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.
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.