Handle with care: Legume seed dressings a fragile lot

Soil microbiologist with the DPI, Ashlea Webster, says the practice of legume inoculation in NSW started in the 1890s with the first field trials conducted in 1950. It wasn't until 1955 that farmers could trust these magic potions to have any quality assurance. Today soil science can prove how live bacteria work in a symbiotic relationship.

At the heart of the matter is trade - with plant sugars swapped for nitrogen. The working gear are the pink nodules on roots, when everything is working effectively.

However these rhizbia are described by Mrs Webster as "delicate snowflakes" that are very sensitive to dry conditions, especially desiccating heat.

"Think of them as your livestock," she advises.

It is important to apply the right strain for the plant, with soybeans for instance requiring group H.

"Well nodulated soy is very efficient when it comes to fixing nitrogen compared to chickpeas for example," she said.

There is currently a new national program investigating improved strains of chickpea inoculant, along with assessing management practices to help boost nitrogen fixing potential.

Successful nodulation is important if a legume is going to sequester its potential of N, with 20 nodules per plant as an average being recommended for soybeans, though this number varies between plants.

"Poor nodulation can come from incorrectly applying the inoculant, or using the wrong strain," she says.

In some cases, especially when inoculation has not been performed, the issue is a low background population of the right bacteria and higher numbers of ineffective rhizobia.

More likely, the reasons for nodule failure down the track is a lack of consideration for the living critters within the inoculant at the time of seed preparation.

For instance incorrect storage can exposes the bacteria to heat and dry. Stock might be past its use-by date.

Leaving a mix exposed to the air for too long, rather than sequestered in the soil, is another simple reason for poor nodulation.

Freeze dried inoculant has no protection once opened and should be used in one go, as exposure to air can limit its life.

"Those products blended with peat have a little more protection but even so, they must be handled with care," she says.

Mixing bacteria with toxic chemicals is particularly tricky.

In sugar cane rotations on the North Coast there is a tendency to apply the broad-spectrum fipronil at planting to control lucerne crown borer with a requirement to coat the seed for best results.

At the same time there is a lack of legume friendly bacteria in soils that have been under cane rotations for four or more years, so inoculation of soybean seed is highly recommended.

Mrs Webster suggests keeping chemicals away from live inoculants, perhaps injecting the bacteria - with no more than 2 BAR of pressure - into the furrow under the insecticide-coated seed.

"The general rule is if you can't put the chemical in your eye then don't mix it with rhizobia," she says. "It's best to keep them separate."

Mixing rhizobia at a higher rate will help boost results - up to the point of clogging the seed delivery system with the sticky mix, especially when using peat formulations or sticker additives.

Some growers add peat inoculants to the seed hopper in dry form, but it is recommended to mix with water for best results.

Trace fertilisers play a role in the development of nitrogen-fixing nodules, particularly molybdenum.

Herbicide use can damage that interaction between bacteria and the plant.

In soils where there is high mineral N, legumes will source that form of nitrogen rather than pursue a trading arrangement with rhizobium.

"It is important to avoid adding too much inorganic N to inoculated legume crops at planting," says Mrs Webster. "You want to encourage optimal N fixation."