Nutrient imbalances and deficiencies can adversely affect the quality of citrus fruit. Excess nitrogen combined with low phosphorus will cause citrus fruit to be misshapen, with thick peel, a coarse and roughly textured rind, coarse pulpy flesh without much juice, and an open centre. The juice will also be more acid in these fruit.
Signs of Phosphorus Deficiency in Citrus
It’s important to first point out that it’s very rare for citrus trees in gardens or orchards to encounter low soil phosphorus and show signs of phosphorus deficiency.
- When it does occur, signs of phosphorus deficiency will show in both the fruit and leaves of citrus trees.
- As previously mentioned, the fruit will be misshapen with thicker peel and a hollow core, and fruit will drop before the normal harvesting time.
- The leaves will be small and narrow, with a dull bronzed green or purplish discoloration, and will shed readily.
- Symptoms will occurs on the older leaves first.
- Flowering will be reduced. Lack of flowers and fruit is more commonly a deficiency of potassium though.
Growth of a citrus tree is reduced when the supply of phosphorus is too low. Phosphorus is highly mobile in plants, meaning that it can be moved around easily by the tree to where it is most needed. In cases where there is a deficiency, the tree can move phosphorus from old leaves to young leaves, and to other areas which are actively growing, energy is needed to form seeds and fruit.
What Does the Nutrient Phosphorus Do?
Phosphorus plays an important role in plants, it is involved with the processes of metabolism, cell division & growth. Citrus trees require phosphorus for good root development and to help their flowers to bloom.
Being part of the macronutrient trio N-P-K (Nitrogen, Phosphorus and Potassium), the main nutrients which plants require to grow, phosphorus is an important ingredient in any balanced fertiliser.
The N:P:K ratio listed on product labels will indicate what proportions of these major nutrients are present in a fertiliser. Natural fertilisers such as blood & bone and chicken manure are very high in phosphorus, but not at the unnaturally high levels found in synthetic chemical fertilizers!
Understanding Soil Phosphorus
The reason why phosphorus deficiencies are quite rare in gardens and orchards is because phosphorus is immobile in soil, it doesn’t leach or wash out of soils, it stays put and accumulates up in the soil. Most phosphorus is present in the upper soil layer.
Australian soils out in the wild are naturally low in phosphorus, and many native plants are adapted to take up as much phosphorus as possible where it is extremely scarce. This is why some, such as those of the Proteaceae family, can be killed from phosphorus toxicity when fed with high phosphorus fertilisers!
The phosphorus in the soil will not all be available to tree roots. Some will be present as an available form which plants and trees can utilise, and some forms will be present which will be unavailable. The unavailable forms act as a soil reserve for the available forms.
The soil pH will determine how much phosphorus is available to plants and trees. Phosphorus is most available in pH range of 6-7, which is from slightly acidic to neutral.
When the soil is more alkaline or acidic, phosphorus becomes less available to plant and tree roots.
- In alkaline soils, phosphorus is fixed in calcium compounds
- In acid soils, phosphorus is fixed in iron and aluminium compounds in acid soils.
When the nutrient is fixed into compounds, it becomes bound up, and therefore made unavailable to plants and trees.
Fixing the Problem
The effect of phosphorus deficiency on citrus fruit is made worse when too much nitrogen fertiliser has been used. The simple solution is to use a balanced fertiliser which has the right ratios of nitrogen and phosphorus. Manures, as well as blood & bone also contain both of these macronutrients, but no potassium (potash), so if you’re feeding citrus trees with these, add some sulphate of potash, or seaweed extract, which is also high in potash for a balanced feed.
Preventing soil loss due to erosion is important as phosphorus is mainly present in the upper layers of soil. In very sandy soils add organic matter to better retain nutrients. Reducing soil compaction is important as compacted soil reduces uptake of phosphorus.
Overfeeding a tree is as bad as underfeeding it, as both erroneous practises will cause problems.
Symptoms of excess phosphorus include:
- Smaller fruit
- Thinner peels
- Higher percentage of juice
- Increased chance of regreening (where overripe citrus fruit begin taking on a green tinge).
These are the effects on fruit quality.
Excess phosphorus won’t reduce yields, but a citrus tree’s ability to absorb certain micronutrients will be reduced, which can accentuate the effects of low zinc availability, induce iron deficiency and affect copper uptake. Zinc and iron deficiencies will lead to weakened and discoloured plant tissues.
Too much is never a good thing, and an overabundance of phosphorus in the soil may also damage or kill surrounding plants, so it’s best to feed citrus trees moderate amounts regularly.
Feeding citrus every 6-8 weeks, from the start of spring to the start of autumn, with a balanced organic fertliser, is a good way to maximise growth without harming the soil or the tree!
Other articles on citrus problems and how to fix them:
- Citrus Nutrient Deficiency – Yellow Leaves
- Citrus Problems – Why Is My Citrus Tree Dying?
- Citrus Problems – Why Citrus Fruit Splitting Occurs and How To Prevent It
- Citrus Problems – Why Citrus Fruit Drops and Flowers Fail to Develop
- Citrus Nutrient Deficiency – Yellow Leaf with Green Veins
- Citrus Problems – Citrus Yellow Veins on Green Leaf in Winter
- Citrus Problems – How to Control Citrus Gall Wasp, Methods That Work