Organic Fertilizers by the Numbers: How Much Blood Meal, Bone Meal or Manure a Bed Needs

Organic fertilizers are often sold as gentle and hard to overdo. Some are, but the amount that a bed needs still varies enormously from one product to the next, because their nutrient content varies enormously. Blood meal is mostly nitrogen. Bone meal is mostly phosphorus. Manure and compost carry a little of everything. Choose the wrong one for the job and you either starve the plants of what they need or pile up a nutrient they do not.

The GardenStuffs Fertilizer Calculator includes five organic products with typical analyses: compost (2-1-2), manure (3-2-3), blood meal (12-0-0), bone meal (4-12-0) and fish emulsion (5-2-2). Real products vary, so if your bag gives a different analysis, choose Custom NPK and type the numbers from the label.

The same bed, five products

Here is a 100 square foot vegetable bed in the vegetative stage, spread evenly (Broadcast). The calculator's target is 0.2 pounds of nitrogen, 0.1 of phosphorus and 0.15 of potassium, and it sets the amount by nitrogen:

Product	Amount	Nitrogen	Phosphorus	Potassium
Blood meal (12-0-0)	1.67 lb	0.2 lb ✅	0 ⚠️	0 ⚠️
Fish emulsion (5-2-2)	4 lb	0.2 lb ✅	0.08 lb ⚠️	0.08 lb ⚠️
Bone meal (4-12-0)	5 lb	0.2 lb ✅	0.6 lb ⬆️	0 ⚠️
Manure (3-2-3)	6.67 lb	0.2 lb ✅	0.13 lb ✅	0.2 lb ✅
Compost (2-1-2)	10 lb	0.2 lb ✅	0.1 lb ✅	0.2 lb ✅
For comparison: 10-10-10	2 lb	0.2 lb ✅	0.2 lb ✅	0.2 lb ✅

Blood meal: nitrogen and nothing else

At 12 percent nitrogen, blood meal is the most concentrated nitrogen source in the list, so the amount is small: 1.67 pounds for 100 square feet, or 0.27 pounds for a 32 square foot herb bed in the vegetative stage. It supplies no phosphorus or potassium at all, which is why both rows read ⚠️. That is fine if a soil test shows those two are already adequate, and it makes blood meal a reasonable choice for a leafy crop that is pale and slow. It is easy to overdo because the amounts are small, and too much nitrogen burns roots and young leaves. Weigh it rather than scooping.

Bone meal: mostly phosphorus

Bone meal is the clearest example of why setting the rate by nitrogen matters. To reach the vegetative nitrogen target with a 4-12-0 product, you would need 5 pounds, and that would deliver 0.6 pounds of phosphorus against a 0.1 target, six times what the stage calls for. The calculator marks it ⬆️.

Bone meal is not meant to be a nitrogen source. Its use is to supply phosphorus where a soil test shows phosphorus is low, typically worked into the planting area. Even at the flowering stage, where the vegetable target leans toward phosphorus (0.1 nitrogen, 0.2 phosphorus, 0.2 potassium per 100 square feet), the calculator gives 2.5 pounds of bone meal, meeting nitrogen, delivering 0.3 pounds of phosphorus (✅, within twice the target) and no potassium. Many garden soils already hold more phosphorus than plants use, and excess phosphorus that washes out of gardens feeds algae in lakes and streams. Test before you add it.

Manure and compost: a little of everything

Manure and compost meet all three targets in the vegetative stage because their ratios are close to what the stage asks for. The catch is weight: 6.67 pounds of manure or 10 pounds of compost for 100 square feet. And the nitrogen in both is released slowly as soil organisms break down the organic matter, so the plants do not see all of it this season. That is why compost is better measured as a layer by depth than dosed by nitrogen; see how much compost a bed needs and the Soil Volume Calculator.

At the flowering stage, both fall short on phosphorus and potassium when set by nitrogen. Manure gives 3.33 pounds (0.07 phosphorus against 0.2), compost 5 pounds (0.05 against 0.2). If a soil test shows those nutrients are low, a separate phosphorus or potassium source will be needed; if it shows they are adequate, the ⚠️ marks do not mean the plants are hungry.

Fish emulsion: liquid, and labeled by dilution

Fish emulsion is a liquid concentrate. The calculator gives a weight (4 pounds for 100 square feet in the vegetative stage) because it treats every product the same way, but in practice you dilute fish emulsion in water at the rate on the bottle and apply it as a drench or spray. Follow the bottle. Its value is speed: the nitrogen is more readily available than in meals and manure, so it is useful for a quick feed of seedlings and containers.

When not to fertilize

Choose Trees/Shrubs and the dormant stage, and the calculator returns 0 pounds for every product, manure included. Plants that are not growing do not take up nutrients, and what you spread is washed away. The same logic applies to vegetable beds in winter. Spread compost or composted manure in fall as a soil conditioner if you like, but save the fertilizer for when plants are growing.

Food safety with manure

Fresh animal manure can carry bacteria such as E. coli and Salmonella that cause illness. On beds where you grow food:

• Use manure that has been properly composted, or bagged manure products that say they have been treated.
• If you use raw manure at all, work it into the soil well ahead of planting and follow your local extension service's guidance on the minimum interval between application and harvest. The interval matters most for crops that touch the soil or are eaten raw, such as lettuce, carrots and strawberries.
• Never use dog, cat or pig manure, or human waste, on food beds.
• Wash produce before eating, and wash hands after handling manure.

Pets and storage

Blood meal and bone meal smell attractive to dogs. Dogs that dig them up or eat them from the bag can become ill, and a large amount of bone meal eaten at once can form a hard mass in the gut. Work meals into the soil rather than leaving them on the surface, store bags sealed and off the floor, and call your veterinarian if a pet eats a quantity of fertilizer. Fish emulsion attracts animals too; cap the bottle and rinse spills. Keep all fertilizers, organic or not, out of reach of children.

What the calculator does not cover

• Real analyses. The built-in organic analyses are typical values. Manure in particular varies widely with the animal, the bedding and how long it has been composted. Use the label figures through Custom NPK when you have them.
• Release speed. The math counts all nitrogen as available now. Meals and manures release theirs over weeks to months.
• Soil test results. Only a soil test tells you whether the ⚠️ marks for phosphorus and potassium matter in your bed.
• Label rates and intervals. Organic products carry labels too, and their rates and directions come first.

The short version: blood meal for nitrogen, bone meal only for a measured phosphorus shortage, compost and composted manure for the soil itself. Run the numbers for your bed before you buy, and never mix products in the hope of covering everything at once. For a synthetic comparison, see reading N-P-K on a vegetable bed.