Guano, the accumulated droppings of seabirds or bats, has a long history as a prized agricultural input. Its value was first recognized by prehistoric Peruvian farmers and later sparked the global “Guano Age” in the 19th century due to its potent nutrient content. Today, guano remains a favored organic fertilizer for gardeners seeking a natural, concentrated source of plant nutrition.
The Distinct Composition of Guano
Guano is a highly effective fertilizer because it delivers a rich profile of compounds that support plant growth and improve soil health. Its composition includes the three primary macronutrients—nitrogen (N), phosphorus (P), and potassium (K)—in significant concentrations, often exceeding those found in other organic manures. The specific nutrient profile is variable, depending on the animal’s diet, the guano’s age, and the environmental conditions where it was deposited.
The value of guano extends beyond its macronutrient content to include essential micronutrients like calcium (Ca), magnesium (Mg), iron (Fe), and sulfur (S). Calcium and magnesium help regulate soil pH, which is important for nutrient uptake by plant roots. Guano also contains a high percentage of organic matter, which stimulates the activity of beneficial soil microbes.
The organic matter content helps improve the physical structure of the soil, assisting in loosening compacted earth and increasing water retention. The nutrients within guano are released slowly over time as the organic matter breaks down, providing a steady, sustained feed for plants. This slow-release mechanism reduces the risk of nutrient leaching, keeping the nutrients available to the plants for a longer duration.
Understanding Guano Types
The source of the guano, primarily bat or seabird, dictates its nutrient bias and subsequent use in the garden. Bat guano generally originates from cave deposits and is higher in nitrogen (N). The high nitrogen concentration, often ranging from 6% to 15% in commercial products, makes bat guano an excellent choice for supporting vegetative growth, such as the development of lush foliage and strong stems.
Seabird guano, collected from coastal environments or remote islands, often exhibits a different profile, with higher concentrations of phosphorus (P) and potassium (K). The high phosphorus content is beneficial for root development, flower production, and fruit setting. While bat guano is preferred for the early, leafy growth phase, seabird guano is applied during the middle and later stages to encourage robust blooming and fruiting.
This difference in nutrient composition is a direct result of the animals’ diets and the decomposition process. Bat guano, especially from insectivorous bats, contains chitin from insect exoskeletons, which contributes to its nitrogen content. Seabird guano, derived from fish-eating birds, often has soluble compounds leached away by rainfall, which concentrates the less soluble phosphorus and potassium compounds.
Safe and Effective Application Techniques
Guano’s high concentration means that correct application methods and cautious dosing are necessary to avoid nutrient burn or over-fertilization. One common method is using guano as a soil amendment by mixing the dry material directly into the earth before planting. For new garden beds, a general guideline is to incorporate 1 to 2 tablespoons of guano per 100 square feet into the top three inches of soil.
For established plants, top-dressing provides a slow, continuous feed throughout the growing season. This involves sprinkling a small amount of dry guano, such as a teaspoon per gallon of container soil, onto the surface. Lightly work the guano into the topsoil, followed by a thorough watering. This allows the nutrients to slowly leach into the root zone with each subsequent watering.
A third, faster-acting method is creating a liquid guano “tea” for a quick nutrient boost or foliar feeding. To prepare a tea, mix 1 to 2 tablespoons of guano per gallon of water and allow it to steep for up to 48 hours, agitating it periodically. This liquid solution can be applied directly to the soil around the base of plants or used as a foliar spray on the leaves. Always start with a lower dose, such as half the recommended amount, and observe the plants’ response before increasing the concentration.
Environmental and Safety Considerations
The use of guano requires specific safety precautions and an awareness of its environmental impact. A primary safety concern, particularly with bat guano, is the potential for exposure to the fungal spores that cause histoplasmosis, a respiratory disease. The fungus, Histoplasma capsulatum, thrives in large, accumulated deposits of dried bat or bird droppings.
To minimize the risk of inhaling spores, it is recommended to wear a respirator mask, such as an N95, and disposable gloves when handling dry guano, especially when the material is dusty. Spraying the guano lightly with water before handling helps suppress dust and keeps spores from becoming airborne. Individuals with compromised immune systems should avoid disturbing large accumulations of guano altogether.
From an environmental perspective, the sustainability of guano harvesting is a concern, as the aged deposits are not a rapidly renewable resource. The extraction process, particularly from remote islands or caves, can disturb fragile ecosystems and animal habitats. Like any concentrated fertilizer, over-application can lead to nutrient runoff, with excess phosphorus and nitrogen leaching into local waterways and causing ecological harm.