Pastures the answer to beat long-term N decline

Author: | Date: 31 May 2013

For grain growers who thought they were already shovelling out the nitrogen every year, here’s a sobering thought.

In 10 years’ time, the CSIRO estimates continuous croppers will be applying 40 per cent more fertiliser N just to keep up with current yields.

This assumes the crop’s demand is constant at 94kg/ha and takes up half each of the mineralised N and fertiliser N.

And if you are still farming in 40 years’ time – or your children have taken on the farm – the rate of N application must double to maintain yields. With N prices climbing, the bill will be more than twice its current value too.

CSIRO research fellow John Angus says the reason for this is two-fold – the fact continuous cropping leads to an exponential decrease in soil total N and the absence of pasture N fixation.

N in continuous cropping

The rate of N decrease depends on inputs and outputs. Dr Angus says the rate of decrease appears to be linear over 10 to 30 years but after about 50 years, turns out to be curved and soil N eventually flat-lines.

Soil N levels can be expressed as a half-life, in a similar way to radioactive elements. For example, research has shown the level of total soil N decreases naturally at about 2-3 per cent a year – or 5-10kg/ha – in moist temperate regions when there is no other N input. A 2pc/year decrease gives a half-life for total soil N of 34 years.

At the Waite Institute in Adelaide, long-term trials of continuously cropped wheat with no added fertiliser gave a half-life of total N in the top 10cm of soil of 27 years.

But N soil measurements taken at a CSIRO tillage experiment near Harden, NSW, has shown more N was removed than first thought, where N’s half-life was only 14 years.

An experiment at Wagga Wagga showed that adding fertiliser N each year at 50 kg/ha only increased the half-life by 4 years.

The Harden trial compared four systems - zero tillage, one-pass tillage, stubble burning and stubble retention – but showed little difference in the decline in soil N between the four practices.

The level of soil total N is extremely sensitive to moisture - in lower rainfall areas, changes in total soil N are equal to the N removed by crops but in wetter areas, denitrification means more soil N is lost than crops remove.

This was widespread in southern NSW in autumn 2012, where waterlogging led to more than 50  kilograms a hectare of N disappearing from soils as microbes responsible for denitrification found perfect conditions in warm and anaerobic soil conditions after floods.

Biological N fixation

Dr Angus says the short-term price of N fertiliser will continue to fluctuate but in the long term, the price is likely to rise as fertiliser manufacture approaches a maximum efficiency. 

“If that happens, cropping in phased rotation with lucerne-based pastures growing at water-limited potential is likely to be more profitable and stable than continuous cropping,” he said.

Research last year by Dr Angus and CSIRO’s Mark Peoples evaluated the value of N fixation by phased pastures – both in soil and in dollars.

They found a farm with 60pc crop and 40pc pasture maintained a stable amount of soil N. Less pasture meant total soil N decreased (see graph). This assumed lucerne-based pastures and wheat grew at their water-limited potentials.

Dr Angus says in farming systems that include pasture phases, there is generally sufficient soil mineral N for crop growth up to stem elongation. Then top-dressed N can supply the rest of the crop’s needs.

“As soil N levels go down, the crops will start to experience N deficiencies earlier in their life cycle so there will be a need for more N at sowing.

“That’s clearly a risk because if a lot of fertiliser is added to the top soil at the time of sowing there will be rapid early growth and if the season goes pear-shaped you have a problem with haying off in spring.

“The way around that is for continuous croppers who are starting to use more N at or before sowing, need to consider low-release forms of N.”

He says there is likely to be more interest in polymer-coated fertiliser and denitrification and urease inhibitors and the less expensive option of banding urea or anhydrous ammonia at sowing for slow release N.

Profit from pastures

Using data from NSW Agriculture, Dr Angus and Dr Peoples found farm gross margins were relatively insensitive to the proportion of crop on the farm.

They compared the 60:40 crop pasture system with continuous cropping from 1980 to 2010.

While the gross margin for a continuously cropped farm was considerably greater than for the 60:40 farm in the early 1980s, by 2010, the gap nearly closed. This was because of higher returns for animal products.

Dr Angus says it is likely that gross margins underestimate the profitability of the mixed pasture-crop system by not accounting for other synergies between pastures and crops.

These synergies include overcoming herbicide-resistant weeds and the hydrogen fertilisation benefit.

“It has long been known that N fixation by rhizobia is accompanied by release of gaseous hydrogen,” he said.

“Normally, when that hydrogen is released into the soil it only moves a few millimetres where it is picked up by other beneficial microbes which stimulate the growth of the following crop. It looks as though that’s worth about 10pc on the yield of the following crop and it’s not a N effect.

“It is interesting that we are seeing a growth effect on the following cereal, apart from the N benefit, which may mean in some circumstances that the crop can respond to fertiliser N.

“This is possible in favourable seasons when topdressing may be profitable on a cereal growing after a legume. Part of the reason for a big crop after a legume is because hydrogen stimulates growth-promoting microbes.”

Dr Angus says this means croppers will receive the main benefit of pastures in the first year after a legume.

“Don’t fret about getting extra yield in second and later years after a legume, you have to cash in on the first crop, try to make a winner out of that.

“Phased pasture systems appear to be more profitable than continuous cropping. Minimising strategic inputs of fertiliser N will provide a comparative advantage over cropping systems with a high dependence on fertiliser N. The essential role of fertiliser N in a variable climate is topdressing and other tactical applications in response to favourable seasons.

“If you continuously crop, soil N will fall almost no matter what you do. To minimise these falls, you have to apply sufficient sulphur and phosphorus to maintain the stable ratios of C:N:P:S in humus, as CSIRO's Clive Kirkby has shown. In practical terms, the only way you can do that is with pastures or green manure.”

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