With the price of fertilisers skyrocketing over recent seasons, grain growers are searching for other ways to improve their nitrogen levels, including the planting of legume crops, but the Department of Agriculture and Fisheries has found the need for additional fertilisers cannot be effectively replaced by adding mung beans to the rotation.
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A trial was run at the Central Queensland Smart Cropping Centre which investigated how the application of nitrogen impacted the mung bean crop's nitrogen fix system, with results showing the more nitrogen present in the soil, the less the crop fixes.
DAF agronomist Doug Sands said trial findings also showed the mung bean crop exports more of its above ground nitrate into the grain than first thought, from 60 to 90 per cent of the total nitrogen uptake.
"We've got this situation where, if you're trying to manage your total nitrogen fertility, the mung beans are actually exporting more N than it's actually contributing," he said.
"The assumption with grain legumes is that they fix so much and that compensates for the amount of N that's being exported off in grain, and that some legumes would fix more so you'd get a net increase in your soil N, but mung beans is not one of those crops.
"What we're finding is that we're actually in net deficit of between 20 and 60 kilos of N after our mung bean crop in a number of situations.
"Where that has an implication for our farming systems is that if you don't put any further nitrogen fertiliser in to compensate for that, it's basically drawing down on mineralised N, or your organic matter pool.
"So, if you don't put any N in, the longer the fallow, the more mineralised N you end up with. If you then go and plant a mung bean crop on that, it then exports a heap of N off the paddock, so then you end up with a net deficit.
"We want to keep enough N in the system and at least balance it - we don't want it to drop, because as soon as it drops, then it starts knocking our organic matter levels down as well."
While legumes such as mung beans, have been found to fix nitrogen efficiently when planted in soil which is deficient in the nutrient, Mr Sands said the trial results showed that a mung bean crop would not contribute enough to be relied on as the sole source of nitrates.
"The assumption was that legumes are good for your fertility and they are in a way, but with a crop like mung beans, we found that, depending on how much N it's planted on will directly impact on how much it decides to fix, and that then impacts on whether you have a net deficit or contribution," he said.
"It will use as much nitrates as you can get a hold of, and then if it's a shortfall, it will just fix whatever it needs to so you can't get a yield response from applying N, but if you don't contribute some N to the system, you get this negative deficit.
"So, you've got this conundrum where you can get a direct yield response from putting that N on because the plant will just compensate whichever way it needs to.
"Basically what it's doing is it's complicating how you manage your N fertility when you've got legumes in the system. It's makes it a little bit harder to predict than if you were just doing back to back cereals, where there's no N fixation.
"So instead of legumes simplifying the story, they actually complicate it a bit more."
Although global fertiliser prices have fallen so far this year, Australians are still paying historically big money for urea, the most commonly used nitrogen input for broadacre farmers.
