Key points

• The major constraint to productivity needs to be correctly diagnosed before selecting an implement.
• Implements need to be customised to both soil constraint and amendment material.

Due to the variable nature of soil constraints across Australia and the range of ameliorants that are applied to address them, there is no universal application implement.

"Growers must firstly diagnose the most significant soil constraint affecting their crop productivity and its depth within the profile. They must then decide on the material they want to use to address the constraint and investigate its properties. This will influence their choice of implement," says Dr David Lester, from the Queensland Department of Agriculture and Fisheries (DAF).

Dr Lester is a member of a new GRDC multi-agency-invested research program in southern Queensland and northern NSW addressing these issues.

The field research component of the broader project is being led by Associate Professor John McLean Bennett, of the University of Southern Queensland.

Key collaboration in the work is coming from Soil Management Designs (Dr David McKenzie) and Dr Chris Guppy, from the University of New England.

The overarching project seeks to:

• provide diagnostic methods for constraints;
• provide empirical information on ways to combat constraints through research trials;
• deliver an economic assessment framework for growers and advisers to evaluate amelioration options against soil constraints; and
• deliver a collaborative learning component with growers.

"Addressing multiple soil constraints in a cropping system is a real challenge and it helps to take a step back and consider fundamental soil, water and plant interactions," Dr Lester says.

Increased water holding

Dr Lester and his collaborators are considering how to improve the soil's capacity to hold water to help avoid crop failure.

By considering fallow efficiency and ways in which the soil water profile can be replenished as efficiently as possible and potentially enlarged, the team is looking to make gains for growers.

Northern region cropping soils have high clay content and therefore potentially good water-holding capacity, but this can be compromised when soils are constrained by physical or chemical limitations such as sodicity, compaction, acidity and salinity.

Additionally, the climate can mean the surface soil dries out quickly, restricting plant access to surface nutrients and water.

"For these reasons we are taking a multi-faceted approach with a modified implement," Dr Lester says.

"Originally it was developed to apply up to four different fertilisers, making multi-nutrient experiments a one-pass operation.

"We've now traded-off some of that fertiliser capability to deliver amendment product instead.

"We have been particularly interested in deep phosphorus and potassium applications, as these are important nutrients which are immobile in soils and also becoming less accessible in no-tillage systems."

To this end, the use of composted manure as a nutrient source and organic amendment is particularly attractive for growers due to the accessibility of cattle feedlots in southern Queensland.

Typically, manure fertiliser is applied using a spreader and may be ploughed into the topsoil where, particularly in dry soils, the phosphorus and potassium may remain and be unavailable for plant roots.

Delivery systems

With local and international expertise, Dr Lester's team has modified an implement that can deliver free-flowing material at multiple depths.

"Our deep nutrient research used tynes on 50cm spacing for application. We've retained this layout for our amendment application, but boosted it up for the much larger amounts of material we are applying," Dr Lester says.

The delivery system was changed with a 75mm-diameter hose running down the back of each tyne to place materials from 20cm depth downwards into the soil.

The tyne layout allows fertiliser application to be separated from gypsum in the soil profile.

Testing revealed a wide application range with the metering system, allowing application rates from one to 20 tonnes per hectare.

Prescription maps are created to control what product goes on where. This makes it relatively easy in the paddock to establish the experiments.

"Yield data from our nutrient placement work has shown excellent responses, but the effects with our soil amelioration research may need more time to deliver full effect," Dr Lester says.

More information: (prototype) Dr David Lester, David.Lester@daf.qld.gov.au; (soil constraints project) Associate Professor John McLean Bennett, John.Bennett@USQ.edu.au