Nitrogen drives yield responses from subsoil manuring

Yield increases following subsoil manuring are more likely to be driven by the nutrients supplied than the deep ripping process or the manure, according to a major subsoil manuring experiment in south-eastern Australia.

Corinne Celestina presented her PhD research project on subsoil manuring at Incitec Pivot Fertilisers’ Agronomy Community forum in Melbourne in July.

She told more than 100 agronomists gathered at the meeting that the positive results from subsoil manuring were largely due to better nitrogen nutrition.

“My take home message to agronomists and growers is to focus on crop nutrition first,” she said.

Subsoil manuring involves the incorporation of large amounts of nutrient-rich organic matter into the soil by deep ripping to improve its physical structure.

“As you would know, some subsoil manuring trials have recorded yield increases of up to 70%,” she said.

“There are many soils with subsoil constraints across the medium and high rainfall zones. They have physical and chemical properties which could restrict the movement of air and water and limit root growth, such as sodicity and high bulk density.

“We know there are some places in the high rainfall zones where growers are only achieving 50% of water limited yield. Subsoil constraints have frequently been blamed for this gap between potential and realised yield, but are they the problem?”

In reviewing previous subsoil manuring trial results, Ms Celestina said the treatments which gave the best yield results were always the ones with the highest levels of plant nutrients, using amendments such as Dynamic Lifter or poultry litter.

But the design of the experiments did not allow her to determine whether it was the manure or the nutrients which improved crop performance.

This prompted Southern Farming Systems and Hart Field Site Group to establish new field experiments to assess crop yield responses to subsoil manuring. The researchers added a fertiliser treatment which supplied nitrogen, phosphorus, potassium and sulphur to match the nutrients in the poultry litter.

Quite high rates of fertilisers were needed to match the nutrients supplied in the poultry litter – between 590-630 kg/ha of nitrogen, 130-300 kg/ha of phosphorus, 270-400 kg/ha of potassium and 80-90 kg/ha of sulphur.

The treatments were also replicated at the surface to check whether there was a benefit in the deep placement.

The trials were conducted over eight sites from 2014 to 2016 on varying soils with subsoil constraints including alkalinity, sodicity, boron toxicity and low organic matter.

Key results

Ms Celestina told the Agronomy Community forum that these trials were “very different from the earlier subsoil manuring work”.

“We saw that subsoil manuring did not increase grain yields compared with other treatments,” she said.

The results also showed no response to tillage type, with the broadcast of poultry litter on the surface providing yields equal to the deep rip treatments.

The crops did not differentiate between the poultry litter and the matched nutrition from fertilisers, performing equally as well whichever was supplied.

“The yield results from the poultry litter were equal to using fertiliser with the same total nitrogen, phosphorus, potassium and sulphur rates,” she said.

“The carbon or microbial component of organic amendments did not show any advantage over chemical fertiliser.

“The yield increases we did see were most likely due to the alleviation of nitrogen deficiency.” 

While Ms Celestina said structural changes in the soil likely did occur following subsoil manuring, they did not appear to be driving the yield increases.

“There was limited change in soil structure and microbes in the bulk of soil away from amendment lines, which suggests that plant roots are driving the changes,” she explained.

She believes more properly designed experiments are needed to understand and manage subsoil constraints.

Ms Celestina’s paper ‘Crop yield responses to surface and subsoil applications of poultry litter and inorganic fertiliser in south-eastern Australia’ has been published in the CSIRO journal ‘Crop & Pasture Science’.

Agronomists and growers interested in subsoil manuring can also follow the progress of a $5 million GRDC project ‘Understanding the amelioration processes of the subsoil application of amendments in the southern region’. This will include a look at machinery design, monitoring of long-term experiments, pot trials and new field experiments.