Land: Soil fertility
- Authors
- Reviewer
- Key findings
- Indicators and summary of status
- Importance
- Pressure and condition
- Response
- References
Authors
Philip Bloesch and Phil Moody, Department of Natural Resources and Water
Reviewer
John Armour, Department of Natural Resources and Water
Key findings
- Soil fertility decline occurs when nutrient removal from an agricultural system exceeds nutrient inputs. The depletion of the soil's nutrient reserves will eventually result in crop yield declines.
- Soil fertility increase occurs when nutrient inputs to an agricultural system exceed crop demands. This may result in nutrient imbalances and off-site nutrient movement, causing pollution of groundwater by nitrates and deterioration in the quality of surface water as a consequence of nitrogen and phosphorus in runoff and sediments.
- Many industries are promoting adoption of best practice nutrient management, but often these practices (particularly fertiliser application rates) have not been well defined because of a lack of underpinning research.
Indicators and summary of status
Indicator |
Status of indicator |
Soil fertility risk |
In general, areas producing sugarcane and horticultural crops are in surplus with respect to nitrogen and phosphorus, and about 60% of the sugarcane areas are in deficit with respect to potassium. The rain-fed broadacre cropping areas of the Burnett and central Queensland are generally in deficit for nitrogen and phosphorus. |
Nutrient management initiatives |
Some catchment management bodies in the Great Barrier Reef catchment have introduced incentives to adopt best practice nutrient management on farms, developed codes of practice for nutrient management, and supported awareness and education programs. |
Importance
In all agricultural systems, significant quantities of nutrients are removed over time in harvested products, such as grain. Off-site losses of nutrients can also occur through soil erosion, runoff, leaching, and burning of crop residues. Gaseous losses of nitrogen may occur through denitrification and volatilisation. The usual management response to nutrient removal or loss is to apply fertiliser.
When nutrient removal exceeds nutrient inputs, the soil's nutrient reserves are depleted and eventually crop yields decline because the soil's nutrient reserves are inadequate to meet crop demands. The result is soil fertility decline. Conversely, when nutrient inputs exceed crop demands, soil nutrient levels increase. Off-site nutrient movement may occur, causing pollution of groundwater by nitrates and deterioration in the quality of surface water as a consequence of nitrogen and phosphorus in runoff and sediments. These are unacceptable environmental impacts.
Pressure and condition
The pressures affecting on-farm nutrient management are the need to produce economic returns in terms of productivity increases as a result of fertiliser inputs, and the need to demonstrate to government and community that nutrient management practices are not having harmful impacts on the environment. Both pressures will increase in the future and it is crucial that the land manager be able to make informed decisions about nutrient management that balance economic returns and environmental risk.
In the National Land and Water Resources Audit (NLWRA 2001), partial nutrient budgets (nutrient applied in fertilisers and irrigation water minus nutrient removed in harvested product) were calculated for each Statistical Local Area using Australian Bureau of Statistics yield data and fertiliser input data provided by the fertiliser industry. Because of the unavailability of updated statistics (particularly fertiliser inputs by crop sector), it is not possible to calculate current partial nutrient budgets for particular industries to track trend in condition.
An alternative approach has been adopted in this report to allow future updates as land use/nutrient management practices change. The framework for assessing the risk of under- or over-application of nutrients is based on a consideration of the recommended rates of fertilisation for a particular land use and average nutrient removal rates in harvested product for that industry. The Queensland Land Use Mapping Program (QLUMP) database provides the current land use according to the Australian Land Use and Management (ALUM) classification scheme.
Figures 4.6, 4.7 and 4.8 show current soil fertility risk assessments for nitrogen, phosphorus and potassium for catchments draining into the Great Barrier Reef lagoon (Bloesch et al. 2006). For each of these nutrients a partial budget (only fertiliser inputs, leguminous nitrogen inputs, crop removal and losses on burning were considered) was calculated to determine if nutrients were insufficient, in balance or in excess. In cases where nutrient inputs were insufficient to meet nutrient export, two risk categories were assigned, based on the soil's ability to supply these nutrients. Another category was assigned for the balanced state (nutrient input equals nutrient export). When nutrients were in excess (nutrient input exceeded nutrient export), risk categories were assigned based on the following criteria:
- for nitrogen, soil properties were used to determine if denitrification or leaching was the main process causing off-site movement of nitrogen; and
- for phosphorus, soil properties were used to assess the likelihood of phosphorus losses through erosion.
In general, areas producing sugarcane and horticultural crops are in surplus with respect to nitrogen and phosphorus, and about 60% of the sugarcane areas are in deficit with respect to potassium. The rain-fed broadacre cropping areas of the Burnett and central Queensland are generally in deficit for nitrogen and phosphorus. In particular, recent research has highlighted the emerging problems of potassium and phosphorus deficiency in rain-fed grain systems when limited rainfall during the growing season forces the crop to obtain its nutrient requirements from subsurface soil.
Response
Some catchment management bodies in the Great Barrier Reef catchments are initiating incentives for land managers to adopt best practice nutrient management on-farm. Several industries (sugar, cotton, vegetable) have documented codes of practice for nutrient management to guide fertiliser use. The sugar industry, in particular, strongly supports an awareness and training program for canegrowers that adopts a site-specific approach to nutrient management. Wider adoption of a regular soil testing program by landholders will allow monitoring of the soil fertility status of individual production blocks over time, allowing adjustment of nutrient inputs to achieve a balanced situation. While validated diagnostic soil test values for the major nutrients are available for some industries (sugar and grain, for example), very limited diagnostic data are available for most horticultural crops. This is a substantial constraint to improved nutrient management in some horticultural industries, which tend to be high users of fertiliser because its cost is relatively low when compared with the cost of other inputs such as labour and pest control.
Figure 4.6Nitrogen (N) fertility risk for catchments draining into the Great
Barrier Reef lagoon
Source: DNRW
Figure 4.7Phosphorus (P) fertility risk for catchments draining into the Great
Barrier Reef lagoon
Source: DNRW
Figure 4.8Potassium (K) fertility risk for catchments draining into the Great
Barrier Reef lagoon
Source: DNRW
References
Bloesch, P., Le Grand, J., Moody, P., Moss, J. and Searle, R. 2006, Soil erosion and soil condition hazard maps for Great Barrier Reef catchments of Queensland , QNRM06214, Department of Natural Resources, Mines and Water, Brisbane.
NLWRA 2001, Nutrient balance in regional farming systems and soil nutrient status , report to National Land and Water Resources Audit, Canberra, Australia, viewed 16 March 2007, http://audit.deh.gov.au/anra/land/farmgate/Nutrient_Balance.pdf.
Return to State of the Environment Queensland 2007 content page
Last reviewed 18 May 2011
Last updated 4 September 2007