Summer 1998
Control the factors you can to minimize nitrogen losses
by Jeff Lorimor, Extension agricultural and biosystems engineer
Proper management can minimize nitrogen losses during land application of swine manure. The amount of time the manure remains on the surface of the soil is a critical factor, but others play a role as well.
The first factor is the percentage of nitrogen that is in ammonia form, because ammonia is the only form that is lost through volatilization. Recent swine manure samplings by ISU ag engineers show that the percentage of ammonia can be 80 percent or higher in anaerobic lagoons, 60 to 70 percent in slurry pits, and 10 to 20 percent in solid manure.
The remaining nitrogen in manure is organic, which is not lost during land application. For solid manure that starts out with 25 percent of the N in ammonia form, 25 percent is the maximum that can be lost. The organic N will remain.
In a slurry pit with 10 percent solids, ammonia nitrogen would make up about two-thirds of the total nitrogen. Therefore the pit might contain about 75 pounds of nitrogen per 1,000 gallons; 50 pounds of which would be ammonia. Figure 1 shows the relationship.
Loss of ammonia N is increased by temperature, humidity, and wind speed. As those conditions increase the ammonia losses occur faster. Under warm breezy conditions, much of the ammonia is lost in a matter of hours. Figure 2 shows losses under two different weather conditions. Nearly 80 percent of the ammonia was lost under warm conditions. The greatest loss occurs in the first 24 hours, more occurs in the second 24 hours, and losses continue slowly after that.
Surface condition of the ground also affects nitrogen losses. More nitrogen is lost from crop residue than tilled soil. The positively charged ammonium ion (NH4+) will attach to negatively charged particles. Soil is negatively charged, so it will prevent losses as compared to crop residue. By combining the percent ammonia with the crop residue cover, a chart of overall losses can be shown for normal weather conditions, Figure 3.
Time is the critical factor that can be controlled. Losses occur only when manure is broadcast on the surface and left there. Losses can be eliminated by injecting or incorporating manure immediately.
