How Much Guano to Add to Soil

N-P-K

• The three main nutrients found in guano are nitrogen, phosphorus and potassium, which are known as N-P-K. These are the standard ingredients found in commercial, nonorganic fertilizer that is created specifically for plant growth, but are naturally packaged together in guano. Many gardeners favor organic fertilizers, and they are willing to pay a premium price for guano. It is sold in powdered form.

Nutrient Levels

• Guano from birds and bats has different nutrient levels and should be applied at different rates. Bird guano is richer, at 13 percent nitrogen, 11 percent phosphorus and 2 percent potassium. This would show on the package as 13-11-2 N-P-K. Bat guano has less nutritional value, at 6 percent nitrogen, 9 percent phosphorus and 3 percent potassium.

Adding to the Soil

• Cornell University recommends that bird guano be applied at 2 to 3 lbs. per 100 square feet of garden or lawn, and that bat guano be applied at 3 to 4 lbs. per 100 square feet of garden or lawn. Colorado State University recommends that 5 lbs. of bat guano be tilled into every 100 square feet. Guano can also be used as a compost tea by mixing 3 tsp. into 1 gallon of water and spraying the plants.

Considerations

• Guano is a slow-release fertilizer, with the nutrients becoming available to the plants over four months, depending on the microbe activity in the soil. Guano stimulates the growth of soil bacteria and earthworms, which provides for better soil conditions. The downside to using guano is the high cost, which is balanced by the need to use less of it than other manures. To receive the same nutritional benefit from cattle or horse manure, a gardener would have to add 10 to 20 lbs. to the soil instead of just a few.