Blood meal is a common organic amendment made from dried animal blood, typically a byproduct of the meat processing industry. Gardeners often use this material as a natural fertilizer, but many are concerned about its potential to alter the soil environment. Specifically, a frequent question is whether the application of this high-nitrogen product will cause the soil to become more acidic.
What Blood Meal Is and Its Primary Role
Blood meal is recognized as one of the most concentrated sources of organic nitrogen available to home gardeners. Its nutritional composition is consistently high, often exhibiting an NPK (Nitrogen-Phosphorus-Potassium) ratio around 12-0-0, meaning it contains roughly 12% nitrogen by weight. This abundance of nitrogen is the reason for its primary use in gardening.
The main function of blood meal is to provide a rapid nitrogen boost to plants, supporting vigorous vegetative growth and encouraging deep green foliage. Gardeners apply it to correct signs of nitrogen deficiency, such as yellowing leaves, especially in heavy feeders like corn and leafy greens. Unlike some slow-release organic fertilizers, the nitrogen in blood meal becomes available relatively quickly, often showing visible results in plants within a week of application.
The Direct Impact on Soil pH
Blood meal does contribute to a reduction in soil pH, meaning it has an acidifying effect on the growing medium. However, the magnitude of this shift is generally mild and should not be confused with the powerful, dedicated acidifiers often used to drastically lower pH. Its effect is localized and is highly dependent on the soil’s natural buffering capacity. Soils rich in clay and organic matter, for example, are more resistant to pH changes than sandy soils. The practical impact is typically not enough to fundamentally change a soil type from alkaline to acidic.
Compared to a mineral-based amendment like elemental sulfur, which is used to achieve significant, long-term pH drops, the change from blood meal is less pronounced and more temporary. For gardeners with alkaline soil, this mild acidification can be beneficial, helping to make certain micronutrients more available to plants. Conversely, those with already acidic soil must use blood meal with caution, as excessive application could lead to nutrient imbalances or plant stress. The overall effect is a minor, short-term adjustment that is a byproduct of its primary function as a nitrogen source.
The Nitrification Process Explained
The actual cause of the soil acidification is a natural, two-step biological process called nitrification. The nitrogen in blood meal is in an organic form, which must first be broken down by soil microbes into ammonium (NH4+). This ammonium is a positively charged ion that plants can take up directly, but it is also the starting point for the pH change.
Soil bacteria, specifically nitrifying bacteria, then begin the process of oxidizing the ammonium. The first group of microbes converts ammonium into an intermediate compound called nitrite, and a second group rapidly converts the nitrite into nitrate (NO3-), which is the form of nitrogen most readily absorbed by plants.
Crucially, this oxidation process releases free hydrogen ions (H+) into the soil solution. The presence of these hydrogen ions is the chemical definition of acidity, which is what lowers the soil’s pH reading. Therefore, the soil does not become acidic due to the blood meal itself, but rather as a direct consequence of the necessary microbial activity required to make the nitrogen available to plants.