Foliar application of urea has been common and researched since the mid 1950’s. Foliar urea has been shown to induce flowering by elevating the amino acid arginine biosynthesis for enhanced floral primordia formation as demonstrated by Dr. Carol Lovatt.
Urea is manufactured by reacting ammonia (NH3) and carbon dioxide (CO2) under high pressure at high temperature.
Biuret (bi meaning two) urea is formed during the manufacturing by the condensation-combining of two urea molecules together into one. Standard biuret urea is not acceptable in foliar applications. Biuret is not readily or rapidly metabolized safely in citrus leaves. Chlorosis and yellow spotting may occur due to biuret accumulation. Foliar applications of low-biuret (LB) urea or “lo-bi urea” is a safe and typical grower practice.
Most urea manufacturers produce standard urea with 1% or less biuret which can be safely used for soil applications. Standard biuret urea is soil-applied and is degraded by soil organisms.
Timing of the LB foliar urea application influences a variety of crop responses. Winter pre-bloom is a great time to apply foliar nitrogen to citrus. Foliar applications of lo-bi urea made during this time have been shown to increase set, thereby yield. Summer applications of foliar N as lo-bi urea have been shown to increase fruit size.
The best time to apply lo-bi urea in the southern central valley California is from about mid-December to mid-February. Winter foliar lo-bi urea applications of 0.35 lbs N/tree have been shown to increase yield of mature navel orange trees based on Dr. Lovatt’s research.
Summer foliar LB urea applications should be made between early June and late July. This would be timed before the end of the cell division stage. Lo-bi urea is normally applied at rates of 50 lbs in 200 gallons of water per acre (23 lbs. N/acre). As always, consult with your PCA for exact recommendations.
A combined foliar application of lo-bi urea and other essential nutrients is often more effective in correcting deficiency symptoms and boosting tree health than single element nitrogen spray on its own.
YaraVita® CITREL™ is a highly concentrated flowable formulation containing high amounts of zinc and manganese. YaraVita CITREL has significantly higher micronutrient concentrations than most liquid solutions or liquid chelates.
YaraVita CITREL combines critical nutrients, making it ideal for application to citrus crops to improve flower & leaf quality and crop productivity.
Sales Agronomist at Yara North America - South Central Valley, California