By Mike Bell
Maintenance of soil fertility is fundamental for sustainable farming
systems, with removal of produce (grain, forage) and losses through erosion,
runoff and leaching making nutrient replacement strategies essential to
avoid long-term fertility decline.
Native fertility reserves in the heavy clay soils of the northern grains
region have been depleted due to declining soil organic matter and continuing
negative nutrient balances. (A partial five-year nutrient budget for a
Central Queensland dryland cropping area is shown in Figure 1.)
Figure 1 Partial nutrient budgets for rainfed cropping fields in Central
Queensland from 1999 until 2005. Budgets are calculated as (fertiliser
nutrient input - removal in produce).
While declining nitrogen and phosphorus reserves have prompted increased
fertiliser inputs, recent reports of emerging problems with potassium
infertility in soils in the region have highlighted that sustainable nutrient
management strategies must embrace a more integrated approach to fertility
monitoring and nutrient replacement. Nutrient management must also keep
adapting to changes in the farming system, such as the move towards direct-drill
Most soil test calibration and nutrient response research in the northern
region was conducted more than 20 years ago, when conventional tillage
and resulting nutrient redistribution occurred at regular intervals.
Since then, there has been a widespread change to reduced/zero tillage,
often employing controlled traffic systems. The result is that soils are
becoming depleted of nutrients at depth, and nutrients now tend to be
concentrated in surface soils due to repetitive placement of shallow fertiliser
bands or return of crop residues.
Soil sampling strategies, developed when background fertility was higher
and residues were mixed into deeper layers with tillage, are increasingly
poor predictors of soil nutrient status as perceived by a crop grown predominantly
on stored soil moisture with infrequent (in some cases no) in-crop rainfall.
The impact of these soil and cropping system changes has not been reflected
in nutrient management practices on-farm. This NMI project has been structured
to fill key knowledge gaps to address future nutrient management in the
region, and provide guidance to locally focused, on-farm adaptive research
and development with growers and agribusiness that will ultimately fine-tune
outcomes to local soils and systems.
The project will develop an understanding of the implications of nutrient
stratification in rain-fed, directdrill systems with a concentration on
the major cropping soils across the region. The focus of this work will
be predominantly on phosphorus and potassium, due to their relative immobility
in clay soils and the need to balance their removal with fertiliser inputs.
The phosphorus work will focus on quantification of the decline in phosphorus
reserves (by determining the changes in both organic and inorganic phosphorus
pools down the profile), the role of subsoil phosphorus in meeting crop
demand during dry periods and the impact of alternative phosphorus application
strategies on crop productivity.
Similar work will be undertaken with potassium, with additional work
focusing on validation of a soil test to measure slow-release potassium
reserves (to provide a better picture of overall potassium fertility)
and on the interaction between potassium and other cations like sodium
(often in large quantities in subsoils in this region).
The project will also undertake collaborative work with the fertiliser
industry, regional agribusinesses and farming groups to develop an understanding
of existing nutrient budgets, and from them, identify likely emerging
issues and where they will most likely appear. Regional nutrient balances
will be developed in relation to current nutrient reserves, with this
analysis to provide a useful guide for future investment in monitoring
to ensure continued sustainability.
Dr Mike Bell is a soils agronomist in the Queensland Department of
Primary Industries and Fisheries, Kingaroy.
GRDC Research Code DAQ000084
For more information: Mike Bell, 07 4160 0730, email@example.com
North, South, West