issues, the Trengove Consulting team tested deep ripping, spading, claying and applying chicken litter and synthetic fertiliser as options for soil amelioration, on a sand hill near Bute. The trials, conducted through the South Australian Grain Industry Trust (SAGIT), GRDC, and Northern and Yorke Landcare investment, highlighted deep ripping as having the highest return on investment for growers.
“Each dollar spent on deep ripping returned between $9 and $13 per hectare over the first three seasons, with responses continuing to be observed now, into the sixth season since treatment,” Mr Trengove says. “Averaged over the first three years, deep ripping saw a 600 kilogram/ha lift in grain yields each year, with the biggest yield gain observed in lentils, which appear to be particularly sensitive to soil compaction.
“Measurements of soil water after lentil harvest showed the crop had extracted significantly more water from deep-ripped soils compared with unripped soils, and from deeper in the soil profile. Higher-yielding treatments were also associated with increased lentil growth and biomass, which in turn is potentially linked with increased nitrogen fixation.”
This finding is being investigated further in 2020.
Sandy soils are prone to acidification and suffer from more rapid pH decline than heavier soils. Acidity restricts crop root growth and access to water and nutrients, and pulse rhizobia are also particularly sensitive to low pH.
“The emergence of acidity within these sandy patches is causing premature senescence in lentils,” Mr Trengove says. “We have several trials evaluating improved acid-tolerant strains for lentils and faba beans, as well as enhanced ways to ameliorate the acidity with the addition of lime.”
Caution is needed when using herbicides on crop legumes grown on sandy soils. The low clay content, organic carbon and cation exchange capacity of sand hills predispose these areas to increased risk from herbicide damage.
Mr Trengove says his research has highlighted management issues when it comes to safety margins for commonly used broadleaf herbicides on lentil crops on sandy soils. He was alerted to the effects of stacking pre and post-emergent chemical applications throughout the season causing damage to lentil crops through field trials in 2015.
Further research, funded by SAGIT, has shown some herbicides caused a significant reduction on crop biomass and grain yield on sandy soil types. For example, a yield loss of up to 20 per cent was measured in lentils when label rates of group B and C herbicides were used in sequence in the same season.
“Herbicides can have a cumulative effect when used together, with a larger negative impact on crop biomass, nitrogen fixation and, ultimately, yield. Herbicide strategies normally considered to have reasonable crop safety on heavy-textured soils caused significant crop damage in trials on sandy soils, which complicates the management of paddocks with variable soil types.
“On sandy soils it is often a tight balancing act to achieve adequate weed control while maintaining crop safety on sensitive crops like lentils.”
The same SAGIT research also investigated whether there were particular lentil varieties, or varietal traits, that perform better on sandy soil types. Mr Trengove says the research showed there is a strong connection between biomass and yield on sandy soils.
“Varieties with higher vigour tended to produce the best yields in our trials. National Variety Trials (NVT) are generally located on heavier-textured soils in the region and while PBA Ace is not a stand-out performer in these trials, it was the best lentil variety in terms of biomass and grain yield at our sandy sites.
“Conversely, PBA Blitz has been a reliable performer in NVT locally but exhibited low yields at the sandy soil sites.”
He says varieties with improved vigour on sandy soil types is an area for future investigation.
Their varietal evaluation work on sandy soils is continuing this year in a new project with SAGIT support.
“Identifying varieties that perform the best in this environment across a range of soil types is important in optimising the result across the whole paddock.”
More information: Sam Trengove, 0428 262 057, firstname.lastname@example.org