No one has to tell a farmer that poorly drained soils are a problem throughout many regions of the United States, and can have significant negative impact on crop production.
“Poorly drained soils mean poor crops,” said Dr. Jeff Strock, University of Minnesota. “If we can get good drainage it can help remove excess water from the root zone of growing plants.”
Strock says waterlogged soils have detrimental impacts on plant production. “One is slow gas exchange at the soil surface; particularly oxygen,” he said. “It creates problems for the plant roots to be able to take up water and nutrients. Nitrification can be inhibited or even prevented when we have waterlogged soils.”
When soils are waterlogged, the incidence of plant diseases, especially fungal diseases, rises. If plant roots are waterlogged early in the season, the crop may be stunted. Strock says if soil becomes dry later in the season due to drought, the plant suffers further stress.
“We know that wet soils are more prone to compaction,” said Strock. “We also know that wet soils take much longer to warm up and plant germination can be delayed.” Higher evaporation rates of wet soils can lead to the accumulation of salts near the soil surface.
Benefits of a good drainage system include reduced soil erosion, earlier seeding dates and more cropping flexibility. Crops in well-drained soils have higher germination rates and optimum overall establishment, which means optimum plant growth and yield, and better resistance to disease.
Although there are numerous benefits to having well-drained soil, there are several drawbacks. Nutrient loss, mostly nitrogen and phosphorus, along with sediments and pesticides can be lost in drainage systems. This can affect local water bodies and watersheds. It’s important to be aware of both the benefits and drawbacks when determining how and what to drain.
Strock explains there are three main types of field drainage systems: surface drainage, subsurface drainage and managed or controlled drainage. No matter what the system, it’s important to remember that any drainage system will function only as well as its outlet.
“When we’re draining, particularly subsurface draining, we are removing gravitational water,” said Strock. “That’s the water that is held least tightly in the soil profile. When we use a drainage system, it’s the gravitational water we’re trying to affect.” It’s important to understand that the amount of gravitational water changes when soil texture changes.
Strock points out the fact that surface drainage is the removal of water that collects on the land surface. “This is usually done with shallow, in-field ditches,” he said, “and oftentimes land leveling or land-forming so that we crown the fields and allow water to run off into ditches and out of the field.”
Water is removed at a rate expressed as a drainage coefficient, a figure that NRCS or other planner will work with when planning a drainage system. “These systems are designed with consideration of crop, soil type, location and topography,” said Strock. “We can manage water up or down in fields. Subsurface drainage can affect the hydrologic response of a field.”
Several considerations when designing a subsurface drainage system include drainage coefficient, drain depth and spacing, and drain diameters and gradient. Extension personnel, soil and water district personnel, contractors and other trained personnel will have access to computer generated tools to assist with drain design.
Peter Kleinman, USDA-ARS, discusses environmental losses of phosphorus in tile drain systems. “We’re talking about phosphorus as a liability rather than a resource in ag production,” he said. “Its something that farmers and nutrient managers feel doesn’t play to the usual nutrient management principles. There’s a disproportionate impact because very small losses of phosphorus can have a large downstream environmental impact. Losses that are agronomically insignificant can be environmentally significant.” For perspective, think about how when soil is managed for fertility, we’re dealing with parts per million (ppm) of phosphorus in the soil solution; while off-site, in a stream or a lake, parts per billion can cause an algal bloom or other environmental issues.
Kleinman says keeping phosphorus in fields and out of waterways can be a challenge. Tile drainage creates a hydrologic connection — it routes water away but also moves phosphorus and liquid manure. Kleinman noted the majority of phosphorus that creates an environmental concern comes from a minority of a given area.
“We have an idea of the critical source areas,” said Kleinman. “Phosphorus sources can be soils, fertilizers, amendments; but for those to become an environmental problem, we need to have a transport mechanism. It’s where we have a phosphorus source and a transport mechanism that we want to target management efforts — the most bang for our buck.”
What’s interesting about phosphorus is that it isn’t moving all the time, like nitrate. Management strategy should be based on separating the phosphorus from the transport mechanism. As landscapes are drained to move water to get productive soil, phosphorus moves with drainage water. Kleinman explains tile drainage serves as hydrologic connectivity and a route for surface runoff.
Numerous studies about how phosphorus moves in the soil and in drainage systems show that the more phosphorus that exists in the surface of the soil, the higher the concentrations of phosphorus in drainage.
Kleinman says while in ‘manure management mode’, what happens is that we inadvertently build up soil test phosphorus because of the ratios of nitrogen to phosphorus. “We end up putting much more phosphorus on than we harvest with crops,” he said. “We want to pursue land grant recommendations for soil phosphorus.”