Once foliage on a lemon tree loses its deep green color and old leaves turn yellow-green, timely action is needed to prevent further deterioration. A dousing of a chelated, or immediately absorbable, liquid fertilizer is applied. The year-round fertilizer regimen must be modified as well to prevent future yellowing of foliage resulting from nutrient-poor soils.
Purchase a chelated liquid fertilizer from your garden center. Ask if there is a special formula available for citrus plants. The chelated fertilizer is a concentrate with easily absorbed micronutrients, such as iron (Fe), magnesium (Mg) or manganese (Mn), that is diluted into water and sprayed onto yellowing leaves. If no special "citrus mix" is available, any chelated iron fertilizer in liquid form will suffice.
Read the product label directions and dosages. Gather the appropriate materials, such as a bucket, sprinkling can, spray bottle or small pump or backpack sprayer. These items allow you to mix the appropriate dosage and amount of solution to apply to your lemon tree, based on its size.
Place water in a bucket with designated pint or gallon increments to an amount directed on the product label. Add the proper amount of liquid chelated fertilizer into the water-filled bucket and stir.
Pour the chelated fertilizer solution into the spray bottle or spray canister. Reapply the lid to the spray applicator and take it to the lemon tree.
Douse the foliage across the entire tree with the solution until you seed beads of liquid beginning to drip off the leaves. Focus on both new and old leaves, even wetting leaf undersides and green stems. Avoid spraying the foliage when direct sun reaches the leaves or when rain is expected within 12 hours. Spray the lemon tree at dawn.
Pour out leftover chelated fertilizer solution onto the soil under the lemon tree so that tree roots absorb nutrients, too. This is a sound way to dispose of excess solution from the mixing bucket.
Create an annual fertilization regimen and schedule for the lemon tree to prevent leaf yellowing in the future.
Apply a granular, slow-release fertilizer to the soil around the base of the lemon tree outward to 2 feet beyond the reach of the branches in early spring. Follow product label directions for specific timing and dosages of the application. Look for either a "citrus special" blend of granular fertilizer or an all-purpose landscape formula (such as 10-10-10) with micronutrients.
Scatter magnesium sulfate around the tree base in a similar manner at least twice a year, once in spring and again in late summer. Magnesium sulfate is commonly known as Epsom salts. Apply 1 to 2 cups Epsom salts under the tree, spreading it out like you are sprinkling sugar on cookies. Don't worry, you can't over-fertilize or harm plants with Epsom salts. Just place the crystals at least 6 inches away from the trunk and outward.
Broadcast cured manure or other organic matter to a depth of 1 to 2 inches on the soil under the lemon tree at least once annually, such as in late spring. Keep the manure or mulch at least 2 feet away from trunk of the lemon tree.