Long-term yield gain from fertiliser not changed by strategic tillage

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Key points

  • Soil inversion to ameliorate soil water repellence does not appear to change the residual value of phosphorus and potassium fertilisers
  • Tissue testing is a better monitoring tool for nutrition than soil testing after soil inversion
  • Surface soil testing for phosphorus is of limited value
  • Residual value of phosphorus and potassium is unaffected by soil inversion in water-repellent soils.

Soil inversion using a mouldboard plough or rotary spader to ameliorate soil water repellence has been shown to increase the availability of soil phosphorus, nitrogen and potassium.

However, little has been known about the long-term impact on soil nutrient availability and the residual value of phosphorus and potassium fertiliser in the years following application to inverted soils.

With the high cost of applying these fertilisers, residual value is an essential component of profitability.

As part of the GRDC’s More Profit from Crop Nutrition initiative, the Western Australian Department of Primary Industries and Regional Development (DPIRD) established a long-term field experiment at the Badgingarra Research Station, WA, at a site where the soil is a gravelly sand over gravel at 40 centimetres and severely water-repellent (MED 3.2).

Three cultivation treatments (untreated control, rotary spading and lime incorporated with rotary spading) were applied at the start of the trial in 2013.

Six nutrient treatments were applied to the same plots for the four years to 2016: nil nutrient control (rapid rundown), 20 or 40-kilogram phosphorus rundown (phosphorus fertiliser applied in the first year only), 40 or 80kg potassium rundown (applied first year only) and high nutrient control (fertiliser applied each year to ensure grain yield wasn’t limited by nutrients).

After four years, there is no evidence of a decline in residual value due to rotary spading or lime (Figure 1).

In 2016, grain yields in the phosphorus and potassium rundown treatments were not significantly different to the high nutrient control regardless of the cultivation treatment, despite the rundown treatments receiving no phosphorus or potassium fertiliser for three consecutive years.

The only interaction that occurred for cultivation treatments was for the nil nutrient control, where grain yield in the rotary spading treatment was less than in the lime plus rotary spading.

Comparing the grain yield and shoot nutrient concentrations at anthesis for these treatments showed that shoot potassium concentration was higher in the lime plus rotary spading treatments.

Tissue testing is a valuable monitoring tool for crops grown on ameliorated soils because it can be used to identify which nutrients are limiting growth by comparing tissue test results with the concentration required at each growth stage for maximum growth.

In the trial, rotary spading and lime plus rotary spading plots had the highest yields despite having soil test results for Colwell P (zero to 10 centimetres) that were consistently below the critical level of 18 milligrams per kilogram. This indicates that zero to 10cm soil testing may be of limited value after rotary spading because the nutrient-rich topsoil has been partly buried and the nutrient-poor subsoil has been brought to the surface.

Results also suggest that rainfall in February and March has a major effect on whether there is a nutritional benefit from ameliorating water-repellent soils. Researchers also observed a statistically significant yield benefit from the rotary spading done in 2013 for grain yield in 2014 and 2016 when February–March rainfall was limited to less than 20 millimetres. In 2013 and 2015, when there was about 50mm of rainfall during this period, there was no yield benefit from rotary spading.

Figure 1 Soil inversion treatments in2 013 had no impact on the residual value of phosphorus and potassium fertilisers to Magenta wheat in 2016 at Badgingarra, WA.

SOURCE: Craig Scanlan

GRDC Research Code DAW00222

More information:

Dr Craig Scanlan
08 9690 2174