The size and quality of pecans are determined by the number of leaves for each nut. A medium-sized pecan requires about 10 compound leaves, each containing nine to 13 leaflets; large nuts need more leaves. Strong crops of large nuts require fertile soil in order for trees to yield the necessary leaves. Pecan trees like a soil pH of 6 to 6.5. To best determine the fertilizer needs of pecan trees, the soil should be sampled by an agricultural extension service.
Soil and Foliage Testing
Soil samples should be taken from the surface to 8 inches deep and from 8 to 16 inches deep. In mid- to late July, a sample of at least 100 undamaged leaflets should be taken from the middle of compound leaves growing in full sun. The leaflets should be placed in an open paper bag and allowed to dry. They should not be collected after applications of nutrients or pesticides.
Ammonium sulfate is a recommended source of nitrogen because it decreases the soil pH, temporarily increasing the zinc, iron and magnesium available to pecan trees. Nitrogen fertilizers are converted to water-soluble nitrates that are easily leached from the soil, so they should be applied in several applications. The fertilizer should never be applied within 12 inches of the trunk.
Nitrogen on Young Trees
Beginning with the second year of growth, agronomists at New Mexico State University recommend applying ¼ to ½ lb. of ammonium sulfate to one tree every six weeks until June. If the tree grows slowly, the nitrogen should be reduced to 1/3 lb. A young tree fertilized after June will retain its leaves too long in the fall, risking winter injury. From the third year until the tree bears pecans, 2 lbs. are applied twice a year. Another formula is apply ¼ lb. of nitrogen per inch of the trunk diameter measured 3 feet above the ground.
Nitrogen on Mature Trees
The terminal branches of trees bearing pecans should grow 6 to 12 inches a year. Pecan trees bear nuts in alternating years. For trees bearing nuts, agronomists at North Carolina State University recommend applying 4 lbs. of 10-10-10 fertilizer in February or early March for each inch of the diameter of the trunk measured below the scaffold branches. For trees not bearing nuts, 1 lb. of 10-10-10 fertilizer for each year of the tree's age should be applied in late February or early March.
If soil is not tested, agronomists at the University of Georgia recommend a mid- to late March application of up to 8 lbs. of ammonium nitrate at the rate of 1 lb. per inch of trunk diameter.
If a soil analysis shows a deficiency of zinc, zinc sulfate is best applied as a foliar spray. Zinc chelates release zinc slowly so that the roots can absorb it before it gets tied up in the soil. Trees should be sprayed four times, once a week beginning when the leaves first unfold. After that young trees should be sprayed every two weeks until the end of July.
New Mexico State agronomists recommend 2 to 3 lbs. of 36 percent zinc sulfate per 100 gallons of water plus 1 ½ quarts of urea that is 32 percent liquid nitrogen. The nitrogen helps the leaves absorb the zinc. The spray should be applied from the ground, aimed at the bottoms of the leaves.
If soil tests reveal a deficiency of phosphorus, New Mexico State agronomists recommend that an 18-48-0 super phosphate be incorporated before pecan trees are planted. If yellowing leaves indicate an iron deficiency, iron sulfate can be sprayed together with zinc sulfate.