Soil inversion needs nutrition tracking

Photo of DAFWA researcher Steve Davies

DAFWA researcher Steve Davies in the trial pits near Badgingarra, Western Australia, which are designed to assess nutrient availability after soil inversion techniques are used.

 PHOTO: Cox Inall Communications

Cultivation methods used to overcome soil water repellence can also change the distribution and availability of soil nutrients and moisture as the season unfolds

To optimise crop yields when cultivation has been used to overcome soil water repellence, researchers are recommending deep soil testing first, then surface soil testing after cultivation to get a better idea of subsequent nutrient needs.

A four-year trial near Badgingarra, in Western Australia’s northern agricultural region, is assessing the impact on nutrient availability, plant establishment, growth and yields of mouldboard ploughing to 35 centimetres, spading to 30cm and banding soil wetters in the base of the furrow.

Mouldboard ploughing and spading to invert the soil can change the distribution of phosphorus, potassium, organic carbon, sulfur and pH in the top 40cm.

Banded soil wetting agents are applied at the base of the furrow to help water entry into the seed zone and to potentially improve crop establishment.

It is hoped that research funded by the GRDC and the Department of Agriculture and Food, WA (DAFWA), now into its third year, will lead to more accurate guidelines for managing crop nutrition where these practices are used. Researchers have found at the Badgingarra site that cultivation has different effects on different nutrients.

Phosphorus uptake

Unlike most other key nutrients, the phosphorus profile in the soil does not appear to change radically with mouldboard ploughing or spading. This has been attributed to relatively high and even phosphorus concentrations to 20 to 30cm depth.

The soil being brought to the surface has a similar phosphorus concentration as the surface soil being buried. This could mean phosphorus fertiliser regimes can remain relatively unchanged after mouldboard ploughing or spading.

Uptake of phosphorus by HindmarshPBR logo barley plants in the 2012 trials at eight weeks after sowing was consistent at about two kilograms per hectare for mouldboard ploughing, spading, banded wetting agent and nil cultivation with no extra phosphorus.

Key points

  • Deep soil testing helps optimise results from mouldboard ploughing and spading
  • Phosphorus availability to crops is easier to predict post-tillage than nitrogen availability
  • Research is underway to better determine how soil inversion changes fertiliser advice

But when 20kg/ha of phosphorus was applied, there was a greater response from the nil cultivation plot in 2012 and 2011, suggesting the cultivated plots had more access to soil phosphorus.

The same trend in nutrient uptake was seen for potassium.

Nitrogen uptake

Water-repellence management techniques had a greater benefit for early plant nitrogen uptake in the trials.

DAFWA research officer Dr Craig Scanlan says plants in the mouldboard plough and spading areas accessed significantly more soil nitrogen as early as eight weeks compared with plots that were not cultivated.

Where 10kg/ha of nitrogen was applied in 2012 trials there was a five per cent increase in uptake by plants on the mouldboard plough areas and a significant increase in uptake on spaded areas eight weeks after sowing compared with the control and banded wetting agent areas.

With regards to pre-cultivation soil testing, Dr Scanlan says the trial results indicate that deep soil testing to 40cm appears to be more reliable for predicting phosphorus availability after mouldboard ploughing or spading than it is for predicting nitrogen availability.

He says surface soil tests after cultivation would also be more reliable for indicating phosphorus than nitrogen levels and availability at depth after treatment.

It is hoped the continuation of the Badgingarra trial this year and in 2014, coupled with other research being undertaken through the GRDC and DAFWA project ‘Delivering agronomic strategies for water repellent soils in WA’, and the GRDC and CSIRO project ‘Novel solutions for managing non-wetting soils’, will improve the prediction of deep soil nitrogen availability.

Soil moisture

Root measurements at the Badgingarra site during 2012 and 2011 found the treatments that resulted in the most roots following old root channels were nil cultivation and mouldboard ploughing. Few roots followed old root channels after spading and the application of the banded wetter.

In the 2012 nil-cultivation, spading and banded wetter treatments, 65 to 70 per cent of roots were found in the top 10cm of soil.

Where mouldboard ploughing was used, only about 26 per cent of roots were in the surface 10cm, indicating a large proportion was able to go deeper.

The increased root depth (10 to 20cm) in the mouldboard plough areas is significant because moisture levels at 10 to 20cm were 50 per cent higher than the nil-cultivation plots.

By comparison, there was about 50 per cent less soil moisture content in the top 10cm at eight weeks after sowing compared with the nil-cultivation and banded wetter treatments.

It is thought this difference in soil moisture storage was a result of changes to organic carbon distribution in the tillage treatments. Dr Scanlan says mouldboard ploughing reduced organic carbon at the soil surface, but increased levels at 20 to 30cm below the surface.

This means crops may not access nitrogen mineralised from this source during early growth, but may be able to access it later in the season.

Yield response

Grain yield responses using an average for all nutrition treatments at the trial in 2012 were highest in the nil-cultivation and mouldboard plough plots at 2.4 tonnes/ha, dropping to 2.2t/ha for banded wetter and 1.9t/ha for spading.

Crops grown on mouldboard ploughed areas performed better than those on banded wetter and spading areas after applications of phosphorus, potassium and nitrogen up to the maximum rates of 50kg/ha nitrogen, 20kg/ha phosphorus and 0 to 10kg/ha potassium.

At this level of fertiliser, grain yields from crops in the banded wetter treatments caught up.

Researchers speculate the higher grain yields in the nil-treatment plots compared to mouldboard ploughing, spading and banded wetter plots were driven by less soil water being available in these tillage treatments later in the season.

That is at odds with previous trial work and grower experience in the region, which has shown mouldboard ploughing and spading consistently yield higher than no cultivation in the first year.

Crop establishment has been poor at the Badgingarra trial site where tillage has been used in 2012 and 2011. This has been attributed to poorer seed–soil contact and surface soil moisture drying out more rapidly than in the nil-cultivation plots.

The Badgingarra trial has attracted significant local grower interest and good attendance at spring field days at the site. Research will continue until at least 2014 to further assess the impacts of non-wetting soil management on soil nutrient availability and crop performance.

More information:

Dr Craig Scanlan,
08 9690 2174,

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