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Fertilising Citrus Trees: Australian Gardening Guide

Citrus trees grow on free-draining but rich soil, and can cope with anything from chalky soil to sandy loams, though clay can cause some serious problems. Regardless of where you’re growing these intensely flavoured citrus trees, you’ll need to feed them well and feed them regularly, to get the most fruit possible. 

Here is an in-depth guide to fertilising citrus, and some of the more common garden feeds that make a handy fertiliser for citrus trees if you’re ever in doubt.

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Understanding Citrus Trees: The Basics

Fertilising Citrus Trees

Citrus trees flower in spring summer, and fruit quickly after, forming fruits that can continue to ripen well into winter in the right conditions. The closer you can give them to their native Mediterranean habitat, the better. 

There are a few native citrus trees including the iconic finger lime that will obviously fare well in most Australian gardens, but any citrus can be grown here with the right soil preparation and ongoing feeding regime.

In colder parts of Australia, you may need to grow citrus in containers, and feed it more regularly throughout the growing season, before moving it indoors for winter to protect from any risk of frost.

What Nutrients Do Citrus Trees Need?

Citrus trees need the basic macronutrients (Nitrogen, Phosphorus and Potassium) that are generally labelled as NPK values on fertiliser bottles. The amounts can vary, and you can either respond to signs from the tree itself, or simply give balanced, gentle, feeds throughout the year. 

More importantly than that though, they need Magnesium, to support the production of sugars through photosynthesis (thus swelling the fruits). Other micronutrients, like calcium, iron and zinc can be added in through mulches and compost, and are naturally present in most soils.

Nutrient amounts and NPK balance for citrus trees

In terms of quantities, magnesium is a trigger to photosynthesise, not necessarily an ingredient of it. That means it needs to be present in fertilisers, but not in any high concentration. So trace magnesium in fertiliser, or composts with trace magnesium will support healthy growth.

Nitrogen is the next most important piece of the citrus puzzle, and key to a long growing season, supporting healthy roots, new leaves, and fruit development in equal measure. 

Phosphorus and potassium are important, but shouldn’t be over-fed, so balanced feeds like a 10-10-10 liquid fertiliser can offer too much of both in contrast to nitrogen.

We’ll have a look at specific fertilisers that have proved useful for citrus trees and fruit development below, but as a general rule, a regular feed of any fertiliser with a ratio equal to 2:1:1 NPK (e.g. 10:5:5) will be beneficial to citrus trees.

What to Use When Fertilising Citrus

The best fertilisers for citrus trees are nearly always the traditional methods, but some inorganic citrus feeds do offer the right nutrients if you’re on a budget.

Organic Citrus Fertilisers

Organic citrus fertilisers often vary in strength, but they have been used for millennia to support the development of citrus fruit across the world.

Fish, Blood and Bone for Citrus Trees

Fish, blood and bone is a good citrus fertiliser, but not perfect. It’s made from meat industry by-products, so is generally considered sustainable, and it offers balanced nutrition right throughout the growing season with a single application in early spring.

While its nutrient contents can vary between brands, it generally offers an NPK of around 3:5:3, meaning it has higher levels of phosphorus than anything else. 

The real benefit of fish blood and bone is when it comes to addressing magnesium deficiency, as it contains both magnesium, and calcium, which supports the uptake of magnesium, and helps to support the development of stronger fruits.

Rotted Manure for Citrus Trees

Rotted manure is a good base for a compost mix when planting, or re-potting citrus trees. Depending on the manure, the NPK will vary (from animal to animal, and from batch to batch).

The most reliable for planting into is horse manure, with a general NPK of roughly 0.6-0.5-0.5, but it must be well-rotted, and paired with plenty of grit and sandy top soil to provide adequate drainage.

Using rotted manure as an annual mulch is a good option in warm climates to retain moisture on very well-drained soil (and improve soil health).

Chicken manure

Chicken manure for fertilising citrus trees

As an alternative to rotted cow or horse manure, chicken or poultry manure makes a wonderful fertiliser for pretty much any part of the garden, with much stronger nitrogen content than other manures, and a manageable slow release of nutrients for up to three months after application.

Our soil is really quite healthy here, so any citrus in the ground is fed with chicken manure rather than rotted manure in spring. Those in pots are only ever fed with chicken manure and liquid seaweed.

Liquid Seaweed for Citrus Trees

Liquid seaweed is the most common organic liquid feed for citrus trees but it doesn’t actually feed them. What liquid seaweed does is to unlock trace and macronutrients that are already held in the soil, creating a more porous soil structure, and retaining moisture without restricting drainage.

This makes it a wonderful addition to general slow release feeds and mulch feeds, as it reduces moisture loss, and prevents the soil from drying to a crisp, which can completely prevent fruiting. 

Note: If you can legally collect seaweed from the shoreline where you live, it can be dried and used as mulch directly on citrus trees. There is some saltwater content, but not enough to damage plants, and as it rots down, it will provide moisture retaining mulch and condition the soil.

Branded Citrus Fertilisers

Some of the best, and most relied on brands of citrus fertilisers are the normal household names, like Osmocote and Charlie Carp. 

Using Tomato Feed on Citrus Trees

Tomato feeds have low Nitrogen content, but will still work to support fruit development once flowers on citrus trees have been pollinated. Avoid feeding with solely tomato fertiliser if you can, but don’t worry if you need to use it once in a while.

Osmocote

Osmocote is a trusted brand and one of the most established in Australia. There are two of their slow-release products that are worth using with your own citrus trees, one is organic and the other isn’t. 

Sadly, I have to admit that the non-organic Osmocote is generally more effective, thanks to its slow release of nutrients through the whole season, but the organics do work well, and the inclusion of seaweed seems to reduce the need for watering.

Charlie Carp

Charlie Carp is probably one of the most sustainable companies in Australia, catching and using invasive European Carp that have invaded our waterways, and made life hell for native species, and using them to produce a sustainable, nature-friendly garden fertiliser.

Their all-purpose liquid fertiliser can make up to 300L of garden feed or can be used in higher concentrations when fruits start to form.

We used this once a month on a selection of potted fruit trees last summer and they performed exceptionally well, with very few dropped flowers, and tons of fruit.

With an NPK of roughly 10:1:6 it is ideally suited to citrus.

DIY Citrus Fertilisers

While the comfort of pre-made and predetermined NPK does make life easier for gardeners, making your own liquid feeds and compost is a wonderful way to keep costs down as well as recirculating nutrients from your own garden and putting soil bacteria back into the ground in natural amounts.

DIY Citrus Fertilisers

Home Made Compost

Composting isn’t hard at all. You just need a good spot in the garden in partial shade, but with about 6 hours of direct sun a day to heat it up and promote bacterial action, and a roughly even mix of dry and fresh materials.

Fresh grass clippings rot down quickly, and mixing them with kitchen waste and shredded paper, animal bedding, egg shells and fallen leaves will produce a wonderful compost after about twelve months.

If you turn your compost regularly, it will rot down faster, and speed up the process, but with patience, any 1mx1m compost bed will produce a usable mulch within a year.

Home-Made Liquid Citrus Feed

One of the best liquid feeds you can make at home is either nettle or comfrey tea. If you decide to grow your own comfrey, choose Bocking 14 as it is a non-invasive sterile hybrid and won’t spread by seed. 

At any time of year, you can cut it right back to the ground and place the foliage in a bucket or old water butt to rot down. Seal the container so moisture can’t escape, and in a few weeks you’ll have a liquid that smells disgusting, but feeds your garden.

The same method works for nettles, but you can also shred them and mix them with water for a less offensive smell. Leave the nettle tea brewing for a few weeks before use.

How to Fertilise Citrus Trees

How to Fertilise Citrus Trees

Fertilising citrus trees, as I touched on briefly above against each fertiliser, is about managing their use, and balancing when to do it. If you’re unsure whether your citrus trees need feeding, it’s worth leaving them without for a week as overfeeding can actually be detrimental, particularly with some organic feeds that can over-acidify the soil.

When to Feed & Fertilise Citrus Trees

Citrus trees in Australia tend to be evergreen and will continue flowering and cropping continuously in warmer regions, so managing when to feed them can be tricky.

Personally, I just stick to spring, when they are most active in leaf growth for a general-purpose, slow release feed. Beyond that, it’s about responding to the individual tree. 

As they are developing fruits, it’s worth offering them a balanced liquid feed, with accessible nitrogen, or a liquid soil conditioner like liquid seaweed every 3-4 weeks.

How to Get Citrus Trees to Fruit

There are plenty of tricks around to encourage more fruit on your citrus trees, and most are variations on soil conditioners. Liquid seaweed, wood ash and gypsum are all generally considered useful ways to help refresh your soil structure for citrus trees but there is one key thing that is nearly always forgotten: citrus flowers on new growth.

Getting Mature Citrus Trees to Fruit

Flowers and fruits on citrus trees develop on new growth. Once bark begins to develop that slows down, and new shoots on mature citrus trees can be few and far between. By far the most reliable way to encourage older citrus trees to fruit is to prune them.

Prune mature citrus trees considerately by about one-third of their total size. This will trigger a flurry of new growth, and new shoots, and much healthier leaf development.

The combination of all of these triggers makes for better photosynthetic, fresh, stronger stems, and energy and nutrients diverted from the roots. That results in fruit. It’s that simple.

Fertilising citrus trees will make bigger, better, healthier fruit, but if your mature citrus tree has stopped fruiting, it is more likely to be from limited new growth than anything else.

Encouraging Larger Fruit on Citrus Trees

Magnesium and calcium support the development of stronger fruit in nearly all fruit trees, citrus included. Adding these essential nutrients into the soil can support the healthier development of fruit, and prevent flowers from dropping off prematurely in cold snaps.

Citrus Fertiliser Frequently Asked Questions

Citrus Tree Fruits

How often should you feed citrus trees?

Some say you should feed citrus four times a year with slow release, or with every other water for liquid feeds. A more deliberate approach to feeding citrus trees is to apply a single slow release feed in spring, followed by monthly liquid feeds.

How do I encourage new growth on citrus trees?

To encourage new growth on citrus trees, prune them. Citrus trees that have slowed down will be triggered into new top growth, which will bear new fruit, if they are cut back in spring.

When should I water my citrus plants?

The best time to water your citrus plants is in the morning. It offers more stable moisture throughout the day and can be taken up during the active hours when the sun is beating down. In the height of summer, do this twice a week, or whenever the soil dries out.

Should I cut the top off my lemon tree?

Lemon trees will grow back from their base if cut back, so it is possible to cut the top of a lemon tree. However, it depends on what you want from the mature tree. Traditional, single-stemmed citrus trees can look beautiful, and if you cut the top back, rather than pruning carefully to the crown, that will no longer be possible.

Wrapping Up Our Guide to Fertilising Citrus Trees in Australia

Citrus trees are one of the most exciting things you can grow in the garden. Their fragrance in flower, and the aroma of lemon rinds on warm days, can transport you to another part of the world in an instant, and the fruit you get from a harvest is unmatched by anything you’ve ever eaten.

Fertilising, and conditioning the soil around your citrus trees is essential for happy healthy growth and maximum citrus fruit. Their structure and stature, whether mature or young and containerised add formality to gardens too. But, if you’re lazy, forgetful, and blaze about your citrus trees, they’ll mirror you. 

Last Updated on November 19, 2023

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About the author 

Gary Clarke

Hi, I'm Gary Clarke, gardening enthusiast and former landscaper. I have had privilege of sharing my gardening knowledge at Aussie Green Thumb since early 2020.

I have a passion for using native Australian plants in Aussie gardens and I always try to promote growing fruit trees and vegetable gardens whenever possible.

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