Manure Nitrogen Calculator

How it works

Manure and compost carry nitrogen, but most of it is in organic forms that soil microbes release slowly. Only part of the total becomes available to plants in the season it is applied; the rest is released in later years or lost. Extension manure guides express this as a first-year availability percentage.

The calculator converts the amount applied to pounds (a cubic yard is converted with the weight per cubic yard you enter), multiplies by the nitrogen content as is to get the total nitrogen, and multiplies that by the availability to get the plant-available nitrogen. This is the method of Worksheet C in Pacific Northwest Extension publication PNW 533, Fertilizing with Manure and Other Organic Amendments (2016).

A dry-matter analysis is first converted to as is: %N as is = %N dry × (1 − moisture ÷ 100). Applying a dry-matter percentage to a wet weight would overstate the nitrogen by the water the material holds.

Turned around, the target nitrogen for the area divided by the available nitrogen per unit of material gives the amount of manure or compost that would supply it.

Formula

weight (lb)    = amount × lb per unit (2,000 per ton; weight per cu yd)
%N as is       = %N dry × (1 − moisture ÷ 100)   (dry-matter analysis)
total N        = weight × %N as is ÷ 100
available N    = total N × availability ÷ 100
target N       = target rate × area ÷ rate area (1,000 sq ft or 43,560 sq ft)
amount needed  = target N ÷ (%N as is ÷ 100 × availability ÷ 100) ÷ lb per unit

Example

PNW 533's Worksheet C: broiler litter with 50 lb N per as-is ton (2.5%) and 50% plant availability supplies 50 × 0.50 = 25 lb of available N per ton, so a recommendation of 100 lb N per acre takes 100 ÷ 25 = 4 tons per acre. Entering 4 tons, 2.5%, 50%, 1 acre and 100 lb N per acre gives 200 lb of total N, 100 lb available, and 4.00 tons needed: it meets the target.

In a garden, 300 lb of manure at 1% N as is with 25% availability on 1,000 sq ft supplies 300 × 0.01 × 0.25 = 0.75 lb of available N. A target of 2 lb N per 1,000 sq ft would take 2 ÷ (0.01 × 0.25) = 800 lb, so the 300 lb is 1.25 lb N short.

Assumptions and limitations

• Nitrogen content and availability vary widely with the animal, bedding, storage, composting and how soon the material is worked into the soil. The typical values in the field help are published averages from PNW 533, not facts about your material; a lab analysis is more reliable.
• A single availability percentage is applied to all the nitrogen. Detailed guides split it into ammonium nitrogen, whose availability depends on how quickly the manure is incorporated, and organic nitrogen; Missouri Extension's G9330 gives 0.25 to 0.6 for organic nitrogen in the year of application.
• Only first-year availability is counted. Repeated applications build up organic nitrogen that releases in later years, so ground with a history of manure needs less.
• Phosphorus, potassium and salts are not considered. Applying manure to meet a nitrogen target often supplies more phosphorus than plants need.
• The nitrogen target is an input; this calculator does not recommend a rate. Raw manure can carry pathogens, and food-safety guidance on the interval between application and harvest, and nutrient-management rules for your land, are separate from this calculation. The result is an estimate, not a substitute for that guidance.

Frequently asked questions