IRRIGATION FREQUENCY AND NITROGEN FERTILIZERS MODIFY COTTON YIELD AT EMERALD, CENTRAL QUEENSLAND

Four irrigation frequencies and six nitrogen (N) fertilizer rates (0-3 00 kg ha-1) were applied to cotton (Gossypium hirsutum L.) grown on th ree Vertisols in the Emerald Irrigation Area, central Queensland. The purpose was to describe lint production responses to the plant availab le water before irrigation and N fertilizer, in terms of the crop N co ntent and the efficiency of crop N use for lint production. Lint yield was greatest when the plant available water before irrigation was 50- 80% of the plant available water capacity (PAWC) of each soil. The rat e of N fertilizer for maximum yield varied with plant available water and soil type. Plant available water before irrigation >60% and <37% P AWC, and rain after irrigation reduced the crop N content at the time of maximum leaf area index. Relative yield generally responded to 130 kg crop N ha-1, although the range from 101 to 141 kg crop N ha-1 refl ected differences in the maximum yield of each treatment. If the crop N was < 130 kg ha-1, yield was mostly determined by the crop N content , whereas if the crop N content was >130 kg ha-1, yield and the effici ency of crop N use for lint production was determined by the plant ava ilable water before irrigation and soil type. Nitrogen fertilizer stra tegies to achieve the maximum yield of cotton (var. Deltapine 61) shou ld focus on obtaining 130 kg crop N ha-1. This crop N content produced maximum yields for a range of plant available water contents before i rrigation, and for three soil types.