The 2 most common reasons for yellowing leaves on your citrus tree are nutritional deficiency and inadequate drainage of the soil.
Citrus trees are evergreen, which means that they keep their leaves all throughout the year. The rootstock that we use for our Citrus trees is Trifoliata, which has a deciduous growth cycle meaning growth slows down in Winter. During Winter, the roots will reduce the amount of nutrition they provide. This can leave the evergreen part of the tree hungry for food. Leaves with a yellow border or mottled yellow colouring indicate a nutrient deficiency.
What type of yellowing do you have on your tree?
Light green to yellow leaves all over the tree indicates a Nitrogen deficiency – Treatment: Blood and Bone/Organic Matter
Yellowish-green blotch at the leaf base that spreads outward - Magnesium deficiency – Treatment: Dolomite or Epsom Salts.
Yellow leaves with green veins (generally soil high in alkaline - common in coastal areas) - Zinc or Iron deficiency
Treatment for Zinc deficiency: foliar spray (zinc sulphate) on the spring flush leaves.
Treatment for Iron deficiency: you will need to test your soil and reduce the pH, see below.
So how do we fix this?
The first thing to do would be to perform a pH test on your soil. If the pH of your soil isn't within the correct range, your tree will not be able to absorb the nutrition it needs from the soil. Thats why we need to test and correct this before applying fertiliser. Follow the guide on pH testing below.
pH Testing
pH testing means potential of Hydrogen and is a measure if acidity or alkalinity of water soluble substances. A pH scale goes from 1 to 14, where 1 is most acidic and 14 being most alkaline - 7 is neutral. We recommend a soil pH for Citrus of 6-7. For recommended pH for other Fruit Salad Trees, click on Stone Fruit Variety Information or Apple Variety Information.
Is it difficult to test my soil?
A pH reading might sound scientific, however it is quick and easy. Purchase a kit from your local nursery or garden supplier. Inside your kit you'll find a bottle of indicator, a slide for the soil, a colour chart, mixing stick and Barium Sulphate powder. When taking a sample, remove grass and rocks from the topsoil and dig 10-15cm into the soil. Try not to use your hands to collect the sample as this may contaminate your reading.
How do I perform a pH test on my soil?
Place the soil on the clean, dry slide and then place 2-3 drops of Barium Sulphate on the soil. Wait a few minutes and check the colour of your soil against the colour chart.
How do I change the pH of my soil?
If your pH is too acidic you could add calcium carbonate (lime) and organic matter. Lime moves through the soil slowly, so it takes time to see the effects of liming soil. If your pH is too alkaline, then you can add elemental sulphur or aluminium sulphate and organic matter. Sulfate is available immediately and is recommended for faster results. Elemental sulfur takes up to a year to be processed and create a change in tree health.
Once you have established that the pH of your soil is with in the required range, you know that any fertiliser you add will be absorbed by your tree as it should be. Wait until Spring to give your citrus tree a good feed with fertiliser to encourage new growth. This is because the rootstock won't absorb a lot of food during the winter months. Also, fertilising citrus in the cooler months can be tricky, as it is best to avoid encouraging a flush of tender new growth that could easily get frost or cold damaged.
Tip: A great way to improve the nutritional elements in your soil in winter is to feed your tree with a mix of dolomite, lime and gypsum, which will increase nutrition and improve leaf appearance without stimulating a growth flush.
Good drainage is very important for the health of your citrus tree, or any Fruit Salad Tree. When you water your tree or it rains, this water needs to be able to run through the soil, then out the bottom of the pot, or away from the tree if your tree is planted in the ground. It is important that the roots of your tree are not continually saturated (sometimes referred to as "having wet feet"). This effectively chokes the roots and will eventually kill your tree.
Did you know: When trees have poor drainage, their leaves turn completely yellow, shrivel and begin to drop off. This can appear like the tree looks “dry” and people then water the tree more, thinking they are helping, but they are only making it worse. Don't be one of those people!
So how do we fix this?
Trees in a pot
Make sure the pot has enough drainage holes in the bottom to allow water to drain out. If your tree is placed on soil or grass, these drainage holes can become clogged due to the fine soil particles flushed out with water. We recommend that you raise the pot up a few centimeters to aid in keeping those holes free draining. We don't recommend DEEP dishes under your pots as these retain water inside the pot to the same level as the dish.
Trees planted in the ground
If you have planted your trees in the ground, and you suspect the soil around your tree is not very well draining or contains lots of clay, we recommend carefully digging up your tree and planting on top of the ground. Then mount up free draining soil or potting mix around your tree, allowing water to flow away from the roots. See our planting guide and videos HERE.
References: QLD Department of Agriculture, Fisheries & Forestry and of course of FST team.
