Manure, the waste product of animals, has been used as a natural soil amendment for thousands of years to enrich farmland and gardens. It functions by supplying nutrients to plants and improving the physical and biological health of the soil itself. Determining which animal manure is the “best” depends on the specific needs of the garden, such as the goal of rapid growth or long-term soil structure improvement. The choice requires understanding the nutrient profile and physical properties of each type of animal waste.
Criteria for Determining Quality
The quality of any manure is measured by its concentration of the three primary macronutrients: Nitrogen (N), Phosphorus (P), and Potassium (K). This concentration is expressed as the N-P-K ratio; higher numbers indicate a more potent fertilizer. Nitrogen supports vegetative growth, phosphorus aids root development and flowering, and potassium helps with overall plant health and water regulation.
Beyond the nutrient ratio, secondary characteristics like organic matter content and moisture levels also dictate a manure’s utility. Manures high in organic matter are superior for building soil structure. Fresh manures contain high concentrations of soluble salts and ammonia, which is why they are referred to as “hot” and can damage or “burn” plant roots if applied directly.
High-Concentration Manures
Manures derived from poultry and small herbivores are considered the most nutrient-dense, making them ideal for gardens needing a swift nutrient injection. Rabbit manure is notably concentrated, often containing four times the nutrient levels of common cow manure. It typically provides an N-P-K ratio around 2.4-1.4-0.6, making it one of the highest in nitrogen and phosphorus among readily available farm manures.
Chicken manure is another potent source, with a typical N-P-K profile near 1.1-0.8-0.5, and is particularly high in nitrogen, promoting lush, rapid foliage growth. Because of their high nitrogen content and salt levels, both poultry and rabbit manure carry a risk of burning plants if they are not properly composted or aged. Bat guano, often mined from caves, is also a highly concentrated fertilizer, prized for its high phosphorus content, which promotes flowering and fruiting in plants.
Bulk Manures and Soil Structure
In contrast to concentrated types, bulk manures from cows, horses, and sheep are lower in nutrient density but offer superior physical benefits to the soil. Cow manure has a relatively mild N-P-K ratio, often around 0.6-0.4-0.5, and is valued for its dense organic matter. This high organic content is instrumental in improving soil structure, also known as tilth.
When added to heavy clay soils, the organic matter helps loosen compacted particles, improving aeration and drainage. Conversely, in sandy soils that drain too quickly, these manures act like a sponge, increasing the soil’s water-holding capacity and reducing nutrient leaching. Horse manure, with a slightly higher nitrogen content (around 0.7-0.3-0.6), also excels at building soil structure, though it often contains a higher number of viable weed seeds due to the horse’s less efficient digestive system.
Safe Handling and Preparation
Regardless of the source, all fresh animal manure must be prepared before being safely incorporated into a garden. Fresh manure contains pathogens, such as E. coli and Salmonella, which pose a human health risk if they contaminate food crops. The high ammonia and salt content in fresh waste can also chemically burn plant roots, a phenomenon often referred to as being “too hot.”
Composting or aging the manure is the necessary process that mitigates these risks. Composting involves maintaining a high temperature, ideally above 131°F, which effectively kills most pathogens and weed seeds. This process also allows unstable nitrogen compounds to convert into forms that are slowly released and available for plant uptake. Furthermore, gardeners must be cautious about the animal’s diet, as persistent herbicides used on hay fields can pass through the digestive system and remain active in the finished manure, potentially damaging susceptible garden plants.