Fertiliser N and P applications on two Vertosols in north-eastern Australia. 2. Grain P concentration and P removal in grain from two long-term experiments

Within north-eastern Australia's grain-production region there are few reports outlining nitrogen (N) and phosphorus (P) fertiliser effects on grain P concentration and P removal in grain. Two long-term N×P fertiliser experiments with different cultivation durations were conducted, one at 'Colonsay' on the Darling Downs in southern Queensland (commencing 1985 after 40 years of cultivation), and the other at 'Myling' on the north-west plains of New South Wales (commencing 1996 after 9 years of cultivation). Applications of N and P fertiliser independently influenced both grain P concentration and P removal for a range of summer and winter cereal and legume crops. Generally, if N fertiliser application increased grain yield, the grain P concentration decreased as grain yield increased; however, if grain yield did not respond to N fertiliser, grain P concentration was unaffected. P fertiliser applications typically increased grain P concentration. Wheat and barley grain P concentrations were generally higher in this subtropical region than reported values from temperate regions in Australia. Grain sorghum values were similar to those from subtropical areas overseas, but were greater than reported values from more tropical production zones. Mungbean and chickpea grain P concentrations were consistent with other reported values. Experimental results indicated grain P concentrations for estimating grain P removal in the northern grains region of 3400mg/kg for sorghum, 3500mg/kg for wheat and barley, and 4000-4500mg/kg for mungbean. At both sites, grain P removal was greater with summer and winter cereals than with legume crops. Larger grain yields with N fertiliser application had the largest influence on grain P removal at the Colonsay site, with an additional 23.3kgP/ha removed from plots with 80kgN/ha applied compared with nil N over 5 analysed crops from 1998 to 2003. Grain P removal was 20.9, 17.1, and 19.7kgP/ha in the 3 sorghum crops at this site in this period. Thus, application of P at 10kgP/ha.crop for this 5-crop study period did not replace P removed. In the predominantly winter-cropped Myling experiment with a shorter duration of cultivation and smaller N fertiliser response, cumulative removal was more influenced by P fertiliser, with 10kg fertiliser P/ha.crop generally sufficient to provide replacement P. These results support findings of negative P balances recently reported for grain production in this region and suggest a need for further investigation into the implications of a continuing negative P balance on the sustainability of grain production