Stuck in the Mud: How Wet Soils Impact Nutrients
In wet areas across the Prairies, this year has brought up a lot of questions.
When a crop sits in water, roots are starved of oxygen and development is restricted. As the roots struggle to grow and function, the crop has less ability to access nutrients, which can lead to stunting and uneven growth. High humidity can compound the problem. Reduced sunlight, from cloudier conditions, and higher humidity also reduces the transpiration potential of a crop meaning less water is being pulled from the soil through the plant.
This moisture map is based on data from an internal network of SWAT Water Stations and shows regional maximums of rainfall accumulation from May 1 to August 13, 2026, reaching up to 450mm in areas:
The decisions that come with uneven maturity, harvesting through wet spots, grain quality and increasing weed pressure are stressful enough. But what is happening beneath the surface?
Water is essential—but too much changes the soil
In reasonable amounts, water is critical to healthy crop growth. It supports mineralization, helps transport nutrients through the soil and provides structural support to plant cells through turgor pressure.
But when soils become saturated, many important soil processes can be disrupted.
Nitrogen
Excess water can have significant implications for nitrogen.
When soil becomes saturated and oxygen is limited, microorganisms can use nitrate as an alternative source of oxygen. This process, known as denitrification, can result in nitrogen being lost from the soil as nitrogen gas and nitrous oxide. The risk is particularly important in heavier clay loam and clay soils.
Excess water can also move nitrate below the crop's rooting zone, where it is no longer accessible to the crop. This is more likely to be a concern in lighter, sandy soils where water moves more readily through the profile.
Sulphur
Sulphur, like nitrogen, is relatively mobile in the soil and can move with water.
Prolonged saturated conditions can reduce available sulphur in the rooting zone, contributing to yellowing and poor early crop growth. Where excess water has moved nutrients through the profile, sulphur deficiencies may become more apparent as the crop develops.
Phosphorus
Phosphorus behaves differently from nitrogen and sulphur, but water still plays an important role.
The processes that influence phosphorus mineralization and immobilization are affected by temperature, moisture and aeration – much the same as nitrogen. Under saturated conditions, changes in soil chemistry can increase the solubility of phosphorus that would otherwise be bound to iron and aluminium oxides and other minerals.
Phosphorus can also be moved with soil through erosion, which can increase with excessive rainfall and runoff.
So, what does this mean for next year?
While we manage our way through this growing season, it will be important to understand what happened beneath the surface—and where it happened.
Not every part of a field will respond the same way. Nutrient losses and deficiencies will vary depending on where water collected, how long the soil remained saturated, soil type, drainage and what nutrients have been applied in previous years.
This is where SWAT MAPS and soil testing by SWAT Zone can provide a valuable report card.
Accurate testing by zone can help identify where nutrients are now low, where nutrients may still be present in excess, and where future fertilizer applications are most likely to provide a return on investment. Rather than treating the field as one uniform area, understanding the differences between zones can help us make more targeted decisions.
Subsurface water doesn't always behave the way we may expect or infer from what see it on the surface. The image below provides a conceptual illustration how excess rainfall water can create features such as a perched water table. Where and when perched water tables intersect a crop's root zone, and the soil situation implications on crop health, are very dynamic in time and space.
Wet conditions create challenges that we can see from the cab—but some of the most important impacts are happening below our feet.
Understanding where those impacts occurred this year can help us make better, more proactive decisions for the next crop. If we can collect data that actually drives decisions, it will deliver results.
