Fish emulsion or other organic fertilizers both liquid and dry are excellent. Fertilize only when the plant is growing new leaves. Nitrogen is the main requirement, a diluted amount of fertilizer every time you water works well. Yellowing leaves are a sign of insufficient nutrients or poor drainage. Citrus trees benefit from minor nutrients zinc, iron and magnesium.
Magnesium deficiency occurs frequently in potted citrus.
Add 1 Tsp. of Epsom salt for every 3 gallons of soil and water to dissolve the Epsom salts. Sometimes the plant will sucker near the root graft at the base of the tree. Remove all growth below the graft. This is especially true with younger trees. Suckers are very vigorous, thorny and are a different plant than the top of the tree.