MACROPORES CONTRIBUTE TO TILE FLOW

These pathways may play a key role in tile drainage systems.

by Eric Cooley

Discovery Farms is an on-farm water quality research and outreach program that is part of the UW-Madison Division of Extension. Tile drainage monitoring data collected on farms in Wisconsin has shown that macropores may play a key role in tile drainage systems.

Macropores are preferential flow paths in the soil that can provide major transport pathways from the soil surface to tile drainage systems. Macropores can be formed by a variety of factors, including earthworm burrows, decayed root channels, shrinkage cracks, and the structural porosity of the soil.

As observed when methylene blue dye is applied to the surface of the soil in Figure 1, the dye moved through the soil using a combination of macropores. Most of the dye entered the soil through shrinkage cracks in the soil surface, then moved laterally along the plow layer, and finally moved deeper in the soil profile through earthworm burrows. Water and nutrient transport through the soil matrix is much slower than through macropores, allowing for enhanced nutrient retention, whereas macropores can rapidly transport water and soluble nutrients to tile drains.

The development of macropores in soil varies significantly with soil type and management. Soil that has been no-tilled long term typically has greater structural porosity, which supports macropore development. Earthworm activity also tends to be greater in no-till fields compared to fields that are annually tilled and can provide considerable macropore paths to tiles.

Soils with higher clay content often develop large shrinkage cracks that occur as soil dries, and they can go deep into the soil profile. Nutrients and organic material from the soil surface can be transported rapidly through these shrinkage cracks, and if they intersect tile lines, can provide a direct conduit from the soil surface to the tile.

May increase tile flow

Evidence of macropore flow to tile systems has been observed in two tile drainage studies with concurrent surface water runoff and tile flow monitoring in a basin. The first example is a long-term, no-till system where surface runoff during nonfrozen soil conditions rarely occurred, unless the tile system was flowing near maximum capacity.

As shown in Figure 2, when a rain event occurred, there was often a rapid response in the tile system. This response was so rapid during intense rain events that tile flow was observed in less than 15 minutes from when the first raindrop fell.

The combined macropore pathways of structural porosity and earthworm burrows in this no-till system provided rapid transfer of water from the soil surface to the tile system. Surface runoff didn’t initiate until the tile system was flowing near maximum capacity (see dashed line in Figure 2). As tile flow began to subside below the maximum capacity, the surface runoff ceased.

In this no-till system, phosphorus loss to the tile system was predominately in the dissolved form (77%) and was similar to surface water chemistry, indicating a strong link between the soil surface and tile system. If the flow was greater through the soil matrix, lower dissolved phosphorus in the tile would be observed.

The second example is a conventional tillage system with macropore flow through a frozen soil. The winter of 2004 to 2005 was bitterly cold, with low snowfall through most of the winter. These factors drove frost deep into the soil. As snowmelt began in late March, there was still over 2 feet of frost in the soil. When daytime temperatures began to rise above freezing (32°F) during the day and drop below freezing at night, flow was observed in the tile 24 to 48 hours prior to enough snowmelt water accumulating to result in surface water runoff seen in Figure 3. It is hypothesized that ice plugs that form in the macropores melt with a combination of the warming soil surface and warmth from the heat coming up through the soil from the tile system.

Macropore flow to tile systems also happens by the daily cyclic heating of the soil and resulting tile flow, even when the soil matrix is frozen to a depth of over 2 feet. The chemical analysis of the water in both surface runoff and tile drainage were nearly identical for the monitored constituents. Both factors indicate a strong link between the soil surface and the tile, which are indicative of macropore flow.

Prevent nutrient loss

Tile-drained agricultural land requires additional considerations when applying nutrients for crop production. The following are management considerations on tile-drained land with well-developed macropores.

1. Assess soil conditions prior to liquid manure applications. Both high and low soil moisture can be problematic for liquid manure applications to tile-drained land. Flowing tiles are often a good indicator of high soil moisture conditions, and well-developed soil surface cracks are an indicator of low soil moisture conditions.

Manure applications should be avoided during high soil moisture conditions. During dry soil conditions with surface cracks apparent in the soil, either utilize pretillage before a manure application or reduce the initial application rate to slowly add moisture to the soil to facilitate closing of the cracks.

2. Review forecasted weather and avoid applications when rainfall is predicted to occur soon after application. Soil moisture levels are elevated by liquid manure applications and subsequent rainfall can result in tile flow and release of manure to tile drains.

3. Monitor tile outlets before, during, and after liquid manure applications for potential discharge of manure. Tiles flowing before applications are an indication of high soil moisture conditions, in most circumstances, and applications should be avoided.

4. Restrict tile discharge prior to manure application, if able. If water level control structures are installed in tile systems, insert stop logs to prevent flow from tile drains before application.

5. Use tillage to break up preferential flow paths prior to or concurrent with application. Pretillage for surface and injected liquid manure applications or application methods that concurrently disrupt preferential flow paths below the manure injection depth should be used to prevent manure entry to tile drains.

6. Take precautions when surface applying liquid manure to no-till or perennial crops. Preferential flow paths are more developed in no-till systems and in later years of perennial crops. Split applications or reduced rates should be considered for liquid manure. ■

More detailed information on understanding and managing tile drainage systems can be found at uwdiscoveryfarms.org or learningstore.extension.wisc.edu

The author is co-director of the University of Wisconsin-Madison Division of Extension Discovery Farms Program.

Wyoming

The Laramie County Board of Commissioners voted to ease up on a long-standing requirement for Concentrated Animal Feeding Operations (CAFO). Previously, CAFOs wanting to locate in the Wyoming county had to get permission from all landowners within a three-mile radius of their farm.

Now, after months of debate, it has been decided that any CAFO looking to build in the county will only need permission from neighbors within a one-mile radius. This change matches the current state statute.

the requirement in NR 151.07 to comply with a NMP during the emergency if a farmer demonstrates to the DNR that they do not have sufficient available storage capacity for unused milk and cannot comply with their current NMP when land-applying the milk.

The agencies did indicate that these emergency actions should not cause an unpermitted discharge of pollutants to waters of the state. More information about land-applying milk can be found on page 9.

renewable fuel.

This renewable natural gas offloading station is the first of its kind in the nation. A station such as this gives biogas producers greater access to the pipeline.

Sweden

Wisconsin

Some dairy farmers have been asked by their dairy processor to dispose of, or dump, milk as a result of disruptions in the milk processing chain. This milk often ends up in manure storage structures and is eventually land-applied.

In Wisconsin, the Department of Agriculture, Trade, and Consumer Protection (DATCP) and the Department of Natural Resources (DNR) offered the following to assist dairy farmers during this emergency:

For permitted CAFOs, on a case-by-case basis, DNR can reduce the number of days required for public comment for modifications to a nutrient management plan (NMP), such as adding fields, which will allow CAFOs flexibility to quickly change their land application plans if necessary.

For non-permitted farms, DATCP staff will assist with updating NMPs at no cost. If staff are not able to assist directly, DATCP will determine how to get help with updates to NMPs and the best way to assist with any cost.

For non-permitted farms, the Governor’s Emergency Order No. 17 suspends

Michigan

The Michigan Department of Environment, Great Lakes, and Energy (EGLE) issued a revised general permit for Concentrated Animal Feeding Operations (CAFOs). The start date of the permit is April 1, but EGLE planned to delay issuing Certificates of Coverage to the state’s 260 permitted farms for 60 days due to the COVID-19 pandemic.

One notable change is that manure cannot be applied to farm fields in January, February, or March when the ground is typically frozen and runoff is more likely. Manure transportation is allowed during those months as long as it is not land-applied.

The Swedish city of Lund used manure as a way to encourage social distancing amid the COVID-19 pandemic. City workers applied 2,204 pounds of chicken manure across the city’s main park as fertilizer to discourage groups from congregating during a public holiday. The nation has banned gatherings of more than 50 people, so city officials hoped that the smell of the poultry litter would keep people from visiting the park.

Wisconsin

A renewable natural gas offloading station at the Dane County, Wis., landfill received its first load of biogas in midApril. The biogas comes from a digester that converts manure and other substrates into methane. Once the biogas reaches the landfill, it is injected into the interstate transmission pipeline to be used as

United States

The American Society of Agricultural and Biological Engineers (ASABE) has revised its standard for operating mobile liquid and slurry tanks. The standard, ASAE S317.1 JAN2020 “Improving Safety on Enclosed Mobile Tanks for Transporting and Spreading Agricultural Liquids and Slurry,” provides a guide for uniform practice and is intended to reduce risk of personal and public injury during normal servicing and operation of enclosed mobile tanks for transporting and spreading liquids and slurry. Originally developed in 1968, the revision updated key reference documents and made clarifications within the standard.