How much fertilizer should I apply?

Charles J. Graham, Pyzner, John R.  |  3/30/2007 2:04:24 AM

There is not a blanket fertilization rate for all pecan orchards. Fertilization rates will vary with orchard location and conditions. Alluvial soils have more natural fertility than upland soils and require less fertilizer.

Nitrogen is the nutrient most commonly used in pecan orchards. Addition of nitrogen to a pecan orchard will normally increase tree growth and yield. An application of 100 pounds of nitrogen is frequently recommended on producing orchards. This rate is sometimes modified due to the natural fertility of the soil. Upland soils may require 120 to 150 pounds of nitrogen for maximum nut production while a bottomland orchard may need 90 pound or less. An application of 50 or 60 pounds may be adequate for native groves on good alluvial soils.

Leaf analysis taken in July is the best way to determine the nutrient requirements of pecan trees. Traditionally 100 pounds of nitrogen per acre are applied when leaf samples indicate nitrogen levels of 2.5% to 2.75%. For each 0.1% below 2.5% an additional 10 pounds of nitrogen is added per acre.

Ammonium nitrate has been the traditional nitrogen source for pecan orchards in Louisiana. It loses little nitrogen to the air under Louisiana conditions and has been easily obtained and has usually been competitive in price. Ammonium nitrate is 33 - 34% nitrogen.

There have been some concerns about the availability of ammonium nitrate due to the fear of terrorism. Ammonium nitrate sold in bags may be less available.

Urea (45-46% nitrogen), anhydrous ammonia (82% nitrogen) and ammonium sulfate (21% nitrogen) are possible alternative nitrogen sources, however they have some disadvantages.

Urea can volatilize (loose nitrogen to the air) when applied to the soil surface if it is not incorporated by tillage or rainfall within a few days. Volatilization occurs most rapidly with higher soil temperatures, higher soil pH, surface plant residues and moist soils. Urea is best used before expected rainfall with soil temperatures below 70 degrees and soil pH below 6.5. Do not apply urea to wet soils.

Anhydrous ammonia is a compressed gas that has to be knifed (applied) below the soil surface. The soil must be moist enough to retain the gas but not wet enough to form cracks in the soil that allows the gas to escape. Knifing the ammonia into the soil can cut surface roots. The knifing operation also produces soft areas in the soil that can cause equipment to become stuck following rains.

Ammonium sulfate is best used on higher pH soils. The soil acidity produced by ammonium sulfate is three times the acidity produced by ammonium nitrate. It would take 535 pounds of pure calcium carbonate to neutralize the acidity produced by the ammonium sulfate used to supply 100 pounds of nitrogen per acre.

Zinc deficiencies can be corrected with 2 to 3 foliar applications of 36% zinc sulfate at 4-5 lbs. per acre or NZN applied at 5 quarts per acre. Zinc is usually added to the early fungicide sprays. Foliar zinc applications should be made before leaves reach maturity. Very little zinc is absorbed through the leaves once they reach maturity. Dry zinc sulfate salt may be applied to the soil at 0.25 to 0.5 lb. per inch of trunk diameter in acid soils. Use the smaller amount on sandy soils. Applications of zinc salt to alkaline soils are not effective. Leaf zinc levels should be determined by foliar analysis in July to determine if pecan trees are obtaining adequate zinc.

Potassium and phosphorus fertilizer applications are generally not needed every year. Foliar analysis during the summer should be used in determining need. Soil analysis can sometimes be misleading due to some nutrients in the soil being unavailable to the plant.

Additional information on fertilizing yard trees can be found in the Homeowners Guide for Fertilizing Pecan Trees in Louisiana located on the General Orchard Maintenance page of the Pecan Station website. 

Question answered by Dr. John Pyzner, LSU AgCenter pecan and fruit extension specialist.

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