Uneven germination puzzle after soil work
GroundCover™ Issue: 126 | Author: Jo Fulwood
Growers in WA’s northern grainbelt are seeing unexplained germination inconsistences up to seven years after mouldboard ploughing, spading and deep ploughing
While much has been documented over the past few years about the benefits of soil amelioration, some growers are also reporting inconsistent germination on post-ameliorated paddocks.
Mingenew grower and GRDC Western Panel member Paul Kelly suspects pre-emergent herbicides used on soils that have been mouldboard ploughed, spaded or deep ploughed are still available for uptake by the germinating crop, with the effects seeming to last several years after the initial amelioration.
But this ‘increased plant-available herbicide’ theory is only one of the possible causes being considered while the issue is being investigated further. Another Mingenew grower, Stuart Smart, says he too has noticed his lupin crops (Mandellup and Gunyidi) have been slower to come out of the ground in the mouldboard-ploughed paddocks. However, he is wondering if the cause is a lack of available built-up fertiliser in this top layer because of the change in soil structure.
Stuart plans to trial liquid nitrogen on ploughed paddocks to see if this has any effect on germination.
“The soil microbes could be attacking the small amount of fertiliser that is there, meaning the plant can’t access it,” he says.
“We are hoping to see some positive results from a trial where we spray liquid nitrogen as soon as we sow to see if it feeds the plant enough to overcome this slower germination problem.”
Paul Kelly says his lupins (Coromup and Gunyidi) planted in the second year after deep ploughing with a one-way modified disc plough over three different paddocks also showed staggered germination. He wonders if this could be a result of a few factors, particularly the lack of organic material left on the surface, and in the topsoil layers, to help absorb the pre-emergent herbicides that had been applied.
“Normally the chemicals, especially Brodal® because it is water activated, get bound up in the organic matter in the topsoil.”
His theory is that because there is limited organic matter in a ploughed paddock, the crop plant could be taking everything up – creating an ‘increased plant-available herbicide’ effect.
Paul says growers do not fully understand the issue at this stage and that anyone planning to undertake soil amelioration needs to be aware of the potential effects of a ploughed paddock combined with pre-emergent herbicides on seed germination – if the ‘increased plant-available herbicide’ theory proves to be the case.
Despite the puzzle, all growers who are reporting germination issues on ploughed soils are quick to point out that the benefits of soil amelioration still outweigh these germination irregularities.
“Despite the staggered germination, the crop did compensate, and the long-term benefits of ploughing were still obvious,” Paul says.
“For example, we ploughed a paddock that had some severe non-wetting problems, and in fact that particular paddock had only produced one successful crop in my farming career, but after deep ploughing we are now seeing wheat crops of around two tonnes per hectare,” he says.
CRT senior agronomist Owen Mann has been assisting growers in WA’s northern agricultural region to implement soil amelioration strategies for several years.
Owen has witnessed the challenge of staggered and slower germination in many crops in ploughed and spaded paddocks across the region.
“Since ploughing can bury a lot of the organic matter, along with less mobile nutrients available in the topsoil, it always takes a while for the plant to get their feeder roots into the bubble of the nutrient-rich soil,” he says.
“Also, organic matter holds and binds not only nutrients, but also has a large influence on the water-holding capacity of the soil.”
Owen says pre-emergent herbicides work better in moist environments, so he is wondering if the leaching of some of these herbicides into the root zones may also be affecting plant growth.
He also points out that one of the biggest issues after soil amelioration is getting an accurate and consistent seeding depth: “This is so you are not burying too deep for the seed to germinate, or too shallow into pre-emergent herbicides.
“In terms of machinery, ideal seeding depth may not have changed – but because you are changing the soil structure, the accuracy of the seeding depth is much harder to establish.
“Also, since the soil is ‘fluffed up’, you may not be getting the same seed–soil contact as you normally would on an unploughed paddock.”
Another Mingenew grower, Daniel Michael, also saw both staggered germination and what he terms ‘silly seed’ germination (where the coleoptile grows in an odd direction) in his second-year ploughed paddocks.
“The first year after each mouldboard plough, we don’t use any pre-emergent, we just use a normal knockdown to kill any weeds,” he says.
“The ploughing appears to be good enough to bury 95 per cent of the weed seeds for that first year. But in the second, third and fourth years we have seen slower, staggered germination, even after we have cut back on the rates of trifluralin from 1.5 litres/ha to 1L/ha.
“Whether this effect is because of a lack of organic carbon to soak up the pre-emergent herbicides or not, we aren’t sure.”
The Michaels have ploughed 1200ha over the past four years, with plans to plough 300ha each year for the next few years, if the conditions are wet enough in the pre-seeding period.
“We know the seeding depth is really important. You can’t go too deep, but then again, it needs to be deep enough that the pre-emergent doesn’t affect it. We are still trying to get this part of the process right – we are learning every year.”
In explaining his observation of lupin crops being slower to come out of the ground in the mouldboard-ploughed paddocks, Stuart Smart says that when he would expect to see the crop at three-leaf stage (at three weeks), it will be two leaf.
Stuart says he has seen an impact on the germination of his crop up to seven years after ploughing.
“We haven’t seen staggered germination, but the crops are certainly slower to germinate,” he says.
Nonetheless, Stuart says the benefits of ploughing still far outweigh the potential issues associated with germination. He has ploughed 16,000ha over the past seven years.
“We believe the benefits of ploughing will last up to 15 years, and after that time we’ll probably start ploughing again,” he says.
More information:Paul Kelly,
GRDC Western Panel,
0427 275 022,
Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers’
Pre-emergent Herbicide Use Fact Sheet
‘Delivering agronomic strategies for water repellent soils in WA’ – fact sheet – DAFWA project 'Delivering agronomic strategies for water repellent soils in WA' (DAW00204) Liebe Group project 'Improved stubble and soil management practices for sustainable farming systems in the Liebe area' (LIE0006)
‘Pre-emergents on ploughed soil questioned’ – GroundCoverTM November – December 2015