Citrus response functions to nitrogen, phosphorus, and potassium fertilization and N uptake dynamics

The citrus industries in Brazil and Florida account for up to 49% of the total world production. To maintain a competitive edge in the world market, and to respond to the public pressure to minimize the adverse environmental impact of non-point source pollution of nutrients into groundwater, it has been increasingly important to develop nutrient management recommendations to improve nutrient uptake efficiency, minimize nutrient losses and reduce the impact on ground water.; Since current recommendations have paid little attention to nutrient fate studies, responses of nonbearing trees (<5-yr-old) to NPK fertilization in Brazil were investigated. Furthermore, the increasing use of different rootstocks has justified the need for determining the nutritional requirements of trees on new rootstocks.; Field studies showed that rootstocks affected the response of nonbearing orange trees to NPK fertilization. Fertilizer rates for maximum tree growth and fruit yield were greater than those currently used in Brazil. The critical nutrient concentrations in the soil and leaves may need to be reevaluated. Data show that the critical levels of P in the soil, and that of N in the leaf tissue, are greater than those currently recommended for bearing trees. Orange trees on Rangpur lime were less responsive to fertilization than those on Cleopatra mandarin (Cleo) or Swingle citrumelo rootstocks. Fruit yield of trees on Cleo increased with P rates (up to 2200 g P₂O ₅ per tree), while yield of trees on Swingle increased significantly with K rates (up to 1800 g K₂O per tree) during a 5-yr fertilization program.; The results of studies conducted in a sandy Entisol (pH = 7.0) under 6-yr-old citrus trees in Florida demonstrated that ammonia volatilization accounted for 33% of applied urea. Similar results were observed in Brazil, even though soil pH was about 5.4. Nitrogen fertilization affected N mineralization and microbial biomass N as measured in 30-day period after fertilization. Leaching of N below the soil surface was low using a monitored under-the-tree low volume irrigation system. Recovery of 15N by citrus trees from labeled 15N fertilizers applied in the spring to the surface of the soil was 25.5% for urea and 39.5% for ammonium nitrate, 280 days after fertilization.