Revising petiole nitrate sufficiency/deficiency guidelines for irrigated cotton in the Desert Southwest

Petiole nitrate-nitrogen (NO₃–N) sampling and testing remains a popular in-season nitrogen (N) management practice in the western United States for cotton (Gossypium hirsutum L.). However, the present guidelines used by Arizona are greater than 35 yr old and are in need of updating. The objectives of this study were to relate in-season petiole NO₃ levels with lint yields and N deficiencies and to revise the former critical deficiency level guidelines by growth stage. Petioles were sampled between first square and peak bloom in nine site-years of cotton N management field trials in Maricopa and Safford, AZ, from 2014 to 2018. Irrigation type in Maricopa was overhead sprinkler irrigation (OSI) (2014–2015) and subsurface drip irrigation (SDI) (2016–2018). In Safford (2014–2017), surface irrigation (SI) was used. Petiole NO₃ in SDI was dramatically lower than with SI or OSI, mostly in the deficient range. Lower lint yields in zero-N treatments compared with pre-plant soil NO₃ test–based reference treatments occurred in eight site-years (Safford 2016 crop lost to late hailstorm) and were considered N deficient. Critical petiole NO₃–N levels from 1984 were revised downward 1 g N kg⁻¹ because several N-fertilized treatment means of petiole NO₃ were in that range and did not exhibit an N rate–related yield depression. Sampling cotton plants for petiole NO₃ analysis should start within 1 wk of first bloom. Petiole NO₃ dynamics and critical levels in SDI cotton required further study.