Tractor apply fertilizer to field
MORE MACRO-AGGREGATES: In three studies by UNL soil scientists, applying feedlot and swine manure on silty loam or silty clay loam soils resulted in increased water-stable macro-aggregates compared to applying commercial fertilizer.

How manure affects soil aggregation

UNL researchers find improved water stability and higher phosphorus concentrations in macro-aggregates after manure application.

By Rick Koelsch

If manure increases formation of larger and more stable soil aggregates, what might be the benefits of fertilizing with manure as compared to commercial fertilizer?

That's one of the questions addressed by University of Nebraska-Lincoln researchers, who noted these potential benefits:
• reduced runoff and soil erosion
• increased water infiltration into the soil, possibly leading to greater drought tolerance
• partial offsetting of higher soil phosphorus levels resulting from manure application and limiting P loss to local surface water

The Manure and Soil Health (MaSH) blog will unpack some of the potential benefit of manure to soil quality by reviewing research for clues as to manure's benefits. Several research studies have documented manure application impact on improvement of soil physical properties. The article The Effects of Manure Application on Soil Aggregation by Charles Wortmann and Charles Shapiro, soil scientists at the University of Nebraska-Lincoln, examines research conducted in Nebraska.

Wortmann and Shapiro conducted studies at three Nebraska farms to explore the impact of composted beef manure, stockpiled beef manure and swine manure on soil aggregate size. Their hypothesis proposed that manure would produce increased macro-aggregate formation with the intent of learning:
• how quickly these aggregates form
• impact of alternative manure sources
• residual effects of manure on aggregates

Three studies were conducted with silty loam or silty clay loam soils. Feedlot manure solids were applied at 20 to 22 dry tons per acre (roughly 40 to 44 wet tons per acre) and swine manure at 1.2 dry tons per acre (roughly 4,800 gallons per acre if manure is 6% solids). Four conclusions were drawn providing clues about manure's benefits:

1. Water-stable large macro-aggregates were increased two to three times for manured soils compared to commercially fertilized soils.

All macro-aggregates increased by 10% to 20% for manured vs. commercially fertilized soils. This increase was consistent across all soil types evaluated. A Michigan State Extension publication suggests that, "When manure … is added to the soil, it is quickly colonized by millions of bacteria … bacteria producing large quantities of polysaccharides. These polysaccharides function like sticky glue in the soil and can actually stick particles together into aggregates."

2. The formation of macro-aggregates occurred with all manures, with some advantage for compost and similar effects for raw feedlot manure and swine slurry. The swine manure's solids application rate was only 6% of feedlot manure but still achieved very similar benefits. This might suggest that achieving the physical soil property benefits may be more dependent upon the rapid growth in soil microbiology and less dependent on the total solids in the manure.

3. Water-stable macro-aggregates formed within 15 days after manure application persisted over a seven-month observation period, but were no longer found on a separate tilled site that was without compost for five years.

The effect of manure or compost on macro-aggregate formation soon after application (first observation made at 15 days) suggests an immediate value from manure application. The study observed this benefit lasted at least seven months but disappearing five years later. Other research has suggested this benefit lasts longer in no-till fields.

4. Phosphorus was observed to concentrate in the water-stable macro-aggregates, twice as great in the large aggregates as the whole soil P level.

This may provide additional protection resulting from manure for holding P in the soil and reducing P in runoff. However, previous research has documented that P movement from cropland is connected to soil surface P levels. Managing manure to keep soil P levels low, near levels required for optimum crop production, remains our best practice for protecting water quality. But manure's large aggregate building response and P concentration in those large aggregates help protect water quality.

Koelsch is a livestock environmental engineer at the University of Nebraska-Lincoln. This report comes from UNL CropWatch.


TAGS: Manure
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