CSO194 - Nutrient balance of a typical wheatbelt farm in WA and impact on soil properties
|Project Start Date||1 July 1998|
|Project End Date||30 June 2002|
|Supervisor Name||Mike Wong (Senior Research Scientist)|
|Organisation||CSIRO Land and Water|
|Contact name||Mike Wong|
This is the first report in Australia of nutrient balance at farm scale that takes account of the major processes controlling nutrient fluxes. The work included interacting with growers to access direct annual values for nutrient input from fertilisers and export in farm produce from farm records for each paddock dating back to the 1960s. It added value to previous research investments and developed methodologies to estimate (1) input from nitrogen (N) fixation under pasture and legume crops using yield values and knowledge developed from the literature; and (2) leaching loss by water balance modelling, coupled with pedotransfer functions developed from published data to estimate the nitrate concentration of drainage water. In addition to estimating leaching loss, this tool allows the risk of water contamination to be assessed.
In the 38-year period for which there was access to farm records, it was estimated that between 8-9kg N/ha/yr was lost by leaching. The nutrient balance identified depletion of potassium (K) and magnesium (Mg) as a problem confronting Western Australian (WA) grain growers. The effect of nutrient balance was commonly manifested in terms of acidification down the soil profile and accumulation of phosphate (P) in the topsoil. The main source of used was single super phosphate. The relatively high sulphur (S) content of this fertiliser led to positive S balance.
The work highlighted the importance of fixation during the pasture phase in supplying N to the farming system. N balance is negative under intensive cropping (more cropping and less pasture). This imbalance and the current trend towards cropping intensification may conceivably contribute to the long term negative trend in grain protein content measured in Australia.
Analysis of farm records and modelling uncovered imbalances in K, Mg, P and N. Chemical analysis of matching paired bushland and cropping sites confirmed the anticipated impact of this imbalance on soil nutrient reserves and soil properties. The decline in soil K and Mg is easily reversible with fertiliser application. The use of these elements in fertilisers is expected to be environmentally beneficial.
The tools developed for determining nutrient balance, nitrate concentration in drainage water and deep drainage and for financial analysis are valuable for economically and environmentally sound farm management. These tools should be incorporated into an EMS to allow performance to be mapped and managed.
Alleviation of nutrient imbalance is expected to improve yield, grain quality and profits. Progress is underway to address K imbalance. The uncovering of Mg and N imbalance is likely to result in further investigations and subsequent economic benefits. More broadly, the mapping tool for simultaneously measuring spatially variable nutrient recovery in grains, profitability, leaching and deep drainage risks is likely to lift farm management and result in financial benefits.
More efficient use of nutrients and water, resulting in lowered onsite impacts on land degradation by nutrient depletion, P accumulation and soil acidification, and offsite impacts on water quality and secondary salinity.
It is expected that economic benefits would translate in social benefits in terms of local jobs, less pressure to migrate to urban areas and a cleaner environment.
This project publicised the adverse impact of K depletion. This has resulted in a recent rapid increase in K fertiliser use, gradual reversal of the K imbalance and increased profits. The implication of Mg depletion in the wheatbelt is unknown and needs to be investigated, together with the implication of imbalance uncovered for N.
Spatial modelling of yield maps was used to determine the spatial patterns of nutrient removal in grains. In wheat, the lowest nutrient accumulation and removal in grains consistently occurred on white sandy soil types because of low yields. The low yield resulted in a corresponding low dollar margin for this soil type. Water balance modelling using the Agricultural Production Systems simulator (APSIM) showed that these sandy soil types were also the most leaky. For wheat, these are the poorest performing soil types both financially and environmentally. The APSIM model was further applied at a regional scale and scenario modelling showed that up to about a half of the regional area composed of the sandier soil type should be planted to perennial vegetation in order to decrease deep drainage by a third. The inclusion of lupins in the farming system, however, changed the spatial pattern of crop profitability, as this crop consistently performed better on the sandier soil types. This reversal of match between crop performance and soil type complicates the delineation of management zones for land use.
This project's findings were disseminated at industry meetings and published for the wider scientific community. The tool for measuring spatially variable nutrient balance and profits can be implemented as a monitoring tool in EMS.
Development of monitoring tools for EMS based on ISO14001 to take account of spatially variable efficiency of nutrient and water use should be given high priority.
Yield maps are currently accurate. In order to match this accuracy, there is a need for tools for more direct measurement of spatially variable pasture production than current estimates from available water by satellite imagery.
Mg is an important component of chlorophyll and is needed for photosynthesis.There is a need to evaluate the potential implications of Mg depletion on cropgrowth and grain yield.
A priority is scenario analysis of land use options based on climate and longterm sustainability in order to determine impact on drainage, soil properties and profits.
There is a need to investigate the implications of N imbalance on the current negative trend in grain protein content in Australia.
The inputs of Ρ and S exceed removal of harvested products by a factor of two to five. Soil Ρ measurements showed that available soil Ρ (Colwell P) was typically above 20mg/kg in most paddocks on the two farms. Corresponding Colwell Ρ on similar soils in bushland in the region was less than 5mg/kg. Although Ρ is relatively immobile, its accumulation may result in increased risk of Ρ transfer to surface and groundwater bodies.
It was concluded that the traditional wheat-pasture rotation system with adequate Ν inputs in the wheat phase appears to be sustainable in terms of Ν balance and that crop intensification may result in Ν deficiency at current rates of Ν fertiliser use. Accumulation of Ρ in the cropping systems in the wheatbelt may result in increased risk of Ρ transfer to surface and groundwater bodies. Under current cropping management, soil Κ and Mg reserves are at risk of depletion. Low soil Κ is becoming a limiting factor to crop production and fertiliser Κ addition is necessary, especially on sandy soils. The yield implication of Mg depletion from the limited soil stock in the wheatbelt is still unknown.
Current nutrient flows
Yield maps available for the more intensive cropping system (Farm 2) allowed determination of the spatial pattern of nutrient removal in crops since 2000 and an examination of these patterns in relation to soil types, irrespective of paddock boundaries. An example of Ν removal by wheat (blue) and lupins (green) harvests in 2000 is shown below (Figure 4).
Figure 4: Nitrogen removal in wheat based on yield maps and nitrogen content.Only a small area (65ha) of white sand (marked 3 on map) was planted to wheat in 2000. Total Ν recovered in grain from this area was 42kg Ν/ha, compared with 51kg Ν/ha recovered from the remaining soil types.
Changes in soil chemical properties during cropping
The effect of cropping on soil properties was measured on matching paired cropped and bushland sites by chemical analysis of the soil profile. Nutrient imbalances resulted in lower Κ and Mg contents in the soil profile and increased Ρ content (Figure 5). Acidification occurred during cropping and in one case (Department of Agriculture WA site), this was treated by liming. The effect of lime applied at the AgWA site in 1999 was restricted to the topsoil.
Figure 5. Comparison of soil properties by depths (y-axis in cm) under matched cropping (open circles) and bushland sites (solid circles).
The Economics of nutrient balance
Table 2 represents the ratio of fertiliser input to cropping income for each of the five soil types for Farm 2 for the period 1998 to 2001. A higher ratio represents lower income relative to fertiliser use. The white sandy soil type generally had a higher ratio, except for in 2000, when this soil type was mostly sown to lupins. The lower ratio was due to the lower application of fertiliser to the lupin rotation. This apparent efficiency on the white sand soil type was offset by a lower income return from lupins in 2000 (Table 4).
Table 2. Ratio of Fertiliser Cost to Cropping Income by soil type, 1998-2001.
M.T.F. Wong, K. Wittwer and H. Zhang (2000). Historical nutrient balance at paddock and whole farm scales for typical wheatbelt farms in the Dowerin - Wongan Hills area. Crop Update 2000.
|Published Date||29 May 2018|
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