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.