# Organic or synthetic fertilizer: how they differ and what a tree needs

> Why a root takes the same nutrients from either bag, how organic and synthetic fertilizers differ in release, salts and soil life, and when a tree needs any.

Published 2026-10-01. Tags: Protection & care, How trees work. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/organic-or-synthetic-fertilizer/

A tree root takes up the same dissolved ions of nitrogen, phosphorus and potassium whether they came from a synthetic fertilizer or an organic one. Organic nitrogen must first be broken down by soil microbes into ammonium or nitrate. The two kinds of fertilizer differ in what comes with the nutrient, how fast they release it and what they do to the soil. Most trees in the ground need neither until a soil test or the tree shows a deficiency. Yellow leaves with green veins often mean the soil pH has locked up iron, which no fertilizer corrects until the pH is right.

![Two young pin oaks on a grassy roadside strip, the left one pale yellow-green and the right one dark green](https://bluewhaletrees.com/img/articles/organic-or-synthetic-fertilizer-1-1000.jpg?v=bed2e590 "A pin oak with chlorosis, left, beside a healthy one in Baltimore, Maryland, on July 27, 2016. High soil pH can leave pin oak chronically short of iron.")
*Photo: Famartin, [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0) ([source](https://commons.wikimedia.org/wiki/File%3A2016-07-27_10_34_35_A_Pin_Oak_sapling_with_chlorosis_(left)_next_to_a_healthy_Pin_Oak_sapling_(right)_along_Maryland_State_Route_2_(Potee_Street)_just_north_of_Maryland_State_Route_173_(Patapsco_Avenue)_in_Baltimore_City%2C_Maryland.jpg))*

## What the three numbers on a fertilizer bag mean

Nitrogen, phosphorus and potassium are the primary macronutrients, needed in the largest amounts. Calcium, magnesium and sulfur, the secondary macronutrients, come next.

The three numbers on the bag, its guaranteed analysis, give the percentages by weight of nitrogen, phosphate and potash, always in that order. Phosphorus is reported as phosphate, P2O5, and potassium as potash, K2O. A 100-pound bag of 0-20-10 holds no nitrogen, 20 pounds of phosphate, 10 of potash and 70 pounds of filler. A balanced 10-10-10 comes out the same: the three nutrients make up 30 percent of the weight, and the other 70 percent is mostly filler. Filler makes the fertilizer easier to spread and less likely to burn plants.

### Synthetic products

Synthetic fertilizers are made by industrial processes or mined, then purified, and their nutrients dissolve readily in water. The common ones are listed highest nitrogen first:

| Synthetic product | Analysis, N-P-K | Release |
| --- | --- | --- |
| Urea | 46-0-0 | Rapid |
| Ammonium nitrate | 33-0-0 to 34-0-0 | Rapid |
| Ammonium sulfate | 21-0-0 | Rapid |
| Balanced blend | 10-10-10 | Rapid |
| Triple superphosphate | 0-46-0 | Rapid |
| Muriate of potash | 0-0-60 | Rapid |

### Organic and natural products

Organic fertilizers come from things that lived: manure, compost, seed meals and animal byproducts. Mined ores such as rock phosphate count as natural fertilizers too. Here rapid release means under a month, medium one to four months and slow four months to a year:

| Organic or natural product | Analysis, N-P-K | Release |
| --- | --- | --- |
| Feather meal | 11-0-0 to 15-0-0 | Slow |
| Blood meal | 12-1.5-0.6 | Medium to rapid |
| Cottonseed meal | 6-2.5-1.7 | Slow to medium |
| Fish emulsion | 5-2-2 | Medium to rapid |
| Bone meal | About 3-15-0 | Slow to medium |
| Compost | 1.5-1-1.5 | Slow |
| Kelp | 0.9-0.5-1 | Slow |
| Greensand | 0-1-5 to 0-2-5 | Slow |

Organic analyses vary from batch to batch, and the products are dilute, so they go on in large amounts. Bone meal's phosphorus is available only in soil below pH 7.0.

## Micronutrients, and why soil pH matters more than the bag

The micronutrients are boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc, needed in far smaller amounts. Compost and manure carry them along with nitrogen, phosphorus and potassium, and kelp is mainly a micronutrient source. A synthetic label guarantees only the elements it lists.

Iron chlorosis, which yellows young leaves between green veins, is a symptom of soil pH rather than of a soil short of iron. Above pH 7.0, iron in the soil becomes less available to roots, and manganese and zinc grow scarcer too. Chelated iron, iron gripped by a carrier molecule, greens the leaves for a while when sprayed on or added to the soil. The yellowing returns until the pH is corrected.

On alkaline soil, [pin oak](https://bluewhaletrees.com/plants/pin-oak/index.html.md) and [river birch](https://bluewhaletrees.com/plants/river-birch/index.html.md) run short of iron and red maple of manganese. We do not grow red maple. These natives that we grow turn yellow on high-pH ground:

- [Black gum](https://bluewhaletrees.com/plants/black-gum/index.html.md) prefers acid soil and turns yellow on limestone. Zones 4 to 9, 40 to 70 feet.
- [Sourwood](https://bluewhaletrees.com/plants/sourwood/index.html.md) shows yellow leaves with green veins when the soil is too sweet. Zones 5 to 9, 25 to 50 feet.
- [Bald cypress](https://bluewhaletrees.com/plants/bald-cypress/index.html.md) turns its needles yellow on limestone soils. Zones 4 to 10, 50 to 80 feet.
- [Northern pin oak](https://bluewhaletrees.com/plants/northern-pin-oak/index.html.md) takes a higher pH than pin oak, yet strongly alkaline soil can still yellow it. Zones 3 to 7, 50 to 70 feet.

On limy ground, choose a tree that takes it. [Shumard oak](https://bluewhaletrees.com/plants/shumard-oak/index.html.md) is little troubled by the yellowing alkaline soil causes in many oaks, and [blue ash](https://bluewhaletrees.com/plants/blue-ash/index.html.md) prefers thin, alkaline ground over limestone.

## How fast each kind feeds, and where the surplus goes

A synthetic nitrogen salt dissolves in soil water and reaches the roots quickly. Organic nitrogen is held in proteins that microbes must break down first, a step called mineralization, at a pace set by moisture and warmth. Compost releases only about a tenth of its nitrogen in its first season.

Slow-release synthetics fall between the two. Sulfur-coated urea, resin-coated granules and ureaformaldehyde feed through the season, and labels list that share as water-insoluble nitrogen. For trees, a third to a half of the nitrogen in that form is the usual advice.

Fertilizers are salts, and too much at once draws water out of roots and leaves. The leaves brown at the edges, wilt and drop. Synthetics are easier to overapply, while slow-release products seldom burn. Never feed a tree in drought, when the salts can burn roots that lack water.

What the roots miss can be lost. Nitrate is not held by soil, so rain carries it below the roots into groundwater. Phosphorus binds tightly and barely leaches, but it leaves with runoff and eroded soil. In lakes, the extra nitrogen and phosphorus feed algal blooms that use up the oxygen.

![Bright green algae coating the water along a pebble shore, with a boathouse, a dock and trees behind](https://bluewhaletrees.com/img/articles/organic-or-synthetic-fertilizer-2-1000.jpg?v=6f850d3f "A harmful algal bloom along the shore of Kelley's Island, Lake Erie, Ohio, in September 2009. Phosphorus carried off in runoff and eroded soil feeds such blooms.")
*Photo: NOAA Great Lakes Environmental Research Laboratory, [Public domain](https://commons.wikimedia.org/wiki/File%3AHarmful_algae_bloom._Kelley's_Island%2C_Ohio._Lake_Erie_(4075269443).jpg) ([source](https://commons.wikimedia.org/wiki/File%3AHarmful_algae_bloom._Kelley's_Island%2C_Ohio._Lake_Erie_(4075269443).jpg))*

Organic products can overdo it too. Most slow-release fertilizers, natural or synthetic, are complete, so they can add phosphorus the soil already has.

## Cation exchange: how soil holds what you add

Clay and organic matter carry negative charges that hold positively charged nutrients, the cations, against water draining through the soil: calcium, magnesium, potassium and ammonium. As roots draw from the soil water, held cations move off to replace what they take.

The size of that store is the cation exchange capacity, or CEC, which a soil test reports in milliequivalents per 100 grams of soil. Sand holds 1 to 5, loam 5 to 15 and clay over 30. Organic matter holds 200 to 400, more than any clay.

[Soil texture](https://bluewhaletrees.com/articles/glossary/index.html.md) is permanent, but organic matter can be added, and it raises a sandy soil's CEC. Liming an acid soil raises it too.

Nitrate is a negative ion, so the exchange does not hold it. Ammonium is held, but microbes soon turn most of it into nitrate, a conversion that releases acid. Ammonium sulfate acidifies soil the most, ammonium nitrate less and urea least. On sand, with little exchange to hold a big dose, feed less at a time and more often.

## The rhizosphere, where roots trade with soil life

The rhizosphere is the zone of soil around a root that the root itself changes, and it has no fixed size. Roots release 10 to 40 percent of the carbon their leaves fix, as sugars, organic acids, amino acids and mucilage. That food packs the rhizosphere with 1,000 to 2,000 times the microbes of bare soil.

Mycorrhizal fungi partner with the roots of about 80 percent of flowering plants and all conifers, trading nutrients for sugar. Their threads give a root about ten times the absorbing surface and two to three times the phosphorus uptake. Ectomycorrhizae, which sheathe the root tips, occur mainly on forest trees.

High soluble phosphate holds back arbuscular mycorrhizae, the commonest kind. The roots take up phosphorus directly and stop hosting the fungus, and the plant loses its help with water and disease.

![Pale roots spread on a black background, with several round pinkish nodules clustered along them](https://bluewhaletrees.com/img/articles/organic-or-synthetic-fertilizer-3-1000.jpg?v=cc0e828e "Root nodules on a black locust, photographed on August 17, 2012. Bacteria housed in root nodules turn nitrogen from the air into a form the plant can use.")
*Photo: Ninjatacoshell, [CC BY-SA 3.0](https://creativecommons.org/licenses/by-sa/3.0) ([source](https://commons.wikimedia.org/wiki/File%3ARobinia_pseudoacacia_root_nodules.JPG))*

Bacteria in root nodules turn nitrogen from the air into ammonia a plant can use. A legume starts a nodule with a signal it sends when starved of nitrogen. [Black locust](https://bluewhaletrees.com/plants/black-locust/index.html.md) fixes nitrogen, and [smooth alder](https://bluewhaletrees.com/plants/smooth-alder/index.html.md) and [New Jersey tea](https://bluewhaletrees.com/plants/new-jersey-tea/index.html.md) carry such bacteria in root nodules and need no fertilizer at all.

These partnerships are much of why a tree in a wood needs no bag of fertilizer:

> "Mature trees in a natural environment seldom require fertilizer due to their extensive root systems and symbiotic relationships with naturally occurring soil micro-organisms." (University of New Hampshire Extension, *Fertilizing Trees and Shrubs*)

## What synthetic fertilizer does to soil life

Decades of farm trials say no, with a catch. In 2026, Shengwen Xu and others pooled 501 long-term farm experiments, a median of 25 years old. Mineral fertilizer raised soil organic carbon by 14 percent, and microbial biomass rose with it, while the number of microbial kinds held steady. The catch was acid soil, with pH down 0.31 units on average, which reshuffled the dominant bacteria:

> "Long-term mineral fertilization had both detrimental and beneficial effects on agricultural soils." (Xu and others, *Global long-term agricultural experiments reveal consequences of mineral fertilization for soil microbiomes*, 2026)

Those were croplands. Soil microbes live on organic matter and decline without a steady supply of it. Most synthetics carry no carbon, and they may not support beneficial microbes as well as organic fertilizers do.

## What a tree in the ground needs

### Test first, and feed nothing at planting

Start with a soil test rather than a bag, sampling 6 inches deep near the drip line. No lab test measures nitrogen, because it changes form and is lost too quickly.

Feed nothing at planting. A new tree spends its energy on roots, and nitrogen then seems to suppress root growth rather than help it. Water it instead, as the [care calendar](https://bluewhaletrees.com/articles/care-calendar-for-a-new-tree/index.html.md) sets out.

Leaves that decay where they fall return at least a pound of nitrogen to every 1,000 square feet beneath the tree. That is why [leaf litter beats a bag of fertilizer](https://bluewhaletrees.com/articles/leaf-litter-not-wood-chips/index.html.md).

### Signs, rates and timing

Feed an established tree only when it shows a need: pale or yellow leaves, small leaves, early fall color and leaf drop, or short twig growth. Rule out drought, girdling roots and trunk damage first.

Rates are figured on nitrogen, which drives growth most, and run from 1 to 6 pounds of actual nitrogen per 1,000 square feet of root zone a year. Use the low end for evergreens and fast-release products, and split the dose on sand. An older tree may need feeding only every three to five years.

Every tree guide we read accepts early spring, around the time growth begins. They split over late summer. The Maryland, Virginia and New Hampshire extension services warn that late nitrogen can push soft growth that frost then catches, and Clemson's rejects that worry:

> "Those claims are unfounded, particularly with conifers and most hardwoods." (Clemson Cooperative Extension, *Fertilizing Trees & Shrubs*)

### Natives that do better unfed

Some natives of lean ground should not be fed at all:

- [American smoketree](https://bluewhaletrees.com/plants/american-smoketree/index.html.md) is content in poor, alkaline ground and wants no fertilizer. Zones 4 to 8, 20 to 30 feet.
- [Leadplant](https://bluewhaletrees.com/plants/leadplant/index.html.md) flops in rich soil or when fertilized. Zones 2 to 8, 1 to 3 feet.
- [Bristly locust](https://bluewhaletrees.com/plants/rose-locust/index.html.md) fixes its own nitrogen, and rich ground only makes it run harder. Zones 5 to 8, 3 to 8 feet.
- [Black huckleberry](https://bluewhaletrees.com/plants/black-huckleberry/index.html.md) wants no fertilizer, lime, wood ash or hard well water. Zones 3 to 7, 1 to 3 feet.

Blue Whale Trees grows all four. The [tree finder](https://bluewhaletrees.com/tools/tree-finder/?want=nitrogen-fixing) lists every nitrogen fixer we grow, plants that make their own nitrogen.

## Where to start

The short version:

- **Test the soil** before buying anything, and fix the pH before feeding a yellowing tree.
- **Keep the leaves** under the canopy as [leaf litter](https://bluewhaletrees.com/articles/leaf-litter-not-wood-chips/index.html.md) for the tree and its soil life.
- **Plant for the ground you have:** the [tree finder](https://bluewhaletrees.com/tools/tree-finder/?want=nitrogen-fixing) opens on the nitrogen fixers we grow.
- **Ask about a yellowing tree** on the [support page](https://bluewhaletrees.com/support/index.html.md), with a photo of the whole tree and a close-up of the leaves.

## Common questions

### Is organic fertilizer better than synthetic for trees?

Not as food: a root takes up the same nitrogen, phosphorus and potassium ions from either. Organic products release slowly and feed soil life, while synthetics act fast but burn more easily when overapplied.

### Does synthetic fertilizer kill soil microbes?

Not in 501 long-term farm experiments, where microbial biomass rose and the number of microbial kinds held steady. It did acidify the soil, and it does not feed soil life the way organic matter does.

### What do the three numbers on fertilizer mean?

They are the percentages by weight of nitrogen, phosphate and potash, always in that order. A 50-pound bag of 5-10-15 holds 2.5 pounds of nitrogen, 5 of phosphate and 7.5 of potash.

### What is cation exchange capacity?

It is how much positively charged nutrient, such as calcium, magnesium, potassium and ammonium, a soil's clay and organic matter can hold against leaching. Soil tests report it in milliequivalents per 100 grams, 1 to 5 for sand and over 30 for clay.

### Do trees need fertilizer?

Most established trees do not, and a newly planted tree should get none. Feed only when a soil test or poor growth and pale leaves show a lack, and correct the pH first if the leaves yellow between green veins.

## Sources

- [How to Convert an Inorganic Fertilizer Recommendation to an Organic One](https://fieldreport.caes.uga.edu/publications/C853/how-to-convert-an-inorganic-fertilizer-recommendation-to-an-organic-one/): University of Georgia Extension, Circular 853 (Reeves, Gaskin, Kissel, Boyhan, McLaurin and Harris, 2014), with Circular 1040, Cation Exchange Capacity and Base Saturation (Saha, 2022), and the Field Report page Understand the numbers on a fertilizer bag (Clint Waltz): organic nitrogen converted to ammonium or nitrate before uptake, the analyses, variability and release classes of the organic products, micronutrients in compost, manure and kelp, compost's first-season nitrogen, bone meal and pH, the cation exchange, nitrification, the unit and the values by texture, and the rest of a bag's weight as secondary nutrients and filler.
- [1. Soils & Plant Nutrients](https://content.ces.ncsu.edu/extension-gardener-handbook/1-soils-and-plant-nutrients): NC State Extension, Extension Gardener Handbook (Luke Gatiboni, 2022), with AG-613, A Gardener's Guide to Fertilizing Trees and Shrubs (Barbara Fair, 2026): the primary and secondary macronutrients and the micronutrients, the fertilizer grade, phosphate, potash and filler and what filler is for, synthetic and natural fertilizers and four of their analyses, ammonium sulfate's analysis and the release of muriate of potash, mineralization, salts and burn, nitrate leaching, phosphorus runoff and algal blooms, pH and the micronutrient metals, permanent texture and organic matter raising CEC, the soil sample at the drip line, and microbes that need organic matter.
- [Triple Super Phosphate (TSP, 0-46-0): A Good Source of P Nutrition](https://graincrops.ca.uky.edu/sites/graincrops.ca.uky.edu/files/cornsoynewsletter2024vol06issue12_Dec.pdf): University of Kentucky, Corn & Soybean News (John Grove, December 2024): triple superphosphate dissolving readily in moist soil, its phosphorus as available to plants as that of the ammonium phosphates.
- [Fertilizing Landscape Trees and Shrubs](https://www.pubs.ext.vt.edu/430/430-018/430-018.html): Virginia Cooperative Extension (Appleton and Kauffman): the analyses, release and soil reaction of the synthetic nitrogen sources, the signs that a tree needs feeding, rates of 1 to 6 pounds of nitrogen with less for evergreens, and the warning against late-summer feeding.
- [Fertilizing Trees and Shrubs and Nutrient Deficiency Symptoms](https://extension.umd.edu/resource/fertilizing-trees-and-shrubs-and-nutrient-deficiency-symptoms): University of Maryland Extension: the guaranteed analysis, no fertilizer at planting, no feeding in drought, iron chlorosis as a symptom of pH above 7.0, chelated iron as a passing fix, manganese and zinc in alkaline soil, and the warning against late-summer feeding.
- [How and When to Use Fertilizer on Plants](https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/how-and-when-to-use-fertilizer-on-plants/): The Morton Arboretum: iron shortage in pin oak and river birch and manganese in red maple on alkaline soil, water rather than fertilizer at planting, the causes to rule out first, older trees fed every three to five years, and spring feeding.
- [Fertilizing Trees & Shrubs](https://hgic.clemson.edu/factsheet/fertilizing-trees-shrubs/): Clemson Cooperative Extension, HGIC 1000 (Robert F. Polomski, 2023): little or no fertilizer for mature trees on good soil, nitrogen as the basis of rates, fertilizers as salts that draw water from leaves, the slow-release share, lower rates for fast-release nitrogen and split doses on sand, iron chlorosis that fertilizer cannot fix until the pH is right, and the quotation on late-season feeding.
- [Fertilizing Trees and Shrubs](https://extension.unh.edu/resource/fertilizing-trees-and-shrubs-fact-sheet): University of New Hampshire Extension (Catherine A. Neal): the quotation on trees in a natural environment, the numbers always in the same order, dilute organic products, slow-release nitrogen sources and the seldom burn, phosphorus overapplied by complete fertilizers, fallen leaves recycling nitrogen, no lab test for nitrogen, and the warning against late-summer nitrogen.
- [The Rhizosphere - Roots, Soil and Everything In Between](https://www.nature.com/scitable/knowledge/library/the-rhizosphere-roots-soil-and-67500617/): Nature Education Knowledge (David H. McNear Jr., 2013): the rhizosphere as a gradient, root exudates and the carbon they cost, microbe numbers, mycorrhizae, their reach and phosphorus, arbuscular and ectomycorrhizae, chelates, and root nodules, ammonia and the nitrogen-starved signal.
- [High phosphate reduces host ability to develop arbuscular mycorrhizal symbiosis without affecting root calcium spiking responses to the fungus](https://pmc.ncbi.nlm.nih.gov/articles/PMC3810610/): Balzergue, Chabaud, Barker, Bécard and Rochange, Frontiers in Plant Science 4 (2013): high phosphate holding back arbuscular mycorrhizae, roots that stop hosting the fungus, and the benefits the plant loses.
- [Global long-term agricultural experiments reveal consequences of mineral fertilization for soil microbiomes](https://pmc.ncbi.nlm.nih.gov/articles/PMC13454535/): Shengwen Xu and colleagues, Nature Communications 17 (2026): 501 long-term experiments with a median of 25 years, soil organic carbon up 14 percent, pH down 0.31 units through nitrification, microbial biomass up, richness unchanged, dominant bacteria reshuffled, and the quotation.