Applying swine manure to cropland speeds up the nutrient cycling process.

Research by Tom King, a graduate student at the University of Saskatchewan in Saskatoon, Canada, shows that swine manure increases the activity of three soil enzymes — phosphatase, urease and arylsulfatase.

These protein-based enzymes are the catalysts that convert nitrogen (N), phosphorus (P) and sulfur — three of the four major plant nutrients — from an organic state to the inorganic form that plants can utilize. The higher the enzyme level, the faster this conversion occurs.

King offers this simplistic explanation: “Enzymes help reactions move along at a faster rate using less energy; they heat things up really quickly.”

Increased enzyme activity is associated with increased biological activity in the soil, which is considered an important indicator of soil quality.

“If your soil is healthy, you've got lots of microbial activity and lots of enzyme activity,” King says. “The more you've got, the healthier your soil is.”

Application Rates Tested

For his study, King applied the agronomic rate of manure, 220 lb. N per 2.47 acre, at double the rate and four times the rate at four different sites in central Saskatchewan. Enzyme levels were measured in the manured plots and then compared to levels in control plots and in commercial fertilizer plots. Manure from a cattle feedlot was also tested at one location.

Areas that received repeated applications of manure showed increases in urease and phosphatase enzyme activity. The study showed little increase in the arylsulfatase enzyme, but the manure used had low sulfur content.

King found stepped increases in urease activity that corresponded to the amount of manure applied. Levels went from about 250 micrograms (mcg) of ammonium/gram of soil in the controlled plot, to about 300 mcg ammonium/gram of soil at the four-times rate. Enzyme activity in the double rate was comparable to those in plots where commercial urea fertilizers had been applied (290 mcg of ammonium/gram of soil).

“Our Dickson site, about 75 miles east of Saskatoon, found that increasing amounts of manure increased soil microbial activity,” King says. “Microbial activity was highest just after application, when the little microorganisms are saying, ‘it's feeding time,’ and they go to work to break the material down.

“Benefits of adding swine manure are remarkably long-lived. We have one set of plots at our Riverhurst site that only had one application of manure in 1999. Even 5-6 years after application, there is still some carryover effect in some of the higher rates of manure,” he notes.

Fall Application Preferred

Since nutrients can be temporarily tied up in the conversion process, King says fall is probably the best time for application.

The fall application date is more important for cattle manure than it is with liquid hog manure, because cattle manure is very highly organic.

“Hog manure is about 98% water, less than 2% solids,” King notes. “The nutrients in the hog manure are much more inorganic and plant-available than cattle manure. It takes longer for microorganisms to break down cattle manure to make it available to crops.”

King concluded that hog manure was very good for agricultural soils when applied at an agronomically correct rate.

“Soil testing is important to find out how much nutrients you need to meet the needs of whichever crop you want to grow,” he says. “We're not saying that manure will meet all of your fertilizer needs, but it does serve as a substitute for some of the commercial fertilizers.”