Blood Meal, Feather Meal, Fish Meal and Guano: How Their Nitrogen Release Actually Differs

AZ_Growshop_Asia · October 15, 2026, 4:35am

Adapted and edited from the book True Living Organics by The Rev (Green Candy Press; first published 2012, later editions 2016 and 2024). The laboratory figures come from a 2006 study by Hartz and Johnstone, listed under References.

Most living soil recipes say four dry nitrogen sources release at very different speeds. In living soil, soil organisms feed the plant from dry organic inputs instead of bottled nutrients. The recipes call blood meal fast and risky and feather meal slow and long-lasting. A controlled laboratory study found that the gap is much smaller than that.

The difference matters most if you grow a sativa that flowers for ten weeks in a small pot. This article covers what each material is and what the book claims. It also shows what the study measured and what to change in your feeding plan.

1. Why dry organic nitrogen behaves differently from bottled liquid feed

A complete bottled liquid nutrient contains nitrogen as nitrate and ammonium. These are the two mineral forms of nitrogen that roots can take up, so the plant can use them right away. You set the dose every time you water.

Dry organic amendments hold their nitrogen inside protein and other organic compounds. Roots cannot take nitrogen up in that form. Soil microbes have to convert it first (see section 2).

With dry amendments, you do not dose nitrogen at each watering. You put a stock of nitrogen into the soil, and soil life decides how fast it comes out. If you put in too little, the plant runs out around week six of flower. If you put in too much, you burn roots or suppress the microbes that the whole method depends on.

What to do: Before you mix, write down how many weeks your cultivar flowers. You need that number to judge whether one starting mix can last the whole crop or when you must add more (section 7).

2. Mineralisation: how microbes turn a meal into plant food

Mineralisation is the name for that conversion. Soil microbes break down the protein in the meal. They release its nitrogen first as ammonium, then convert it to nitrate. Roots can then take up the nitrogen.

The speed depends on four things: temperature, moisture, the size and activity of the microbial population, and the material itself. The material matters because some proteins are much easier for microbes to break down than others. That is why blood meal, feather meal and guano have different reputations (section 3).

What to do: Write down the date you mix each amendment into the soil, either on the pot or in your grow diary. Release time counts from that date, not from the day you planted.

3. What each material is

The descriptions below are summarised from True Living Organics, a book on organic cannabis growing by a grower who writes under the pen name The Rev. They are paraphrased, not quoted.

Blood meal. This is dried, powdered animal blood. It is very high in nitrogen and also supplies iron and a broad range of trace elements. Its protein dissolves easily, so microbes can break it down quickly.

Feather meal. Poultry feathers are hydrolysed, meaning cooked under heat and pressure, and then ground. Feather meal supplies nitrogen and some calcium. Its nitrogen is held in keratin, the same tough protein found in hooves, horns, hair and nails.

Microbes break keratin down much more slowly than blood protein, which is where feather meal’s “slow release” reputation comes from. Hydrolysis is the step that opens keratin up to microbes. The book lists hoof meal, horn meal and hair as substitutes. It says the animals should be healthy and untreated for fleas, and the hair should not be dyed. Read section 9 before you rely on these substitutes.

Fish meal. This is dried, ground whole fish. It contains more phosphorus than the other three materials. On a fertiliser label, the NPK numbers give nitrogen, phosphorus and potassium content in that order. The Rev gives 11-5-1 for the fish meal he uses.

He notes that the phosphorus in fish meal binds well with calcium, so it does not release too fast. He sees this as a good thing in a living root zone. Fish meal is expensive, so he does not mix it through the whole pot. Instead, he uses it in three ways:

In core feeding, where you fill a few holes in the soil with dry fertiliser.
As a top-dress at the start of flower. Top-dressing means spreading dry fertiliser on the soil surface and watering it in.
In teas (fertiliser steeped in water), in place of liquid fish fertiliser (fish emulsion).

Guano. This is aged bird or bat droppings. Its analysis varies a lot from product to product, and section 4 explains why that matters.

What to do: Write the NPK from the label on every bag you own. You will need these figures to choose the right material for each stage in sections 4 and 7.

4. Guano: the two types, their analysis and safe handling

Guano is sold in two classes that do different jobs:

Class	Example NPK	When it is used
High-nitrogen	9-3-1; 10-8-2 for a more balanced type	Vegetative growth and early flower
High-phosphorus	0-5-0 or 2-10-1	Mid-flower

You cannot swap one class for the other. Buying a high-P guano when you needed high-N is a much bigger mistake than buying a mediocre product of the right class. The original article does not say which of these examples come from bats and which from seabirds. The analysis also depends on how fresh or aged the deposit is, not only on the animal. So choose by the analysis on the label.

Commercial guano is normally sold aged or processed. You can apply it directly, including in guano tea. The real risks of burning and disease come from fresh, raw droppings, so do not put those straight into a pot.

Bird and bat droppings can also carry Histoplasma, the fungus that causes the infection histoplasmosis. People catch it by breathing in dust. The US National Institute for Occupational Safety and Health (NIOSH) has published guidance called Histoplasmosis: Protecting Workers at Risk. It advises keeping dust down when you disturb droppings, for example by wetting them first. It also advises wearing a NIOSH-approved respirator (a fitted, certified dust mask), gloves and eye protection. The current NIOSH web guidance lists a filtering facepiece respirator, such as an N95, as the minimum for medium-risk tasks. For high-risk tasks, it recommends a powered air-purifying respirator or a full-facepiece respirator with 100-series filters.

Translator’s note: Open guano bags and mix them outdoors or in a well-ventilated area, not in a closed grow tent. If a seller cannot tell you whether the guano is aged or processed, treat it as raw.

What to do: Before you buy guano, check which class the label analysis puts it in. When you open the bag, wet the guano first and wear a respirator, gloves and eye protection.

5. What the book claims

True Living Organics draws a sharp line between these materials. In paraphrase: guano and blood meal release nitrogen very fast and very strongly, while feather meal releases much less nitrogen but stays in the soil far longer.

The book’s practical rules follow from that view:

Be very careful with blood meal and guano because they burn plants quickly.
Use feather meal as a backup nitrogen supply for long-flowering sativas in small pots.
Mix feather meal into the soil to prevent nitrogen deficiency late in flower.

The Rev also makes a point that many growers miss. In a very active living-soil mix, blood meal gets used up faster than standard advice suggests. The reason is that the microbial population is larger and more active than in ordinary soil.

What to do: If your plan relies on feather meal to carry late flower, mark that step. Section 7 explains why to change it.

6. What the laboratory study measured

The study is Hartz and Johnstone (2006), published in the journal HortTechnology (details under References). It was carried out at the University of California, Davis, a public agricultural university in the United States.

How the study was run:

The researchers mixed seabird guano, hydrolysed fish powder, feather meal and blood meal into soil from an organically managed field.
They incubated the mixes in the laboratory at 10, 15, 20 and 25 °C.
They measured mineral nitrogen (ammonium and nitrate) at 1, 2, 4 and 8 weeks.

Material	Total N	N mineralised at 8 weeks (average of 10–25 °C)
Blood meal	15.8%	66%
Fish powder	13.7%	62%
Seabird guano	11.7%	61%
Feather meal	14.2%	60%

The main result: within two weeks, 47–60% of the organic nitrogen had been released across all four materials and all four temperatures. In other words, about half the nitrogen came out in the first fortnight.

This corrects the usual picture in three ways:

The gap between “fast” and “slow” materials is narrow. After eight weeks, feather meal averaged 60% and blood meal 66%. The difference is real but far smaller than “fast and strong” versus “slow and persistent” suggests.

All four materials release quickly. None of them behaves like a coated slow-release fertiliser. If you build a mix expecting it to release over twelve weeks, it will run short at the end.

The ranking is right, but the gaps are exaggerated. Blood meal released the most nitrogen and feather meal the least. Guano, which the book groups with blood meal as fast and strong, came out in the middle.

Limits of the study. It used field soil in a laboratory, not a heavily amended, biologically very active container mix. Nothing was tested above 25 °C. The book’s observation that blood meal disappears faster in a living-soil pot points the same way as the data.

No equivalent controlled study in a cannabis container mix was found when the original article was researched. So the exact release pattern in your pot is unverified. Two things are verified. These materials release quickly, and the differences between them are smaller than growers usually believe.

What to do: When you plan, assume about half the nitrogen in any of these four materials is released within two weeks of mixing.

7. What to change in your feeding plan

Do not count on feather meal for late-flower nitrogen. This is the biggest change. For a sativa that flowers for ten or twelve weeks, plan a top-dress late in the crop. Do not assume feather meal mixed in at week zero is still feeding at week ten. A single starting mix cannot carry the whole cycle, and that is why core feeding and mid-flower top-dressing exist.

Stop adding nitrogen after about week four of flower. This rule from the book holds whatever the release rate. From then on, top-dress with high-P guano instead.

Do not blame blood meal’s burn reputation on its strength alone. At 15.8% N it is the most concentrated of the four, but feather meal is close behind at 14.2%. Concentration alone does not convincingly explain the burns, and the study does not identify the cause. For every material on the list, the amount you apply is the thing you control.

Use fish meal for its phosphorus, not its nitrogen. At 13.7% N it is similar to the others. Its phosphorus makes it useful for the switch into flower.

Choose guano by class first. Section 4 explains the two classes and how to handle guano.

What to do: For your next crop, schedule a top-dress date in mid-flower on your calendar before you plant. It prevents a nitrogen shortfall in late flower.

8. Warm rooms and the Thai climate

In this study, temperature changed mineralisation only modestly. Averaged across all four materials, eight-week release was 56% at 10 °C and 66% at 25 °C. Nothing was tested above 25 °C.

A room at 28–30 °C would be expected to release nitrogen somewhat faster. That is an extrapolation, not a measured result. The 10 °C to 25 °C difference was small, so do not expect a dramatic speed-up.

Three adjustments:

Do not assume your mix releases for longer than temperate-climate advice says. If anything, expect it to run out a little sooner.
Make top-dressing a routine step, not an emergency fix. In a warm room, the mid-flower top-dress is a fixed part of the plan.
Watch the plant, not the calendar. Nitrogen fade means older, lower leaves turning yellow as the plant takes nitrogen from them (how to read leaf symptoms). Fade in late flower is normal and desirable. Fade in week five means the plant has run short of nitrogen.

Translator’s note: Many home growers in Thailand have no air conditioning, so rooms often sit at 30–35 °C. That is 5–10 °C above anything the study tested, and there is no data for that range. Check plants weekly from week three of flower and keep top-dress material ready. That way you can respond in days rather than waiting to buy supplies.

Translator’s note: The same applies when a power cut stops the air conditioning: the room heats above the tested range. The study was a controlled laboratory incubation. A pot that stays waterlogged for days in the rainy season is a different situation, and the study does not cover it. Note any long power cuts and waterlogged periods in your diary next to the date you first see fade.

Translator’s note: In the rainy season, store dry meals and guano in sealed containers off the floor. Meal that takes up moisture can clump, smell and attract flies and rodents.

What to do: Record your room’s daily maximum temperature alongside your watering log. Then you can compare the week you see nitrogen fade with how hot the room was.

9. Finding these materials in Southeast Asia

Translator’s note: The table shows typical availability for growers in Thailand and Southeast Asia. The ratings are general guidance, not survey data, and local supply varies.

Material	Availability in the region	What to check
Fish meal	Good	The region has a large fishmeal industry. Check that it is whole-fish meal and not mixed with fillers.
Guano	Moderate	Sold in both high-N and high-P grades. Check the label analysis, and check that it is aged or processed, not fresh.
Feather meal	Moderate	Poultry processing is large across the region. Hydrolysed feather meal exists in the animal-feed supply chain even where it is not sold as a garden product.
Blood meal	Poor to moderate	Often the hardest of the four to find at consistent quality, and often imported.

If you cannot get the exact material, substitute by what it does, not by its name. Be careful with the book’s keratin substitutes for feather meal. The feather meal in the study was hydrolysed, and that processing is why it released almost as fast as blood meal. Raw hoof, horn and hair keratin is not the same. Unprocessed, it breaks down much more slowly, so it will not release nitrogen at the rates in the table above.

Always ask for a guaranteed analysis, meaning the nutrient content the seller guarantees on the label. For dry organic amendments, the label NPK tells you more than it does for a liquid product. These are essentially single-ingredient materials, so their total nutrient content is stable even though their release rate is not.

Translator’s note: Thailand regulates fertilisers under the Fertilizer Act B.E. 2518 (1975), amended by the Fertilizer Act (No. 2) B.E. 2550 (2007). To check a label, see Verifying a Fertiliser Is Legally in Thailand. For supplier experience, ask other growers in the General Nutrients section, or ask the official distributor in Thailand.

What to do: Before you buy, ask the seller for the guaranteed analysis and, for feather meal, whether it is hydrolysed.

Share your results

The published data suggest that a living-soil mix runs out of nitrogen earlier than most growers plan for. This is one of the few things you can test cheaply, either with a soil test or by watching the plant. If you have tracked when your mix ran out in a warm room, post the details below this topic. Include your pot size, mix recipe, room temperature and the week you first saw nitrogen fade. Other growers in the region can then plan their top-dressing from real data.

Legal note

This article is technical horticultural information for readers who operate legally where they live. Cannabis rules differ in every country.

In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025. Flower may be sold only with a PT 33 (ภ.ท. 33) prescription, which covers a maximum of 30 days. Advertising and online sale of flower are prohibited.

Two Ministerial Regulations of B.E. 2569 tightened licensing in April 2026. One covers cannabis and hemp extracts and has been in force since 26 April 2026. The other, Controlled Herbs (No. 2), tightens licence criteria for flower and has been in force since 30 April 2026. The draft Cannabis Control Act is not law yet: the Cabinet approved the draft on 22 September 2026 and sent it to the House. Check your local law before growing. Nothing here is legal advice.

References

The Rev. True Living Organics. Green Candy Press; first published 2012, later editions 2016 and 2024.
Hartz, T.K. and Johnstone, P.R. (2006). “Nitrogen Availability from High-nitrogen-containing Organic Fertilizers.” HortTechnology 16(1): 39–42. doi:10.21273/HORTTECH.16.1.0039.
NIOSH. Histoplasmosis: Protecting Workers at Risk.