Response of safflower (Carthamus tinctorius L.) to residual soil N following cotton (Gossypium spp.) in rotation in the San Joaquin Valley of California

Deep-rooted crops used in rotation can improve the overall water and N use efficiencies of cropping systems and help minimize nitrate leaching to groundwater. Safflower (Carthamus tinctorius L.) is a deep-rooted annual crop grown in Mediterranean regions that might be useful for this purpose. Safflower's response to residual soil N measured to 2-7 m in the soil profile was evaluated in 1998 in field plots in the San Joaquin Valley, California, USA that were used previously for cotton over a 9-year period and had been fertilized with nine N rates from 0 to 230 kg N/ha. Residual soil NO3-N measured prior to safflower planting increased with prior cotton fertilization rates. Amounts present to a soil profile depth of 2-7 m varied from 760 to 2600 kg/ha. Safflower seed yield increased with increasing pre-plant residual NO3-N levels, from 1700 kg/ha in the control to 2200 kg/ha, and then declined to 1800 kg/ha at the largest residual N level. Oil per cent, and oil yield were affected by soil N only at the largest residual N level, while oil composition was not affected. Root growth and N uptake at depth increased in plots with larger amounts of residual N compared to those with less. Results suggest that N fertilization applied to safflower could be reduced or even eliminated following crops previously fertilized at economic levels. Residual N should be accounted in growers' management programmes.