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The development of fertiliser application recommendations provides estimates of the nutrient needs of a crop compared with the likely supply of the nutrient from the soil , with the difference being made up by the application of fertiliser . The current recommendation system supported by the sugarcane industry , known as ‘ Six Easy Steps ’ specifies that : ( i ) The target yield is the ‘ district yield potential ’, which is defined as 120 per cent of the ‘ estimated highest average annual district yield ’; ( ii ) The nitrogen requirement of sugarcane is 1.4 kg / t , for yields up to 100 t / ha , plus 1 kg / t for yields over 100 t / ha ; ( iii ) A range of factors account for supply of nitrogen from mineralisation of a range of organic sources ( soil organic matter , crop residues , etc .). The district yield potential is seldom achieved ( Schroeder et al ., 2010 ), and there have been calls to revise the yield goal to something more closely aligned to yields farmers typically achieve , that is , the block ( or productivity zone ) yield potential ( Bell and Moody , 2014 ; Bramley et al ., 2017 ). This suggestion reflects the spatial ( i . e . block to block ) variability of the productive capacity of fields . While aligning production goals to block or productivity zone yield potential is an attractive concept , determining production goals at finer scales may be difficult . Year-to-year sugarcane yield variability can be greater at fine scales ( e . g . a block ) than large scales ( Schroeder et al ., 2010 ) making identification of yield potential uncertain . High climate variability between years ( Everingham et al ., 2007 ) and variability of crop response to different soil types are also limiting factors to assessing yield potential . To overcome these challenges , the use of model-based decision support systems to optimise nitrogen fertiliser management decisions will be needed and is reasonably common in other cropping systems in Australia and overseas ( Thorburn et al ., 2014 ).
Splitting fertiliser applications can synchronise the supply of nitrogen to match a crop ’ s requirements by applying frequent but small amounts of nitrogen to a field at many times during the growing season . Splitting nitrogen applications to sugarcane in the Tully region was predicted to reduce nitrogen losses , especially in years of above average rainfall ( Thorburn et al ., 2011c ).
The efficacy of burying fertiliser for reducing nitrogen in run-off over some weeks following fertiliser application has been confirmed in rainfall simulator studies ( Cowie et al ., 2012 ; DNRM , 2016 ).
Reducing erosion to reduce particulate nutrient losses . Managing loss of particulate nutrients is achieved through managing loss of fine sediments . Practices for managing particulate nutrient losses from grazing lands are thus addressed by those for managing fine sediment loss .
Factoring in other sources of nutrients to crops . Fertiliser is not the only source of nutrient inputs to crops , and it is necessary to consider the effects of these other sources on nutrient losses . Examples include irrigation water , biological fixation of nitrogen in legumes and application of mill mud . Irrigation water can contain substantial amounts of nitrogen , especially from groundwater sources in the Lower Burdekin which has high concentrations of inorganic nitrogen and irrigation applications are high ( DNRME , 2017 ). In these circumstances nitrogen applications can be appreciable , for example up to 150 kgN / ha ( Thorburn et al ., 2011a ), and so needs to be accounted for in determining nitrogen fertiliser requirements ( Schroeder et al ., 2014 ). This drives the need to monitor nitrate concentrations in groundwater and adjust fertiliser rates accordingly . The use of legume crops across the Lower Burdekin is increasing , although adoption of these practices is still generally lower than in other sugarcane growing areas . The nitrogen rates can be adjusted in the Six Easy Steps framework based on the legume species , biomass grown and whether it was grain harvested . Mill mud application is a common practice for farms close to the mills . The cost of transportation is the limiting factor to whether a farm uses mill mud or not . Again the Six Easy Steps framework has a mechanism by which to account for the application . These adjustments are also being revised in the paddock scale modelling .
Minimising run-off . Existing evidence indicates that dissolved nitrogen losses in run-off from irrigated sugarcane ( Thorburn et al ., 2011b ; Agnew et al ., 2011 ; DNRM , 2016 ) are generally not substantially different from those found in rainfed crops ( Prove et al ., 1997 ; Masters et al ., 2008 ; Webster et al ., 2012 ; Rohde et al ., 2013a ; Rohde et al ., 2013b ; DNRM , 2016 ). However , given that irrigation losses can be controlled by managing the amount and timing of application , there is scope to reduce nutrient losses through surface runoff and deep drainage by increasing irrigation efficiency . This is described further in Section 4.2.3 .
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