Diagnosing and Addressing Ponding Problems in the Field
One of the most visible consequences of an extremely wet year like we've seen in Central and Eastern Ontario is standing water in fields. Standing water has obvious consequences for production: wet conditions can delay planting, result in crusting that impedes seedling emergence and infiltration later in the season, and increase the risk of deep compaction. What isn't always so obvious is why some fields pond more than others, and what we can do about it. This article will address some of those reasons, explain how you can evaluate which ones might be present in your field, and point out some strategies to solve or avoid the problem in the future.
At the most basic level, soil saturation and ponding occur when water input is greater than water output. There's not a whole lot we can do about input How about output? We want to minimize runoff to decrease the risk of erosion, so our other options for output are evapo-transpiration and infiltration. In Ontario, our spring climate makes evaporation a painfully slow process. In a wet spring, over-wintered cover crops can get transpiration going to dry out soils faster, but that's a topic for another article. That leaves infiltration, water entering the soil and percolating through the profile to deeper soil where it is less damaging to crops and field operation timing, or can enter tile drains. Infiltration is a function of soil structure, specifically porosity.
Infiltration and Porosity
Efficient infiltration relies mostly on macropores, large enough (? 0.08mm) to allow water to move freely by the force of gravity. Macropores are most prevalent between aggregates in rounded, granular surface soil structure, but also in good blocky structures. Pores created by organisms like plant roots, earthworms, and other burrowing creatures also fit the bill.
Macropores need to be connected to the surface for water to easily drain into them. Open or continuous macropores drain water up to 100 times faster than closed pores (Shipitalo et al., 2000; Zhou et al., 2012). Compaction, crusting, and macropore and soil aggregate destruction from excessive tillage are the most important causes of reduced infiltration.
Macropores are the first to go when soil is compacted, leaving smaller pores that hold water more tightly. That's clear to see from the big, blocky clods that are often present in in the 5-30 cm depth range in compacted soil (Figure 1). Compacted layers might also exhibit platy structures that break horizontally (Figure 2). While there are some large pores between the clods and plates, they are far fewer and in the wrong orientation for facilitating infiltration. Plow pans in loamy or clay soils where tillage equipment has smeared the soil will create a thin but impermeable layer that keeps water sitting over top.
Figure 1. Big, dense, blocky structure caused by surface compaction in a loamy sand.
Figure 2. Platy structure in this clay loam is another sign of surface compaction.
Depending on the strength and moisture of the compacted layer, a cover crop with a good taproot can break through, and fibrous-rooted grasses or cereals will help make it crumbly again. Chisel plowing can also help to break this layer, but it will be easily re-compacted by the next pass unless the breaks are filled and fortified with roots. Make sure the soil is dry when attempting to mechanically bust a plow layer so that it will shatter and not smear. Chisel sweeps will increase the amount of loosened soil, but also increase smearing risk.
Crusting happens when surface aggregates are destroyed by water and disperse into smaller particles that fill pores and harden when dried, and it can be a problem on any soil finer than sandy loam. Crusting can be avoided by protecting the soil from the destructive force of raindrops, and by increasing the stability of surface aggregates. Both of those goals can be achieved with organic matter, either in the form of living cover crop canopy, or crop residue. The pictures in Figure 3 and 4 illustrate that well.
Figure 3. This bare part of a clay loam field has crusted.
Figure 4. Not two feet away, the same soil under residue cover (removed for the picture) has a rougher surface and evidence of earthworms. This is how infiltration happens.
You might think that figure 3 actually looks drier than 4, until you see exhibit 5:
Figure 5. Only the top 5mm are dry, and the crust keeps deeper soil moisture from evaporating.
Figure 6. Evenly distributed moisture, and earthworms actively making more macropores for drainage.
Tillage can decrease infiltration and lead to crusting and ponding in several ways. First, tillage buries residue that would otherwise protect the soil from rain and feed earthworms. Second, excessive tillage destroys aggregates, which makes the soil more vulnerable to crusting and compaction. Lastly, tillage disrupts macropores, making them less effective at draining water. The literature can be somewhat confusing on the effect of tillage on porosity because researchers often measure total porosity without paying attention to whether those pores are connected to the surface or each other. Reduced tillage favours earthworms and keeps pores made by previous crop roots intact, though the full benefits of no-till may take up to a decade to manifest. Cover crops can help ease the transition to no-till by accelerating improvements in soil structure and organic matter levels, as well as transpiring excess water early in the season.
If you noticed ponding on your fields, first check for compaction. The type of compaction you find will determine the approach for remediation, but ultimately the root causes - uncontrolled traffic, soil structure disruption, and low organic matter - need to be addressed. Crusting can be resolved by keeping residue cover as much as possible, and potentially adding organic amendments to problem areas. More important than sand, silt, or clay content is the pore space between those, especially the larger macropores that rapidly channel water into the profile. Crop residue, organic amendments, and living roots feed the soil organisms that create and maintain soil aggregates and pores, and over-wintered cover crops have the potential to transpire moisture faster than it would evaporate.
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