Another week, another citrus question. This seems to be the lot of gardening writers the world over, and while it's true that citrus trees are host to a broad array of pests, diseases and deficiencies, it's equally true that most issues can be solved with a single, simple cure all. Feed your trees.
Citrus plants are greedy. Like a ravenous teenager they have an almost insatiable appetite for nutrients and will lap up whatever fertiliser you can send their way. In return, well nourished citrus trees will become the picture of health, overcoming trace element deficiencies and putting up natural defences against pest insects and diseases. Most gardeners under-fertilise their trees, and as a consequence, wonder why they're so unhealthy.
It's not hard to set things right. Start with the soil. Citrus have relatively shallow root systems, so it's important to provide soil that drains freely, but is rich inorganic matter. Prepare sites for new trees by incorporating plenty of well rooted manure or compost, and top dress established trees in late winter using the same materials, topped by a feeding mulch such as lucerne. Keep the mulch away from the trunk to prevent collar rot (citrus trees are particularly prone).
Then feed your trees at least once per season. My preferred fertiliser is pelletised chook manure. Some brands have a product designed specifically for citrus trees, but I use a generic certified organic mix made locally here in South East Queensland. A generous handful per square metre applied right out to the drip line under the tree is sufficient to give citrus a boost at the start of each season. However if you live in a high rainfall area or have experienced flooding, nutrients can leach easily from the soil and extra fertilising will be necessary.
After last January's floods I got good results by watering the ground beneath my trees with a watering can of fish emulsion one month, and a watering can of seaweed solution the next month, for about a year. Since the weather has settled down I've gone back to my usual regime of seasonal feeding with chook pellets, washed down with a drink of seaweed solution. My trees are largely pest and disease free, and they're bearing nicely. Apply a similar feeding regime to your plants, and odds are you'll get the same kind of results. Well fed citrus trees are happy trees.