So, what happens to soil nutrients in winter?
No matter your definition of winter – cold and rainy conditions, or snow and freezing conditions – the season has an impact on soil nutrients.
Some nutrients are immobile, like phosphorus, potassium, calcium, magnesium, and many micronutrients. These nutrients typically remain in the soil over winter, as long as the soil is not eroded. Other nutrients, like nitrogen and sulfur (in the nitrate and sulfate forms) are considered mobile in the soil and may or may not remain in the soil over winter. In these forms, the nutrients are not attached to solid particles in the soil and can move out of the soil by leaching if precipitation exceeds the soil’s infiltration rate and water-holding capacity. But if the soil freezes, leaching is minimized until it thaws.
It depends on your environment
In more northern regions where crops don’t grow over the winter or winter annuals like winter wheat and perennial crops like forages are dormant, nutrients have no real function during the winter season. There isn’t any plant uptake or little, if any, nutrient use by microbes, but depending on soil and weather conditions, nutrient losses can still occur.
In southern regions, some crops remain somewhat active during the winter and may not go completely dormant. Crops like winter wheat and forages may continue to take up small amounts of nutrients during the winter and throughout warmer winter weather spells. For these crops and cropping systems, small amounts of nutrients may help the plant remain healthy during the winter and stimulate earlier spring growth. Other soil organisms may also be active and use small amounts of nutrients.
How to maintain your soil nutrients
Do
Consider cover crops. Soil health is best managed in conditions where biological activity occurs, making cover crops during the winter a great opportunity to retain nutrients, protect against erosion, and add biomass to soils. Even in colder regions where the cover crops may die or go dormant during winter, the soil cover provided can be beneficial to soil health. In areas where cool-season plants may continue to grow throughout the winter, the additional biomass produced can add carbon to the soil, improving soil health and structure.
Cover crops can also scavenge soil nutrients before winter sets in or during the winter to prevent nutrient losses. And depending on environmental conditions, the nutrients scavenged could become available for next year’s crop.
Leave crop residue. If cover crops don’t fit into your crop management system, leaving crop residue on the soil surface is beneficial in preventing erosion, improving infiltration, reducing runoff and improving overall soil conditions.
Don’t
Apply nutrients to frozen ground. Soil nutrients, no matter if they are immobile or mobile, can be washed away by runoff if applied on the soil surface and not incorporated. Rainfall or snow events following application can lead to nutrient runoff in the water because frozen soil prevents water from being absorbed.
Apply nitrogen in the fall on sandy soils or where winter and spring leaching is expected. Nitrogen converts quickly to nitrate which is mobile in soil. Nitrate can be easily leached from porous soils with rainfall or snow melt events.
Soil type matters
Soil type influences nutrient behavior over the winter. Sandy soils are more prone to loss of mobile nutrients because they have low water-holding capacity and high infiltration rates. That means nitrogen should not be applied in the fall and sulfate sulfur is also prone to loss from sandy soils.
Heavier soils (with more clay content) and soils with higher organic matter content are less prone to leaching of nutrients because they hold more water and have slower infiltration rates. But they are more prone to runoff and may remain wet longer into the spring, contributing to greater denitrification losses. In heavier soils that stay frozen during the winter, fall nitrogen application may be acceptable, depending on the form of nitrogen used.
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