Building soil fertility is like building equity — what you invest can pay dividends for years to come. But that’s only if you understand how nutrients persist in soil, how roots access them, and why some nutrients offer more residual value than others.
That’s where nutrient mobility comes in — a concept Ray Ward, founder of Ward Laboratories, has spent decades studying and explaining to farmers.
Ward credits the late Roger H. Bray, a University of Illinois soil scientist, with developing the nutrient mobility framework that still guides soil fertility practices today.
The basic principle is this: Some nutrients are water-soluble and move through the soil profile as water flows. Other nutrients remain relatively stationary, locked in place until a growing root comes close enough to take them up.
Mobile Nutrients Ride the Water Highway
Nitrogen in the nitrate form, sulfur in the sulfate form, and chloride all dissolve readily in soil water. When water moves into plant roots, these nutrients travel along with it.
“Anything in the root zone is available to the plant since they’re soluble ions,” Ward explains. This means your (corn) plants can tap into mobile nutrients from a broader area of the soil profile.”
This mobility creates an important practical advantage: You can quantify these nutrients through soil testing. A soil test can estimate the pounds of nitrate-nitrogen or sulfate-sulfur present in your soil profile, giving you a more concrete number to work with when planning applications.
Immobile Nutrients: The Root Must Find Them
Phosphorus, potassium, calcium, magnesium, zinc, and iron don’t move readily through soil water. The roots must grow to them.
Ward calls this root-surface mobility. “Wherever the root is, is all that can be taken up,” he says.
This has major implications for your fertility program. You might have adequate phosphorus levels according to your soil test, but if your crop develops a shallow or restricted root system due to compaction, drought stress, or cool soil temperatures, the plant often can’t reach those nutrients.
Soil microorganisms can help by dissolving nutrients and converting them into more available forms near the root zone. But at the end of the day, root growth is what determines access to immobile nutrients.
Rethinking What Your Soil Test Tells You
Understanding nutrient mobility likely changes how you read future soil test results.
For mobile nutrients like nitrate-nitrogen, a soil test gives you a snapshot inventory. Essentially it says, “here’s how many pounds are in your soil right now.”
For immobile nutrients, though, the soil test number means something different. It’s not an inventory. It’s an intensity reading — an indication of how well your soil can supply that nutrient to the crop over time.
“You can’t quantify it. You can’t say, ‘I got so many pounds of phosphorus in the soil,’” Ward explains. “You have no clue how much is available.”
Instead, soil test levels for phosphorus and potassium are interpreted through research-based calibrations. These calibrations place your soil into categories — low, medium, high, very high — that indicate the likelihood your soil can meet crop demand. A high-testing soil might supply 100% of your crop’s phosphorus needs. A medium-testing soil might supply 80% or 90%, meaning you’d need to apply fertilizer to make up the difference.
This interpretation framework is why two fields with the same soil test phosphorus level might still require different fertilizer recommendations based on crop demand, yield goal and soil type.
Putting Mobility to Work on Your Farm
So, what does all this mean for your fertility program going into 2027? There are at least three considerations:
1. First, don’t treat all nutrients the same. Mobile nutrients like nitrogen can require more in-season management and careful application timing because they move with water — and can be lost through leaching or denitrification.
2. Second, recognize that a soil test for phosphorus or potassium is a predictor of your soil’s ability to supply those nutrients over time. Maintaining adequate soil test levels is about building long-term fertility, not just feeding the next year’s crop.
3. Third, think about root health as part of your fertility strategy. Anything that limits root growth — compaction, poor drainage, soil crusting, cool temperatures — also limits nutrient uptake, especially for immobile nutrients. Managing soil structure, reducing compaction, and promoting early root development all contribute to better nutrient availability.
Understanding why nutrients behave differently in the soil can help you ask better questions of your agronomic advisers, interpret recommendations more critically, and make more informed decisions about when and how much fertilizer to apply.
Get more of the details on nutrient mobility and its importance from Ward in his latest YouTube video:
