Nitrogen and phosphorus fertilizer and residual response in cotton-sorghum and cotton-cotton sequences

Nitrogen and P fertilizer response for cotton (Gossypium hirsutum L.) and sorghum [Sorghum bicolor (L.) Moench] in a reduced tillage rotation system in the Southern High Plains has not been well studied. During 2000 to 2003, an irrigated study of cotton-sorghum rotation vs. continuous cotton evaluated the crop rotation effects on cotton lint yield and assessed N and P fertilizer and residual fertilizer response for the two systems. Preplant soil samples were collected each spring to determine fertilizer rates. Cotton lint yields and cottonseed N were not affected by rotation with sorghum compared with continuous cotton. Nitrogen fertilizer response was observed in lint yields from 2001 to 2003 in cotton following sorghum, but not in continuous cotton. No P fertilizer or soil residual P response in cotton lint yields was found, regardless of rotation. Grain sorghum yields responded to N fertilizer in 2 yr. No grain sorghum response was observed to P fertilizer, but in 1 yr a yield response to residual P fertilizer relative to zero-P plots was noted. Seed N uptake was greater in sorghum than in cotton. Nitrogen fertility level increased seed N in sorghum and in cotton following sorghum. Infrequent crop response to P fertilizer was not unexpected, especially when Mehlich-3 soil P in zero-P subplots was near the 95% sufficiency level of 20 mg P kg(-1). The main finding of this study is that N fertilizer response was more consistent for cotton following sorghum than in a continuous cotton system. In refining N fertilizer recommendations, N debits may be needed for N immobilization in sorghum residue. Nitrogen credit may be appropriate from leaf litter for crops following cotton and for NO3-N in irrigation water.