Citrus Nitrogen Uptake and Nitrification Rates in Sandy Soils.
Kelly Morgan, Johan M. Scholberg, and Thomas Obreza. Univ of Florida, 2686 SR 29N, Immokalee, FL 34142
The optimum timing, frequency, and rate of fertilizer N application for citrus production under Florida conditions have been explored for nearly 60 years with little regard to environmental consequences. Sandy Entisols of central Florida have low water holding capacities in the range of 0.07 to 0.10 cm3 cm-3, require frequent irrigation, and are prone to nitrate leaching. Nitrogen best management practices have been established for citrus on these sandy soils to sustain high fruit production and tree health. The objective of this study was to determine the effect of time of year on fertilizer nitrogen uptake and nitrification rates in sandy soils. Mature productive ‘Hamlin' orange trees [Citrus Sinensis (L.) Osbeck] on Swingle citrumelo [C. paradisi Macf. x Poncirus trifoliata (L.) Raf] or Carrizo citrange (C. Sinensis x P. Trifoliata) rootstocks were used in this experiment. The N fertilizer rates used in this study were approximately 50 and 100% of the monthly rate based on 269 kg N ha-1 yr-1 in 6 monthly applications (February to May and September to October), or 45 kg N ha-1 per application. The 100% rate is equivalent to 500 g N tree-1 yr-1 or 83 g N tree-1 per application. Irrigation was applied by an automated irrigation system using switching tensiometers to trigger irrigations. Irrigation was applied when soil water potential in the upper 30 cm dropped below -10 kPa during the bloom and fruit set period of February to May and -15 kPa for the remainder of the year. Differences in mean cumulative N loss from the soil during the 3 days after application were significant (P=0.05) for month of year, with greatest loss occurring in May when soil temperatures were high and leaf N concentrations were low, compared with late summer when soil temperatures were similar and leaf N concentrations were higher. Percentage of total soil N loss during the 3-day period for the high application rate of approximately 83 g tree-1 were 60.7, 68.6, and 63.4% for March, May, and September, respectively. These means were significantly different (P=0.05) from mean percentage N losses for the lower rate which had respective values of 83.6, 82.6, and 73.1%. The nitrification rate in this soil was rapid, with a mean of more than 50% of the NH4-N converted to NO3-N within the first 24 h. Changes in NO3-N and NH4-N content inside the control pipes were used to estimate N loss and nitrification rates with time. Mean NO3-N content in the upper 45 cm of soil increased 24 h after application to 150.6% of NO3-N applied. The mean NO3-N content decreased during the next 48 h to 125.8% of total NO3-N applied. Content of NH4-N decreased to 34.0% of that applied after 24 h and steadily declined after application to 16.1% of NH4-N applied on day 3. The sum of NO3-N and NH4-N (total N) declined throughout the period to 73.9% of total N applied. Under Florida conditions, nitrogen uptake and nitrification rates were rapid and must be taken into condsideration when developing best management practices to reduce potential leaching in these sandy soils.