Fertiliser decisions in the wake of last year’s waterlogging
Last year’s extreme October and November waterlogging events continue to impact us and this time it’s in our fertiliser decisions.
At the Rokewood pasture site there has been a fall in sulphur below critical levels of 8mg/kg (KCl test) down to 4 to 6 which we can attribute to the leaching rainfall events in October and November and the high mobility of sulphur.
We can find high levels of sulphur deep in the soil profile (around 60cm) which can be a useful source for deep rooted established pastures but not for newly planted seedlings or shallow rooted clovers.
We also expect the autumn flush of mineralised nitrogen to be diminished resulting in sluggish pasture growth rates.
Nitrogen losses would have occurred in the waterlogged soils in October and November. Firstly, by denitrification where Nitrogen is converted into a gas and lost in the oxygen deprived waterlogged soils or through leaching. Sandy or light textured soils would have higher losses. Thirdly the mineral nitrogen originating from legume fixation and organic matter break down could have been used up supporting pasture growth over the wet summer.
That puts us in a dilemma, in terms of how early we apply nitrogen. Autumn application of urea is risky. In the warm conditions of autumn, if the soils are dry or there is only small rain events of 5 to 10 mm, the urea can volatilise and change into a gas and be lost.
Richard Eckard from University of Melbourne found only 3 trials in 18 located across the Western district provided autumn nitrogen responses in 2016. This was because the soils were mainly too dry or there already was enough nitrogen in the soils, so that the urea fertiliser provided no additional response.
To maximise nitrogen fertiliser response, we want good stored soil moisture, at least 50mm to quickly dissolve it and get it into the soil before losses can occur and grasses with a minimum of 2 or 3 leaves per tiller for pastures to respond.
So we will wait for now but look for opportunities to boost pasture growth once we are confident that the risk of nitrogen losses is low.
The other mobile or leachable nutrient is potassium. While we are above critical levels of 180 mg/kg (Cowell K) for pastures on clay soils at Rokewood, it should be one closely looked at in lighter textured soils.
While crops don’t always seem to show potassium responses, this may be due to them being able to access leached potassium at depth. Malcolm McCaskill at Agriculture Victoria found two canola experiments, with an available K of 155-160 mg/ha, recorded a 17% decrease in yield where K was omitted. They speculated that the extended periods of waterlogging these crops were exposed to, limited the ability of canola roots to take up available K from deep in the soil profile. Where responses did not occur, shorter periods of waterlogging enabled the crop to access subsoil K.
If you haven’t already checked out our Nitrogen in pastures seminar series, then go to the following links.
How to use fertiliser N successfully – Lee Menhennett, Incitec Pivot Fertilisers – Southern Farming Systems (sfs.org.au)
How to maximise the potential gains from N in perennial pasture systems? – Lisa Warn, Ag Consulting – Southern Farming Systems (sfs.org.au)
Photo: Potassium response in perennial ryegrass pasture trial at Murroon, Victoria. Muriate of Potash applied in a strip on left versus none or right.