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
Wm. Strong et al., 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, Australian journal of experimental agriculture, 36(6), 1996, pp. 665-674
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.