Blood meal is a traditional amendment in organic gardening, derived from dried, powdered animal blood, typically from cattle or hogs. It is processed to be safe and pathogen-free for agricultural use. Gardeners use this material as a natural way to deliver a powerful nutrient boost to plants in need of vigorous growth. Blood meal is well-known for its extremely high concentration of a single macronutrient.
Defining the Nitrogen Content
Blood meal is high in nitrogen, making it one of the most concentrated organic sources of this nutrient available to gardeners. Most commercial formulations feature an NPK (Nitrogen-Phosphorus-Potassium) ratio of approximately 12-0-0, though some variations may range up to 15% nitrogen. This means blood meal contains 12% by weight of elemental nitrogen, with only trace amounts of phosphorus and potassium.
This nutrient profile establishes blood meal as a specialized, single-nutrient supplement rather than a balanced, all-purpose fertilizer. The high nitrogen content promotes lush, green foliage and rapid vegetative growth, benefiting leafy vegetables like spinach and kale. Because of the absence of phosphorus and potassium, gardeners must use other amendments, such as bone meal or kelp meal, to ensure a complete balance of nutrients for overall plant health, flowering, and fruiting.
The Mechanism of Nitrogen Release
The nitrogen in blood meal is classified as organic nitrogen, chemically bound within the material’s protein structure. Unlike synthetic fertilizers, the nitrogen is not instantly available, defining blood meal as a slow-release fertilizer that provides a sustained supply of nutrients over several weeks.
The release process begins when soil microbes, including bacteria and fungi, break down the complex organic matter through mineralization. This decomposition releases nitrogen as ammonium (NH4+), a form plants can absorb. Specialized microbes then convert the ammonium into nitrate (NO3-) through nitrification, which is the preferred form of nitrogen for most plants.
The rate of mineralization depends on environmental factors. Warm soil temperatures and adequate moisture increase microbial activity, accelerating the nitrogen release. Conversely, cold or dry soil significantly slows down this activity. This biological mechanism provides steady, long-term feeding and reduces the risk of nutrient leaching compared to faster-releasing synthetic products.
Practical Application and Cautions
When applying blood meal, mix it thoroughly into the top few inches of soil before planting, rather than leaving it on the surface. This incorporation ensures contact with the soil environment, promoting microbial action and preventing nitrogen loss. For established plants, gardeners can use side dressing, sprinkling a small amount around the base and lightly working it into the soil.
The high potency of blood meal requires careful measurement to avoid damaging plants. Applying too much can cause nitrogen burn, where excess nitrogen salts draw moisture out of the plant roots, causing foliage to wilt and turn brown. A common application rate for new garden beds is roughly 1 to 2 pounds per 100 square feet, or about a teaspoon for a new transplant. Always follow the specific instructions on the product packaging.
A significant caution is blood meal’s potential to attract omnivorous or carnivorous animals, such as dogs, raccoons, and opossums, due to its strong scent. These animals may dig up the soil to consume the fertilizer, which is harmful to them and disruptive to the garden. To mitigate this risk, mix the blood meal well into the soil and water immediately after application to minimize the smell and deter unwanted visitors.