The olive tree has a reputation of being a hardy tree that can withstand almost severe neglect. These trees respond well to regular watering, but only need to be fertilized in case of significant nutritional deficiency in the soil. In fact, over-fertilization of an olive tree can reduce the size of its fruit. Before you fertilize your olive tree, it is important to test its foliage to see exactly what--if any--nutrients it needs.
Send a few of the leaves from your olive tree into your local county extension office to have a foliar test conducted for a nominal fee. Take the leaves in July from this season's shoots (non-bearing).
Interpret the test results. The nutrient levels in your olive tree will be listed as percentages. If the nitrogen levels in your olive tree are listed as below 1.4 percent, then your olive tree is deficient in nitrogen and should receive it as fertilizer. If the potassium levels are listed as below 0.8 percent, then your olive tree is deficient in potassium and should be fertilized with potassium sulfate. If the boron is listed as below 14ppm (boron is listed in parts per million), your olive tree is deficient in boron and should be fertilized with borax. Phosphorous, calcium, zinc, copper, manganese and magnesium deficiencies are rare in olive trees.
Apply nitrogen, if needed at a rate of .6 to 1.5 lbs of urea per tree per year until annual foliar tests reveal acceptable rates of nitrogen.
Apply 10 to 20 lbs of potassium sulfate to potassium-deficient trees per year until annual foliar tests reveal acceptable rates of potassium.
Apply 1/2 to 1 lb of borax per tree per year in mid to late winter until annual foliar tests reveal acceptable rates of boron.
Things You Will Need
- Foliar test
- Potassium sulfate
- Apply all fertilizers in mid to late winter.
- Olive trees with nitrogen deficiencies have yellowed leaves and inadequate shoot growth (optimal shoot growth is between 8 and 20 inches per anum).
- Olive trees with potassium deficiencies have light green leaves. Severe deficiencies often result in leaf tip burn or even dead sections of the olive tree.
- All ground applications of fertilizers should be spread in a 1 foot band starting at the drip line of the tree and watered in.
- Nitrogen fertilizer can also be given in the form of ammonium nitrate (.8 to 2 lbs), ammonium sulfate (1.3 to 3.2 lbs), CAN-17 (1.6 to 4 lbs), sodium nitrate (1.7 to 4.2 lbs), calcium nitrate (1.7 to 4.3 lbs), potassium nitrate (2 to 5.1 lbs), ammonium phosphate (2.4 to 6 lbs).
- If potassium deficiencies are severe, prepare a foliar spray of 1 lb of potassium nitrate per 10 gallons of water and spray the leaves just before the point of runoff. This will counteract the deficiency more quickly.
- If boron deficiencies are severe, prepare a boron foliar spray of 1 oz. of borax per 14 gallons of water and spray the leaves just before the point of runoff.