Optimizing nitrogen fertilizer rates and time of application for potatoes under seepage irrigation.

Splitting N fertilizer application is recommended to increase crop N use efficiency by reducing risk of leaching. The objective of this study was to optimize the timing and rate of split N fertilizer applications to maximize tuber yield and quality of chipping potatoes grown using seepage irrigation in Florida. A two-year study was conducted on two commercial farms using cultivars Atlantic and FL1867. A factorial of two N rates applied at pre-plant (N pre-pl ) 0 or 56 kg ha −1 30 days before planting followed by four N rates applied at plant emergence (N emerg ) 0, 56, 112, or 168 kg ha −1 were randomized in a complete block design with four replicates. At tuber initiation, all treatments received an additional 56 kg ha −1 of N. All applications of N were of granular ammonium nitrate banded. Total N applied for the various treatments ranged from 56 to 280 kg ha −1 of N. Soil N was monitored in the 0–20 cm soil depth layer throughout the season. Whole plant biomass peaked at 8.5–8.7 Mg ha −1 with N emerg rates between 112–143 kg ha −1 . Plant N uptake range from 66 to 157 kg ha −1 . N uptake use efficiency decreased with increasing N rates. Potato yield ranged from 25 to 42 Mg ha −1 in both years. In 2013, application of 56 kg ha −1 of N pre-pl produced 2.5–5.1 Mg ha −1 higher yield than treatments receiving 0 N. In 2014, there was an interaction between N pre-pl and N emerg on yield. The higher 56 kg ha −1  N pre-pl resulted in higher yield only when N emerg was at or below 56 kg ha −1 . For cv. Atlantic when no N pre-pl was supplied, yield increased linearly in response to N emerg rates, while with 56 kg ha −1 of N pre-pl , yield responded quadratically to N emerg reaching a maximum at 114 kg ha −1 . For cv. FL1867 yield increased quadratically to N emerg peaking at 138 and 126 kg ha −1 of when 0 or 56 kg ha −1 of N pre-pl was applied, respectively. The study shows that while the risk of N pre-pl loss is high, N pre-pl can result in higher yield, especially when subsequent N rates are low. Application of N pre-pl was particularly effective in a dry year. By contrast, when soil mineral N from N pre-pl was largely lost to leaching in a high rainfall year, yield was increased by the N pre-pl application only when subsequent N emerg rates were less than 112 kg ha −1 . When early-season soil N was low, the N emerg had a larger impact on tuber yield due to minimum loss to leaching, maximizing yield when N emerg was 128–168 kg ha −1 . This indicates that applying N fertilizer prior to emergence is necessary to maximize tuber yield. [ABSTRACT FROM AUTHOR]