SUSTAINING PRODUCTIVITY OF A VERTISOL AT WARRA, QUEENSLAND, WITH FERTILIZERS, NO-TILLAGE OR LEGUMES .2. LONG-TERM FERTILIZER NITROGEN NEEDSTO ENHANCE WHEAT YIELDS AND GRAIN PROTEIN

Cereal production in the summer-dominant rainfall region of Australia, especially the north-east, has relied heavily on natural soil fertili ty. Continued cereal production has so depleted the fertility of some soils that corrective strategies are required to restore the productio n of high protein wheat needed for domestic and export markets. Applic ation of nitrogen (N) fertilisers, along with other strategies to impr ove soil N status, was evaluated between 1987 and 1994 on a Vertisol l ocated in an area of unreliable winter rainfall. Responses of wheat gr ain yield and protein content to applied N (0-150 kg/ha) under zero ti llage (ZT) and conventional tillage (CT) were determined each year, ex cept 1991 when severe drought prevented wheat sowing. The ZT practices increased grain yields, particularly in 1988 and 1992-93 when anteced ent soil water supplies were moderate (about 1 m wet soil in 1988 and 1992) or low (about 0.6 m wet soil in 1993), apparently due to increas ed antecedent soil water. Tillage practice had little effect on availa ble nitrate-N (kg/ha) to 1.5 m, but the greater water supply in ZT soi l usually benefitted the wheat crop when N was applied. Applying N inc reased returns from 5 of the 7 crops because of grain yield and/or gra in protein responses. Grain yield responses were inconsistent in the y ear of fertiliser application where no N fertiliser had been applied t o preceding crops. Nevertheless, grain protein usually increased with increasing N application at sowing, except in 1994, when drought after sowing prevented secondary root development and fertiliser uptake. Wh ere N was applied with each successive crop, the crops receiving small N applications (0, 12.5 or 25 kg/ha.crop) produced grain of a low pro tein concentration (<10%) and lower yields (<90% maximum yield) than c rops which received larger N applications (75 kg/ha.crop). Profits wer e substantially reduced where the rate of N applied was insufficient t o raise grain protein concentration to greater than or equal to 11.5%, due to the low market value of low protein wheat, or because of lower grain yields. Routine N application to crops over the period 1987-94, which included the longest drought (1990-94) in the lifetime of most producers, caused similarly inconsistent grain yield increases but inc reased grain protein concentrations (>11.5%) in all except the first c rop (1987). Increased frequency of high protein wheat and a high antic ipated market value of the higher protein grain should encourage great er producer confidence with routine application of N throughout this r egion.