NPK for Fruit Trees: The Best Fertiliser Ratios for Healthy Growth & Sweeter Harvests
Choosing the right NPK for fruit trees can completely transform your harvest.
If your tree has lush leaves but very little fruit, pivot your feeding at the first sign of buds and prioritise a fruit-focused formula.
Why 4-18-38 + CaMg for Fruit Trees?
4-18-38 provides low nitrogen (to prevent excess foliage at bloom), high phosphorus (to support blossoms)
and high potassium (to drive fruit size, colour and sweetness). Added calcium & magnesium improve firmness,
reduce common fruit disorders, and support smooth nutrient flow during fruit fill.
Use CompleteGrow Advanced NPK 4-18-38 + Calcium & Magnesium
When to Switch to 4-18-38
- Build: Early season (vegetative) — establish canopy and structure with a balanced feed (e.g., 20-20-20).
- Pivot: At first visible buds — switch to 4-18-38 + CaMg to prioritise blossom and fruit set.
- Develop: Fruit set → colour — stay on 4-18-38 + CaMg for size, sweetness and firmness.
- Finish: Near harvest — gradually reduce feeding to avoid pushing soft, late growth.
Lots of leaves but few flowers? → Move to 4-18-38 + CaMg at first buds.
Pale or small fruit? → Stay consistent on fruiting feeds and ensure steady watering.
fruit set, size, colour, sweetness and firmness.
Shop CompleteGrow 4-18-38 + CaMg
Optimising Fruit Set & Flavour with 4-18-38 + CaMg
As fruit trees mature, their nutritional priorities shift.
They no longer need large doses of nitrogen for foliage but instead demand a fertiliser that powers roots, flowers, and fruit development.
This is where CompleteGrow NPK 4-18-38 + Calcium & Magnesium stands out.
The formula’s low nitrogen (4%) prevents over-growth of leaves that can shade blossoms and reduce fruit set.
Its high phosphorus (18%) stimulates extensive root networks and fuels flower formation, ensuring more blossoms convert into healthy fruit.
Meanwhile, the very high potassium (38%) improves sugar movement inside the plant, resulting in sweeter, denser, better-coloured fruit.
Phosphorus and potassium act in partnership.
Phosphorus sets the stage for fruiting by strengthening the root system and floral structure, while potassium manages quality—sweetness, juiciness, and disease resistance.
When applied together in the correct ratio, fruit trees experience a visible transformation in both yield and flavour.
Calcium and Magnesium further strengthen this blend.
Calcium reinforces fruit cell walls, preventing cracking and extending shelf life.
Magnesium boosts chlorophyll production and energy conversion, which improves nutrient transport and supports flavour development.
The combination keeps leaves greener for longer and enhances sugar synthesis during ripening.
Across apples, citrus, mangoes, and stone fruit, growers using 4-18-38 report denser blossoms, reduced fruit drop, and noticeably sweeter crops within one or two feeding cycles.
It’s a scientifically balanced ratio that gives fruit trees what they need precisely when they need it.
Feeding Schedule, Best Practices & FAQs
Applying fertiliser at the correct time ensures every nutrient supports the right growth phase.
Here’s how to schedule feeding throughout the year for consistent yields.
Seasonal Feeding Guide (Australia)
- Early Spring: Use CompleteGrow 20-20-20 Liquid to promote new shoots and establish foliage after dormancy.
- Mid-Spring to Summer: Switch to CompleteGrow 4-18-38 + CaMg every 3–4 weeks to power flowering and fruit development.
- Autumn: Apply lighter 4-18-38 feeds to reinforce roots and strengthen wood before winter rest.
Always water before and after fertilising.
Apply product around the canopy’s drip line — where most feeder roots are — and never heap fertiliser near the trunk.
Finish with mulch to stabilise soil moisture and temperature.
Best Practice Recommendations
- Feed early morning or late afternoon to reduce evaporation and maximise uptake.
- Mulch regularly to conserve soil moisture and protect beneficial microbes.
- Watch leaf colour: deep green means balanced NPK; pale leaves can indicate magnesium or nitrogen deficiency.
- After pruning or harvesting, apply 4-18-38 to stimulate recovery and new bud growth.
CompleteGrow Two-Stage Feeding Program
- CompleteGrow NPK 20-20-20 Liquid Fertiliser – Balanced nutrition for root and leaf growth.
- CompleteGrow NPK 4-18-38 + Calcium & Magnesium – High-P & K blend to optimise flowering, fruit set, and taste.
Used in sequence, these two formulations cover the complete lifecycle of your fruit trees — from early structural growth to mature fruiting and harvest.
Frequently Asked Questions
What is the best NPK ratio for fruit trees?
For fruiting phases, 4-18-38 + CaMg is the most effective ratio — it promotes root health, flowering, and fruit sweetness.
Use 20-20-20 earlier in the season to build foliage.
When should I fertilise fruit trees in Australia?
Begin feeding in late winter (August) and continue through autumn (March).
Adjust frequency to climate and tree variety.
Why are high-phosphorus and high-potassium fertilisers recommended?
They increase blossom strength, reduce fruit drop, and enhance colour and flavour by directing the plant’s energy toward fruit instead of leaves.
Can I use the same fertiliser for different fruit trees?
Yes. CompleteGrow 4-18-38 + CaMg suits all fruit-bearing trees including citrus, apples, mangoes, figs, and stone fruits.
Is liquid or soluble powder better for fruit trees?
Powdered formulations like 4-18-38 + CaMg dissolve easily in water and deliver faster, more efficient nutrient uptake than slow granular fertilisers.
