Achieving the “balancing act” in manure application
April 29, 2008 by Jeff Schoenau
We know that manure and compost
have been used to sustain soil fertility and crop production for
thousands of years. All we have to do is look at China for an example
of this. And the value of manure is well known to modern-day farmers;
it can improve soil tilth and crop growth for many years. But key to
all this is using manure in proper quantities.
We know that manure and compost have been used to sustain soil fertility and crop production for thousands of years. All we have to do is look at China for an example of this. And the value of manure is well known to modern-day farmers; it can improve soil tilth and crop growth for many years. But key to all this is using manure in proper quantities.
One challenge lies in the nutrient make-up of the manure. Examples: liquid hog effluent may have levels of from 0.1 percent to 0.5 percent nitrogen, and feedlot manure nitrogen levels may vary from 0.5 percent to 1.5 percent. While these may seem like small variables, in percentage terms they are huge. Compare this with liquid fertilizer, which has a consistent 28 percent level of nitrogen.
While we may want to use manure on crop land, it may not have the exact balance of nutrients we need, and may have to be supplemented with commercial fertilizer.
In terms of best managing manure, we need to know what’s in it and how it behaves. In the case of liquid effluents, we know that it has a high availability of nutrient in the year of application, but not much organic matter. With solid manures, we know that it has slow availability of nutrients, lots of organic matter, and is a long-term soil builder. These are important considerations in what and when to apply manure.
When viewed simply as a matter of manure disposal, there is a tendency to apply manure at the highest rate possible, closest to the livestock operation. But we know that too much manure causes problems. In agronomic terms, there is lodging, haying off, salt loading and toxicity. In environmental terms, there is nutrient loading and pollution due to losses by leaching, erosion and gaseous escape. But manure applied at an appro-
priate rate and method of application is both sustainable and economical.
Clearly, the best route to take is to employ nutrient management planning and achieve what has been termed a “balancing act.” This involves:
- selecting a rate of applied manure nutrient that matches crop demand and nutrient removal over time;
- determining the appropriate rate using manure testing and soil testing;
- ensuring the proper ratio of available nutrients in soil following manure application;
- using application technologies that get the manure in the ground.
Focusing on this last point, there are significant advantages to getting the manure right into the ground, rather than just on the ground. A study showed that swine effluent (100 pounds N/acre/year) applied by broadcast and incorporation had cumulative crop recovery of nitrogen of 31 percent. But the same effluent injected into soil had a higher nitrogen recovery rate—43 percent.
In terms of characteristics, the repeated application of solid manure is shown to be most effective at increasing organic matter. There is a large direct addition. Liquid manures of low organic matter content, however, increase organic matter more slowly and indirectly by stimulating plant production and residue addition.
Manure also stimulates microbial activity and respiration. Additions of manure affect the soil microbial community structure; microbial populations were more diverse in manured soil than in urea fertilized soils. There has been no evidence that the addition of swine manure increases the soil population of pathogenic microbial isolates. And manure had no impact on foliar plant diseases.
In terms of salinity, we know that a high concentration of soluble salts holds back water from the plant. And a high proportion of sodium relative to calcium and magnesium, causes soil particles to disperse, causing crusting. But on well-drained soils, the effects of manure application on soil salinity are minimized. And while surface crusting could be an issue with repeated application, especially with swine manure, it has not been evident in SAR, soil strength or emergence measurements to date. Salinity and sodicity in manure fields should be monitored using benchmark sites, however.
Manures can also influence soil metal bioavailabilty. We know that some metals, such as copper and zinc, are functional plant nutrients. Others, such as cadmium, lead and selenium, are non-functional. Manure, in the case of copper and zinc, can influence metal bioavailability directly, and indirectly, through altering pH and microbial activity. Studies have shown that repeated (11 years) cattle manure applications have increased soil zinc. Further studies showed that mercury, arsenic and selenium concentrations in soils and crops were not significantly influenced by repeated manure applications.
Nutrient and metal loading, salinity and sodicity issues do not appear to be a concern when manure is applied at recommended agronomic rates on normal well-drained soils, but should be monitored over time.
In summary, achieving the manure “balancing act” may take some effort, but it is worth the investment of time. By adding organic matter and stimulating growth through a fertilizing effect, manure addition at agronomic rates has a positive effect on soil quality. The Chinese have known that over time and we know that today.
Jeff Schoenau is senior research scientist and adjunct professor at the University of Saskatchewan.