Organic N in Manure

Organic N in manure becomes plant available over time, some during the year of application and some in the years to follow. Referring back to the N cycle, organic N must be mineralized by soil microorganisms before it can be taken up by plants. The major factors that influence organic N mineralization from manure are:

• The dry matter content (i.e. total solids) of the manure. The more solids, the less N will be mineralized in the year of application.
• Weather and soil conditions. It's the N cycle, again. What conditions most favor rapid mineralization?
• The animal species. Some species have more readily available organic N sources in manure (e.g. chickens) .

Mineralization of organic N in manure varies from season to season and field to field, because of the range in the conditions, above. To estimate the amount of organic N in manure that becomes available for crop production, research-based, average mineralization rates across a range of conditions have been developed, shown in the table, below.

Release rate for organic N in manure (%)
Source	Dry Matter Content (%)	Applied in Current Year	Applied Last Year	Two Years Ago
Cows	<18	35	12	5
Cows	18	25	12	5
Poultry	<18	55	12	5
Poultry	18	55	12	5
Swine	<18	35	12	5
Swine	18	25	12	5
Horses	<18	30	12	5
Horses	18	25	12	5
Sheep	<18	35	12	5
Sheep	18	25	12	5

For instance, if 10 tons/acre of 6% dry matter cow manure is applied in the spring, 35% of manure's organic N would, on average, be used by the crop during this growing season. If the same application was made in each of the previous two years, then 12% of the organic manure N would be available this season from last year's application and 5% from the application made two years prior.

The calculator, below, is designed to determine the N available to crops from organic N in manure. To do this, assume that 10 tons/acre of cow manure with 5 lbs organic-N/ton and 6% dry matter is applied to a field.

1. Enter Cows in the "Animal Species" drop down menu and <18% in the "% dry matter" menu.
2. Enter 10 into the "Application Rate" cell for "This Year".
3. Enter 5 into the "Organic N from Manure Analysis" cell.
4. Hit "calculate it!".
5. Track the differences in N availability.
6. Enter the same manure rate and analysis for "Last Year" and "Two Years Ago" and hit "calculate it" to see the N contribution from past manure applications.

The calculator can also be used to assess organic N availability from manure for other manure types as well as different rates and histories of application.

To read more on this subject: Nitrogen Guidelines for Field Crops in New York (section 3.3).

To download an Excel spreadsheet for calculating the N contribution from manure: Manure Nutrient Calculator.