Feeding citrus

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I have revised this post to provide more detail because there have been a few enquiries recently about fertiliser requirements for citrus, particularly trace elements.
Time to fertilise
There is no set time of the year for fertilising citrus as different species produce fruit in different seasons. Many citrus trees are producing crops at this time of year (through winter) and it is not a good time to give them a good dose of fertiliser. As citrus go through a growth cycle after fruit has matured, a good general rule is to apply fertiliser to the soil surface, under the outer part of the canopy, after fruit has been harvested, then cover the fertiliser with about 5-8 cm of organic mulch. Avoid scratching fertiliser into the soil surface as citrus roots lie close to the soil surface, and never apply fertiliser to dry soil, as it will burn tree roots. Repeat the fertiliser application in approximately six months, but avoid applying fertiliser in very hot weather.
Suitable citrus fertilisers
Citrus trees require a good supply of fertiliser that contains a full range of both major and trace elements for good growth, and pest and disease resistance. Trees to five years of age can use up to 500 g of complete organic fertiliser per year, as the inclusion of organic matter in soil will make nutrients more readily available. This should be divided into 2 applications. Older trees may require a little more. Very young trees should receive a proportional amount, as they will also benefit from the occasional application of manure tea. Too much high nitrogen fertiliser will attract aphids, scale, the citrus butterflies and citrus leaf miner. Over fertilising can also kill citrus trees.
Compost is the best fertiliser for citrus, but worm castings, poultry-based fertilisers, and well-rotted manures will also keep trees healthy.
Major nutrient elements are: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Part of the role of phosphorus in plants is to promote root formation and early growth. Heavy applications of high nitrogen fertilisers can make phosphorus unavailable to plants. Phosphorus is also only available within a narrow pH range. Keeping the soil pH around citrus trees close to neutral will improve their growth, and phosphorus from organic sources is more readily available to plants. Citrus have a fairly high requirement for the major element magnesium. Signs of magnesium deficiency appear first in older leaves where yellowing begins at the outer edges of the leaves and moves inwards, resulting a green V shape at the stalk end. It can also cause cupping of leaves and lack of sweetness in ripe fruit. This problem is common in citrus in autumn when fruit it maturing. This deficiency can be corrected quickly by watering in some Epsom salts: about 250 g for a young tree up to 2 kg for a fully-grown tree. If pale leaves have occurred on your citrus trees in the past, in future, apply one handful of dolomite per square metre of tree canopy when fertilising after harvest. In soils of SE Queensland that contain high quantities of magnesium, this problem will only occur where far too much potassium has been applied, or where soil has become quite acidic.
Yellow leaves in late winter, or early spring, are often caused by cold soils, if the tree has been adequately watered and fertilised. This problem will correct itself as the soil warms, and the tree begins to extract nitrogen from soil.
Trace elements
As mentioned above, citrus trees require a full range of trace elements. These are: iron, copper, zinc, molybdenum, manganese and boron. Their availability to plants is dependent on soil pH and the presence of organic matter in soil.
Copper deficiency will cause fruit drop and can, in more serious cases, cause gum to form inside and outside fruit, and on shoots. Iron deficiency reduces citrus crop size and causes leaves to gradually become pale green, and then fade to pale yellow, preventing the tree from manufacturing carbohydrates. Zinc deficiency reduces fruit bud formation, and manganese deficiency prevents vitamins forming in fruit. Zinc and manganese deficiency both produce yellow mottling between veins on young leaves, but in zinc deficiency leaves are smaller than normal and bunch together. Boron is essential for flower production and fruit quality, but boron toxicity can be a problem where laundry grey water containing borax has been used for irrigation.
Trace element deficiencies can occur if your soil, or the fertiliser you have been using, does not contain a particular micronutrient. However, they are most commonly caused by your soil being too acid or alkaline where nutrients are locked into compounds that plants can’t absorb. Humus in soil has a pH of around 6.5 where all nutrients are available to plants. Humus is also able to hold trace elements in a form that is easily absorbed and prevent nutrients leaching away through soil.
It is unwise to apply trace elements individually because, as the name implies, they are only required in tiny amounts, and excess applications can be toxic to plants, causing another range of problems.
To avoid trace element deficiencies, add a moderate amount of compost to the soil surface around your citrus trees. Organic mulch will also produce humus after friendly microorganisms break it down, and earthworms distribute it through soil. Also apply a liquid seaweed extract, at weak black tea strength, in autumn and spring to ensure your trees have access to a full range of trace elements. Seaweed also contains a good quantity of potassium to improve fruit quality and build plants’ resistance to pests, disease, frost and drought by strengthening cell walls. If you are experiencing serious problems with the health of your citrus trees, I suggest you test and correct your soil pH, or problems will continue.
Reasonably priced Soil pH test kits are available from most nurseries.

7 thoughts on “Feeding citrus

  1. I am new to all of this. I have just moved from a cold climate country to a warmer climate where I can grow citrus trees. I have a clementine tree on the property already. I am sure in the past many applications of pesticides have been used because when we moved here the fruit was perfect the first season. Last season the clementines were all really bad and this year they seem to have little black dots and I find tiny worms in a lot of them. I would like to know what I can do organically to keep the tree and fruit healthy? The only thing my gardener suggests is to spray it but I am sure there are other options.
    Thank you kindly,
    Julia in Israel

  2. Although I am not familiar with pests in Israel Julia, it sounds as though your citrus are being attacked by Mediterranean fruit fly. Fruit fly lay their eggs in the fruit of stressed plants and there is no organic way to control the development of the maggots in the fruit. You can use exclusion bags to protect undamaged fruit, or protect the tree with mosquito netting as long as the netting does not touch the fruit. And you can set up traps to catch fruit fly before they lay their eggs. There is an organic bait called Eco-Naturalure that attracts both male and female fruit fly. The trick is not to put the bait near the citrus trees. Any bait worth its salt will attract the flies, and you don’t want to steer them straight to the crop you are trying to protect. However, I do not know if this product is available in your area.
    Fruit fly larvae pupate in soil, so it is essential that you remove all damaged fruit and kill the grubs by placing it in a sealed black plastic bag left in the sun for a week to cook the grubs. If you don’t break the breeding cycle of these pests, you will have continual problems with them.
    Julia, you may not like my answer to controlling pests as I don’t believe that sprays are the best answer. Simply relying on better sprays whenever you have a pest problem ensures that you will continue to have problems. Many organic sprays will also kill pest predators, ensuring that you will have a different pest problem in the near future. Pest attack is merely a symptom that your plants are stressed. I know that it is difficult to provide adequate water in drought conditions but, if you can provide adequate water for your plants and they are still being attacked, improve plant nutrition with liquid fertilisers. If you have provided them with adequate fertiliser, check your soil pH, as an incorrect soil pH can prevent plants absorbing the nutrients they need for good health. The secret to pest-free plants is to restore health to your soil, and my book “Easy Organic Gardening and Moon Planting” will show you how to keep it that way.

  3. Indian Lime ……….My tree is 18 mounths old and has been planted in a good sunny position. the soil is sandy lome and has been given a good organic treatment over the last few years……..could you advise me when to expect to see flowers and fruit developing. no fertilizer has been given but the tree is growing well. I live in SE Queenland near Caboolture.

    If you have given the tree no fertiliser, I don’t understand what you mean by giving it “a good organic treatment”. Compost is an excellent fertiliser for citrus trees, but they are heavy feeders and some should be applied to the soil surface (under mulch) twice a year. Without adequate fertiliser, flowering and fruiting will be poor – unless the tree is so undernourished it thinks it is going to die and puts in an all-out effort to reproduce before it goes. You sometimes see this happen on very old citrus trees.
    Our Tahitian lime ripens fruits around Christmas time and the flowers appear late winter to spring, but yours may fruit earlier in a warmer climate. Mexican (or West Indian) limes require tropical conditions, but I understand they also take 3-4 months for fruit to mature.
    Citrus trees are ready to harvest when 3-4 years old, and then gradually increase the amount of fruit the tree carries. Harvesting fruit from very young trees, no matter how vigorous they seem, can weaken the trees and cause problems later. Any fruit should be removed from very young trees soon after it sets. Shorten any long branches to encourage side shoots. Fruit on long branches will weigh the branches down and permanently damage growth on young trees. – Lyn

  4. Hi, I have a Tahitian lime in a large pot in coastal Sydney. I applied homemade compost about mid autumn as the leaves started to yellow. The tree improved but now mid winter the leaves are yellowing again and dropping. The tree had a burst in flowers after compost was applied and so now have small fruit forming. I have not watered the tree as it has rained. Should I reapply compost or a slow release fertiliser. Thanks for your help.

    Hi Sandra, A photo of the yellowing leaves would help an accurate diagnosis as different causes produce different patterns on the leaves. Citrus have a high need for magnesium and this can cause yellowing of leaves, especially in autumn when fruit is forming. Or, the mixture in the pot may be too alkaline, and this can cause yellowing known as ‘lime-induced chlorosis’. Check the pH of the potting mix before you add any amendments.
    The fact the leaves are dropping as well as yellowing could indicate water-logging. Also check whether the drainage holes in the pot are clear. Some large pots only have one drainage hole and this can quickly become blocked especially if the pot is sitting directly on the ground. Pieces of tile placed underneath pots help to keep the drainage hole/s clear.
    I’ve found that worm castings are a great fertiliser for potted plants. They are easy to water-in, whereas compost sitting on the surface and not covered by mulch tends to dry out and lose some of its benefits. Also in pots, you don’t have the assistance of earthworms moving the compost (as castings) into the root area as you do in garden beds. If you don’t have a worm farm, an organic liquid complete fertiliser would be the next best thing. – Lyn

  5. Hi, I have a dwarf lemon tree in a pot. It’s been established 3 years this Christmas, so I am dreaming of my first harvest. It has flowered and now has fruit buds but the leaves are yellow with green veins. My research has lead me to believe it is either an iron or magnesium deficiency (young leaves are yellow green veins) I use a citrus fertiliser in spring and autumn. My mum suggested adding Epsom salts which seemed to fix her tree. Doesn’t appear to have helped mine. Any tips? I have a photo but don’t know how to add it.

    If Epsom salts (magnesium sulphate) hasn’t helped your tree, then it is not a magnesium deficiency. Magnesium, nitrogen and iron are necessary for leaves to form chlorophyll (green colouring). However, both nitrogen and magnesium are mobile in plants so that when one of both are deficient, the plant can move the nutrients to new growth and the older leaves become yellow first.
    Yellowing of young leaves with green veins suggests iron or manganese deficiency. Manganese is essential for the formation of carotene and ascorbic acid but too much can induce iron deficiency. Consequently, patterning of discolouration in leaves is extremely helpful in making an accurate diagnosis. You can email the photo directly to me at lyn@aussieorganicgardening.com via the link on the right side of the home page.
    Iron deficiency can be caused by a soil pH higher than 7.0 but is also common when soil is cold and wet. As you haven’t said which area you live in, I will wait to hear further information from you before suggesting a suitable treatment.– Lyn

  6. Hi Janelle, from your photos, it does look like your potted tree has an iron deficiency, as yellowing is showing in the young leaves. This can be caused by a number of conditions:
    a) potting mix (or soil) that is too alkaline from excess bio-char or calcium in the mix or fertiliser containing a lot of poultry manure, b) cold and wet soil or growing mix especially in spring (as it may have been in Melbourne this spring), c) if there is a build up of fertiliser salts from synthetic fertilisers, or d) where there is an excess of potassium from synthetic fertilisers or over-use of seaweed liquid fertiliser.

    The first thing I would do it check that your pot has ample drainage. large pots should not sit directly on a hard surface. While smaller pots usually have ample drainage holes around the sides at the base of the pots, large pots often have only one large hole in the base and this can easily become blocked resulting in poor aeration and/or a concentration of fertiliser salts if synthetic fertilisers have been used. Large pots should have pieces of tile placed under the pot to allow a small space between the base of the pot and the verandah or paving. If you notice crusting around the top of the soil line (fertiliser salts), flush the plant with clean water, once drainage has been improved.

    If poor drainage is not the problem, the next step is to check the pH of the mix with a test kit. A suitable pH is important to all parts of your garden as the pH in soil or mix controls the availability of nutrients. Test kits are very economical to use and readily available from larger nurseries. If you find that the pH is above 7.2, you could repot the tree using an organic-registered potting mix as organic matter is an important source of iron. However, to do so may result in the loss of this crop of fruit.
    The addition of flowers of sulphur (elemental sulphur) is the usual way to reduce pH in soils, but it is easy to overdo this in potted plants. You can apply iron chelates (the form of iron in organic compost) to the mix in the pot at the recommended rate. Citrus trees do not absorb iron chelates well through foliar spraying. Or, you can fertilise the tree with a weak solution of Multicrop’s Ecofish (1/2 teaspoon per litre of water). This is an organic-registered liquid fertiliser that contains soluble iron and has a low pH, which will help to reduce the pH in the pot. Repeat the application in a fortnight.
    As some of the older leaves in one of your photos and bare twigs indicate a possible mild deficiency of other trace elements, I’d be inclined to use the Ecofish as it contains iron, manganese, sulphur and zinc (trace elements needed by citrus) but unlike seaweed fertiliser, it does not contain a lot of potassium. Manganese deficiency is also caused by high pH or poorly-drained soil.

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