12.9: Estimating Manure Nutrient Availability

Nutrient management is challenging when it involves using manure because it is difficult to balance agronomic and environmental objectives. This is especially the case with nitrogen because it is the most dynamic nutrient, one that is easily lost with rain, and its availability from manure is very difficult to predict. Conversely, P, K and most other nutrients stay in the soil and can be assessed through soil testing.

Universities and government agencies offer guidelines for estimating manure N availability, but it is generally recognized that, while they are useful for planning purposes, they are imprecise. The estimated nitrogen availability factors for the northeastern United States are shown in Table 12.3. They reflect the following patterns:

• Spring applications of manure are more efficient than fall or winter applications. The latter can result in considerable losses because the winters in the northeastern United States are wet and can cause N losses. (This also holds for the West Coast and Southeast but is less the case in the midwestern and western United States due to drier winters.)
• The type of manure has a modest impact on N availability.
• Immediate versus delayed incorporation has a big impact on crop N availability because a lot of ammonia is lost when manure is on the soil surface. This effect is especially significant with poultry and swine manure because they contain relatively more ammonium and urea than ruminant manures.
• There is a large benefit of cover crops in conserving the manure N after fall and winter applications. They take up the manure N when it becomes available, store it in their root and shoot biomass, and return it to the soil when they are terminated to grow the following crop.

The estimated N availability values in Table 12.3 are based on total manure N, but they can be improved if the manure analysis separates ammonia-N from organic N. Additional N credits are appropriate if there were manure applications in previous years.

Most farmers follow these guidelines due to nutrient management regulations. However, they are imprecise if they are not based on a manure analysis that shows how much N is actually present. And even when combined with a lab analysis of manure, the guidelines are imprecise because they don’t account for weather factors. In other words, they are reasonable planning tools for manure applications, but the actual N availability may be quite different due to subsequent weather conditions and management practices. Follow-up measurements are therefore recommended, like the pre-sidedress soil nitrate test, weather-driven simulation models, satellite, airplane or drone images, and in-field crop sensors (discussed in Chapter 18). These allow for in-season evaluation of a crop’s N status and a more precise determination of the need for corrective N applications.

Table 12.3 Estimated Nitrogen Availability from Manure Applications in the Northeast United States

Nitrogen Availability Factor*
Application Season	Target crop	Incorporation Management	Poultry Manure	Swine Manure	Other Manure
Spring or summer	All crops	immediate	0.75	0.7	0.5
		1 day	0.5	0.6	0.4
		2–4 days	0.45	0.4	0.35
		5–7 days	0.3	0.3	0.3
		> 7 days or none	0.15	0.2	0.2
Early fall	Winter-spring crops	< 2 days	0.5	0.45	0.4
		3–7 days	0.3	0.3	0.3
		> 7 days or none	0.15	0.2	0.2
	Summer crops after cover crop	< 2 days	0.45	0.4	0.35
		3–7 days	0.25	0.25	0.25
		> 7 days or none	0.15	0.2	0.2
	Summer crops without cover crop	All methods	0.15	0.2	0.2
Late fall or winter	Winter-spring crops	All situations	0.5	0.45	0.4
	Summer crops	No cover crop	0.15	0.2	0.2
		Cover crop harvested	0.15	0.2	0.2
		Cover crop as green manure	0.5	0.45	0.4
*The nitrogen availability factor is the N fertilizer equivalent per pound of manure N. Source: Penn State University (table simplified from original source; for illustration only)