Iowa State University's new nitrogen fertilizer recommendations for corn (outlined in Pm-1714) do not involve calculating credits for N supplied by animal manures. Instead, adjustments for this N are based on information from the late-spring test for soil nitrate and end-of-season test for cornstalk nitrate. The reasons for this change deserve explanation because calculating credits for animal manures has been widely accepted as a best management practice.
The practice of calculating fertilizer credits for N supplied by manures was developed to help producers decide how much commercial fertilizer should be applied to manured fields. Such decisions need to be distinguished from the task of selecting appropriate rates of manure application.
The concept of N credits involves estimating N fertilizer needs for non-manured soils and then subtracting estimated amounts of N supplied by the manure. In the past, estimated N fertilizer needs have been based on yield goals. These estimates may indicate average fertilizer needs, but they have limited reliability on individual fields because they do not address variability--both in the amounts of N supplied by the soil and in the percentages of N fertilizer lost before it is needed by the crop.
The estimated amounts of N supplied by animal manures are based on averages that have no known origin. These estimates have poor reliability for individual fields because they do not consider important the variability caused by site-specific interactions of weather, soil properties, manure characteristics, and management practices. Large amounts of variability are caused by N losses soon after application, differences in carbon-to-nitrogen (C/N) ratios, and the residual effects of manure applied for previous crops.
Substantial amounts of the N in animal manures can be lost through ammonia volatilization, surface runoff, leaching, and(or) denitrification before the N can be used by the crop. Such losses tend to be larger and more variable than N losses following application of commercial fertilizers. One reason is because the times and methods of manure application are not selected with as much concern for N losses. Another reason is that carbon in animal manures can greatly increase losses of plant-available N by denitrification under some conditions.
The effects of C/N ratios are important because C/N ratios determine whether the manure increases or decreases availability of N for the next crop. Although C/N ratios of the manures applied are important, the C/N ratio of the manure after early losses of N are more important. Current recommendations suggest that substantial amounts (often more than half) of manure-derived N often are lost by ammonia volatilization when manure is applied to soil surfaces. Such losses of N change
C/N ratios and can convert manures from N-supplying treatments to N-consuming treatments. This problem makes it more difficult to predict the effects of N losses from manures than from commercially prepared fertilizers.
Much of the N applied as animal manures is not available to crops during the first year. This N remains in organic forms that increase the
N-supplying capacity of the field in the future. Although a single application of manure can have significant effects on N availability to crops for several years under some conditions, the residual effects of manure application in any given field are highly variable and unpredictable. Thus, it's difficult to determine how much N is supplied by "the manure" and how much is supplied by "the soil." This confusion can be a serious problem on soils that routinely receive applications of manure.
This article originally appeared on page 125 of the IC-478(15) -- June 30, 1997 issue.