Fertilizer guide
Granular, slow-release, water-soluble or organic: how each works, which products suit fruit trees, vegetables and pots, and how to read your plant's leaves.
Reading the label: N-P-K
The three numbers on every bag are the percent by weight of nitrogen, phosphorus and potassium. A 20 lb bag of 8-3-9 holds 1.6 lb of nitrogen.
- N
- NitrogenLeaves and growth
- P
- PhosphorusRoots and flowers
- K
- PotassiumFruit quality and toughness
Kinds of fertilizer
How each one works, when it shines, and how to use it.
Granular vs slow-release vs water-soluble vs organic
| Property | Granular | Slow-release | Water-soluble | Organic |
|---|---|---|---|---|
| Starts working | 1 to 2 weeks | 1 to 2 weeks | Days | 2 to 4 weeks |
| Lasts | 4 to 8 weeks | 3 to 9 months | 1 to 2 weeks | 1 to 3 months |
| Burn risk | Medium | Low | High if too strong | Very low |
| Cost per feed | Low | Medium | Medium | Medium to high |
| Lost to heavy rain | Quite a lot | Little | A lot | Little |
| Best for | In-ground trees | Pots, young trees | Seedlings, pots, quick fixes | Organic gardens, soil health |
Common products
What they're good for and how they differ.
- 8-3-9Fruit tree mix 8-3-9Granular (quick-release)
Usually with magnesium and micronutrients
The classic Florida blend for mango, avocado, lychee and citrus: moderate nitrogen, high potassium for fruit, low phosphorus because Florida soils have plenty.
Look for a bag that lists magnesium (2 to 4%) and minor elements.
Fruit treesCitrus - 8-2-12Palm special 8-2-12Slow or controlled-release
4% magnesium, micronutrients
The University of Florida formula for palms, with its nitrogen, potassium and magnesium in slow-release form. Prevents the frizzled and yellow fronds palms get from potassium, magnesium and manganese shortages.
Also good for most tropical fruit trees on sandy soil.
PalmsFruit trees - 5-2-6Citrus-ToneEspoma · Organic (natural) fertilizerOrganic
Calcium, magnesium, beneficial microbes
Natural granular feed for citrus and other fruit trees. Gentle and slow, with extra potassium for fruit.
CitrusFruit treesPots - 4-3-4Holly-toneEspoma · Organic (natural) fertilizerOrganic
Sulfur, acidifying
Natural feed that slightly acidifies soil, for blueberries, gardenias, azaleas and camellias.
Acid lovers - 5-3-3Plant-toneEspoma · Organic (natural) fertilizerOrganic
All-purpose natural feed for trees, shrubs and beds.
Fruit treesVegetablesSoil building - 3-4-4Garden-toneEspoma · Organic (natural) fertilizerOrganic
Natural feed for vegetable gardens, a little higher in phosphorus and potassium for flowers and fruit.
Vegetables - 15-9-12Osmocote Smart-Release PlusOsmocote · Slow or controlled-release
Micronutrients
Coated prills that feed potted plants for months. Releases faster in hot weather.
In a Florida summer, expect about half the months on the label.
PotsFruit trees - 14-14-14Osmocote Flower & VegetableOsmocote · Slow or controlled-release
Balanced slow-release feed mixed into beds or potting mix at planting.
VegetablesPots - 24-8-16All-purpose water-soluble 24-8-16Miracle-Gro · Water-soluble powder or crystals
Micronutrients
The familiar blue crystals. Fast, cheap per feed, and easy to overdo.
Half strength for seedlings and herbs.
PotsVegetablesSeedlings - 20-20-20Classic All Purpose 20-20-20Jack's · Water-soluble powder or crystals
Micronutrients
Balanced water-soluble feed popular with growers for potted trees and nursery plants.
PotsSeedlingsVegetables - 18-18-21Tomato water-soluble 18-18-21Miracle-Gro · Water-soluble powder or crystals
Extra potassium for tomatoes, peppers and other fruiting vegetables.
VegetablesPots - 5-1-1Fish emulsion 5-1-1Alaska · Liquid concentrateOrganic
Liquid fish, high in quick nitrogen. Great for leafy greens and greening up pale plants.
Smells for a day; dogs love it, so water it in well.
VegetablesPotsSeedlings - 2-3-1Fish & seaweed 2-3-1Neptune's Harvest · Liquid concentrateOrganic
Trace minerals from kelp
Gentle liquid feed and transplant tonic. Can be sprayed on leaves.
VegetablesSeedlingsPots - 0-0-1Liquid seaweedLiquid concentrateOrganic
Trace minerals, natural growth hormones
Not much of a fertilizer on its own, but helps transplants root and plants handle heat stress.
SeedlingsFruit treesVegetables - 12-0-0Blood mealOrganic (natural) fertilizerOrganic
The fastest organic nitrogen. Fixes yellow, nitrogen-hungry greens.
Easy to burn plants with; use lightly and keep away from pets.
VegetablesSoil building - 12-0-0Feather mealOrganic (natural) fertilizerOrganic
Slow organic nitrogen that lasts for months as it breaks down.
Fruit treesVegetables - 3-15-0Bone mealOrganic (natural) fertilizerOrganic
About 20% calcium
Phosphorus and calcium for roots and flowers.
Does little in soil above pH 7, and many Florida soils already have plenty of phosphorus. Very attractive to dogs.
VegetablesSoil building - 3-1-2Alfalfa mealOrganic (natural) fertilizerOrganic
Mild feed and soil conditioner that soil life loves.
Soil buildingVegetables - 1-0-2Kelp mealOrganic (natural) fertilizerOrganic
Trace minerals
Dried seaweed with a broad mix of trace minerals.
Soil buildingFruit trees - 3-2-2Composted poultry manureOrganic (natural) fertilizerOrganic
Calcium; analysis varies
Pelleted or bagged chicken manure, composted so it won't burn. A good all-round organic feed.
VegetablesFruit treesSoil building - 1-0-0Worm castingsCompost and aged manureOrganic
Very gentle and rich in soil life. Mix into potting mix or seed-starting mix.
SeedlingsPotsSoil building - —CompostCompost and aged manureOrganic
The best soil builder there is. Mulch under trees or mix into beds.
Soil buildingVegetablesFruit trees - 6-4-0MilorganiteMilorganite · Organic (natural) fertilizer
Iron
Slow-release nitrogen made from treated wastewater solids. Popular on lawns and shrubs; some gardeners prefer not to use biosolids on food crops.
Soil building - 46-0-0UreaGranular (quick-release)
The most concentrated nitrogen. Cheap but easy to overdo and quick to wash away.
Fruit trees - 21-0-0Ammonium sulfateGranular (quick-release)
24% sulfur, acidifying
Nitrogen that also lowers soil pH a little over time. Good for blueberries and alkaline soils.
Acid loversAlkaline soil - 15.5-0-0Calcium nitrateWater-soluble powder or crystals
19% calcium
Quick nitrogen plus calcium. Helps with blossom-end rot when watering is also kept even.
VegetablesPots - 0-46-0Triple superphosphateGranular (quick-release)
Concentrated phosphorus. Only use it if a soil test says you're short; excess phosphorus pollutes waterways.
Vegetables - 0-0-50Sulfate of potashGranular (quick-release)
17% sulfur
Potassium for fruit size, sweetness and palm health, without the chloride in muriate of potash. Some forms are allowed in organic gardens.
Fruit treesCitrusPalms - 0-0-22Sul-Po-Mag (langbeinite)Granular (quick-release)Organic
11% magnesium, 22% sulfur
A natural mineral with potassium, magnesium and sulfur. Allowed in organic gardens.
PalmsCitrusFruit trees - 0-0-0Epsom salt (magnesium sulfate)Water-soluble powder or crystalsOrganic
10% magnesium, 13% sulfur
Fixes magnesium shortage: yellow older leaves with a green triangle at the base (citrus) or yellow-edged old fronds (palms).
It isn't a cure-all; it only helps plants that are short of magnesium.
CitrusPalmsAlkaline soil - 0-0-0Chelated iron EDDHAMicronutrients and chelates
About 6% iron
The only iron chelate that stays available up to about pH 9, so it's the one that works on limestone soil. Turns yellow new leaves green.
Apply as a soil drench. It's red and will stain concrete.
Alkaline soilCitrusAcid loversFruit trees - 0-0-0Chelated iron EDTA or DTPAMicronutrients and chelates
Cheaper iron chelates. EDTA works up to about pH 6.5 and DTPA up to about 7.5, so they suit pots and neutral or acid soils.
Acid loversPots - 0-0-0Minor element spray (Mn, Zn, B)Foliar spray
Manganese, zinc, boron
A leaf spray that University of Florida recommends for young fruit trees on alkaline soil, applied on new flushes of growth.
Fruit treesCitrusAlkaline soil - —AzomiteAzomite · Soil amendmentsOrganic
Trace minerals from volcanic rock
Rock dust with a broad mix of trace minerals. A gentle long-term addition; don't expect quick results.
Soil building - —Dolomitic limeSoil amendmentsOrganic
Calcium and magnesium; raises pH
Raises the pH of acidic soil and adds calcium and magnesium.
Only with a soil test showing pH below what your plants want. Never on limestone soil.
Soil building - —Elemental sulfurSoil amendmentsOrganic
Lowers pH
Soil bacteria slowly turn it into acid, lowering pH over months. Essential for blueberries in neutral soil.
Doesn't work well on soil full of limestone rock: the rock neutralizes it. Use pots or raised beds there.
Acid loversAlkaline soil - —GypsumSoil amendmentsOrganic
Calcium and sulfur; no pH change
Adds calcium and sulfur without changing pH. Helps salty or sodic soils and calcium-hungry crops like peanuts and tomatoes.
Soil buildingVegetables
What each nutrient does
Where the yellowing starts tells you a lot: old leaves first points to nitrogen, potassium or magnesium; new leaves first points to iron, manganese, zinc or sulfur.
- NNitrogenLeaves, shoots and green color.Shows on old leaves
Too little: Older leaves turn pale yellow all over; growth is slow and thin.
Too much: Lush leafy growth, fewer flowers and fruit, more pests.
Sources: Blood and feather meal, fish emulsion, ammonium sulfate, urea, most blends.
- PPhosphorusRoots, flowering and energy.Shows on old leaves
Too little: Dull, dark or purplish older leaves; poor roots and flowering.
Too much: Locks up iron and zinc and runs off into waterways. Many Florida soils already have plenty.
Sources: Bone meal, superphosphate, balanced blends.
- KPotassiumFruit size and sweetness, drought and disease resistance.Shows on old leaves
Too little: Older leaves scorched or yellow at the edges and tips; palm fronds spotted and frizzled.
Sources: Sulfate of potash, Sul-Po-Mag, kelp, fruit tree and palm blends.
- CaCalciumCell walls; firm fruit and strong new growth.Shows on new leaves
Too little: Distorted new leaves, blossom-end rot on tomatoes and peppers, split fruit.
Sources: Gypsum, lime, calcium nitrate, bone meal. Even watering matters as much.
- MgMagnesiumThe heart of chlorophyll.Shows on old leaves
Too little: Older leaves yellow between the veins; citrus shows a green triangle at the leaf base.
Sources: Epsom salt, Sul-Po-Mag, dolomitic lime, palm and fruit tree blends.
- SSulfurProteins and flavor.Shows on new leaves
Too little: New leaves pale yellow all over (like nitrogen, but on new growth).
Sources: Ammonium sulfate, gypsum, sulfate of potash, Epsom salt.
- FeIronMaking chlorophyll.Shows on new leaves
Too little: New leaves bright yellow with green veins. Very common on alkaline and limestone soil.
Sources: Chelated iron (EDDHA for high pH), acidifying the soil, compost.
- MnManganesePhotosynthesis.Shows on new leaves
Too little: New leaves pale between the veins with broad green bands; palm new fronds frizzled.
Sources: Manganese sulfate, minor element sprays.
- ZnZincGrowth hormones and leaf size.Shows on new leaves
Too little: Small, narrow, mottled new leaves bunched at the tips (little leaf). Common in citrus and mango.
Sources: Zinc sulfate, minor element sprays.
- BBoronFlowering, fruit set and new tissue.Shows on new leaves
Too little: Dying shoot tips, cracked or lumpy fruit, poor fruit set.
Too much: Toxic at only a little more than plants need: yellow, burned leaf edges.
Sources: Borax (in tiny amounts), minor element sprays.
- CuCopperEnzymes and strong stems.Shows on new leaves
Too little: Twig dieback and gummy bark. Rarely short; copper fungicides usually supply plenty.
Sources: Copper sulfate, minor element sprays, copper fungicides.
Soil pH
pH decides which nutrients plants can actually reach. A soil test tells you yours.
- pH Below 5.5. Very acid. Blueberries are happy; most trees are short of calcium and magnesium. Lime helps.
- pH 5.5 to 6.5. The sweet spot for most fruit, vegetables and herbs. Nutrients are easiest to get.
- pH 6.5 to 7.5. Fine for most plants. Iron and manganese start to get harder to reach; EDTA iron stops working above 6.5, DTPA above 7.5.
- pH Above 7.5. Alkaline, like limestone soil. Expect iron, manganese and zinc shortages. Use EDDHA iron, foliar minor elements and mulch.
Feeding tips
- Feed little and often. Several small feedings beat one big one, especially in sandy soil.
- Water before and after feeding. Fertilizer on dry roots burns them.
- Spread it out to the drip line and beyond, where the feeder roots are, not in a pile at the trunk.
- Hold off on nitrogen for a few weeks before you expect flowers (mango, lychee), or you'll get leaves instead of bloom.
- Stop feeding cold-tender trees in late fall so new growth isn't caught by frost.
- Yellow new leaves are usually a micronutrient or pH problem, not too little fertilizer.
- Many Florida counties restrict nitrogen and phosphorus on lawns and landscape plants during the summer rainy season. Check your county's rules.
- When in doubt, get a soil test: your county extension office does them cheaply.
Formulas change and products differ by region; the label on your bag is always the authority for rates and safety.
Questions
- Is water-soluble or granular fertilizer better?
- Neither is better everywhere. Granular feeds for weeks and suits in-ground trees; water-soluble works in days but lasts only a week or two, so it suits pots, seedlings and quick fixes. Slow-release granules feed for months with little risk of burning.
- What do the three numbers on fertilizer mean?
- N-P-K: the percent by weight of nitrogen, phosphorus and potassium. A 20 lb bag of 8-3-9 holds 1.6 lb of nitrogen, 0.6 lb of phosphate and 1.8 lb of potash.
- Why are my plant's new leaves yellow with green veins?
- New leaves bright yellow with green veins. Very common on alkaline and limestone soil. Use chelated iron (EDDHA on soil above pH 7.5) and check your soil pH.
- What fertilizer is best for fruit trees in Florida?
- A fruit tree blend such as 8-3-9 with magnesium and minor elements, plus minor element sprays on new growth for young trees on alkaline soil. Small amounts every 6 to 8 weeks for young trees; 3 or 4 times a year for established ones.
Let Gardyn keep track
Log feedings, keep stock of your bags and get reminded when each tree is due, with feeding paused outside its season.
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