Residual Effects of Pelletized Poultry Litter on Cotton Leaf Area Index and Nitrogen Uptake.

Pelletized poultry litter (PPL), with a slower rate of N release than inorganic fertilizer, may provide long-lasting plant growth benefits after applications cease. Additionally, using PPL to meet some of the N requirements of cotton (Gossypium hirsutum L.) can reduce fertilizer costs and risks of NO₃-N leaching into groundwater. Field studies were conducted in 2014 and 2015 to determine residual (carry-over) effects of 6.7 Mg ha^-1 PPL (220 kg N ha^-1 yr^-1) applied as a sub-surface band in 2010-2013 growing seasons on cotton leaf area index (LAI) and N uptake. Fertility treatments consisted of residual PPL, residual PPL with 84 kg N ha^-1 (as urea-ammonium nitrate) at squaring stage, N fertilization with 56 kg ha^-1 at planting plus 84 kg ha^-1 at squaring stage, and no N fertilization (control). Treatments were replicated four times and each strip-plot experimental unit was 28 m long with six, 0.97-m wide rows. Two central rows were sampled for LAI in July – August and for plant N uptake at open-boll stage in mid-September. Residual PPL with additional 84 kg N ha^-1 resulted in LAI values of 3.2 and 4.1 in August 2014 and 2015, respectively, which were significantly greater than LAI of 2.7 and 3.4 achieved with standard N fertilization of 140 kg ha^-1. At these times, LAI was approximately 1.5 in residual PPL only treatment. Averaged across years, N uptake in vegetative biomass did not differ between residual PPL with 84 kg N ha^-1 and the 140 kg inorganic N ha^-1 treatments (112 vs. 116 kg N ha^-1), and was considerably less with residual PPL only (40 kg N ha^-1). Lint yield was greater (P<0.01) in PPL with 84 kg N ha^-1 than standard fertilization treatment by approximately 10% in 2014 (1795 vs. 1633 kg ha^-1) and 5% in 2015 (1898 vs. 1809 kg ha^-1), suggesting residual PPL enhanced N-use efficiency (kg lint per kg applied N). Beneficial effects on cotton LAI, N uptake and lint yield were evident in two growing seasons after PPL applications ceased, and applying 84 kg N ha^-1 at squaring stage in residual PPL plots significantly boosted LAI during the boll-filling period.