Your citrus trees need high-quality soil plus a number of trace elements to meet its specific nutritional needs. Citrus trees in containers are susceptible to malnutrition even when planted in good compost, as every time you water some of the nutrients leak out, and they can only be replaced by you. You can recognise if your citrus tree has poor nutrition by checking for stunted growth, yellow leaves, and low or no fruit production.
Older leaves that yellow in the centre may be an indication of magnesium deficiency and leaves that yellow at their tips can indicate a lack of fertiliser. If the leaves of your citrus tree are yellowing, falling off and are curled and crispy when they fall, it sounds like your tree is thirsty. Give the root zone a good soaking every 4-5 days for 2 weeks and it should remedy this. Leaves may also drop when the soil is too wet.
Happily, nutritional deficiencies in an orange tree, lemon tree, lime tree or other citrus tree can be easily addressed with soil amendments. To prevent malnutrition, fertilise your young citrus tree every six weeks or so in the ground, and every month in containers. After your citrus tree is full-grown, fertilise four times a year.
The three major nutrients that all plants need are nitrogen (N), phosphorus (P), and potassium (K). Industrial, non-organic fertiliser is available that will meet these nutritional needs very well. If you would like to use commercial non-organic fertiliser, look for a 20-20-20 formula of NPK at your local nursery for winter feeding. However in the USA these units are designated as N-P₂O₅ – K₂O whilst in other countries such as Australia and UK, the units are N-P-K. P₂O₅means phosphate in the oxide form, as opposed to phosphorus (used in Australia and UK) and K₂O is the oxide form of potassium.
You will find full details about fertilising your citrus tree and advice and photos on recognising and correcting nutrient soil deficiencies in Grow Citrus: The Insiders Secrets to Growing Great Citrus