Hiya oops!!!,
To keep your lemon tree looking good, use a good all round liquid fertiliser such as 'photostogen', about every 3rd watering during the growing season.
when you come to transplant into a larger pot use 'john Innes #3' mixed with coir fiber to retain moisture and coarse grit to give good drainage and air circulation.
If the leaves start to turn yellow, this could be down to magnesium deficiency ( which they are prone to) this can be easily remeded by giving a dilution of 'Epson salts' at a rate of 1 teaspoon per gallon of water. Good Luck Tbird+