Preserving soil fertility

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).

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 cropping.

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,

Region North, South, West