Spatial Critical Fertilizer – A Dynamic Approach to Phosphorus & Potassium

For several decades, Midwest agronomists and farmers alike have used a standard rule for managing nutrient concentration levels in their soils.  This rule of management has primarily been influenced by utilizing various state or university guidelines. These guidelines have been widely adopted across much of the United States to manage soil fertility on a field-by-field basis to ensure that nutrients are not being over or under applied by producers.  Additionally, these guidelines have also served as a guide for sufficient nutrient concentration levels to sustain crop yield.

As an industry, we often utilize the Soil Survey Geographic Database (SSURGO) to identify soil composition, water holding capacity, frequency of flooding, etc. that can be used to identify areas of risk for nutrient loss. We designate these acres lower in landscape position and closer to the water table of the soil as “at risk” for nutrient loading, but in general we don’t manage this at the sub-field level. However, what if we had a way to identify a critical value for Phosphorus and Potassium that addressed the influence of other factors such as soil landscape position within the field and water availability?  For example, will a Silty Clay Loam at 0-1% slope mineralize various nutrients at the same rate or total amount as the same soil type only 30 yards away with a 3% slope?  Of course, the answer is no. However, when it comes to critical values and managing soil fertility, many recommendations treat these acres the same.

Soil temperature, rainfall, and landscape position all have profound effects on nutrient utilization and uptake. Given a uniform concentration of nutrients in the soil, a plant will respond differently depending on where it stands in the landscape. This can have major consequences for the grower if plant density and soil fertility are not managed together at the sub-field level.

For years, it has been researched and proven that high percentages of Phosphorus and Potassium are taken up by the plant through diffusion. It’s also been researched and proven that there is a near direct correlation between water availability and diffusion. For example, if water availability is reduced by 75%, then diffusion rates are reduced by nearly 75%; if diffusion rates are reduced by nearly 75%, then Phosphorus and Potassium (as well as other immobile nutrients) are reduced by nearly 75%. Therefore, in sub-field environments with less water-availability, fertilizer concentration is of utmost importance.  Inversely, sub-field environments with more water-availability require a lower  fertilizer concentration due to the increased efficiency of diffusion.  This is not a new concept as Roger H. Bray, a pioneer in soil fertility, documented  in a 1953 journal article, “as the mobility of a nutrient in the soil decreases, the amount of that nutrient needed to produce a maximum yield, increases.”

Years ago, Advanced Agrilytics began taking a more dynamic approach to managing soil fertility in a site-specific manner. Instead of using the same static critical level for every acre like most state and university recommendations, we asked the question, “what if we could generate site-specific critical values on each grid cell that would allow us to better address environmental factors affecting plant growth through more efficient use of fertilizer applications?” The multi-year results of this “environmental adjustment” have proven to have had a positive influence on yield, particularly on acres that have historically been a net-loss to the farm.

At the same time, this approach has equated to a greater nutrient use efficiency on the most productive (non-water-limited) acres.  We’ve found that we can produce high yields while keeping the critical values for Phosphorus at the bottom end of the state and university critical recommendation range, while increasing those critical levels in stressful water-limited environments. Additionally, we know that Phosphorus run-off most frequently occurs on the acres closest to the watershed.  Therefore, applying unnecessary amounts of Phosphorus on these environments can not only result in inefficient use of fertilizer spending, but more importantly, contribute to nutrient runoff. On the other hand, by applying higher concentrations of fertilizer on water-limited environment’s furthest from the watershed, we’re able to utilize higher rates of fertilizer more efficiently while reducing environmental run-off at the same time.

In summary, yield results over time have proven that in a dry year, higher rates of fertilizer on the water-limited acre (high ground), had a significant advantage compared average to below average fertility in these same environments.  Alternatively, in a wet year when the high ground does much of the heavy lifting in terms of yield contribution, higher concentrations of fertilizer not only resulted in good yields, but rather took yields to another level in these respective environments.

For more on Spatial Critical Fertilizer Strategy, contact your local Advanced Agrilytics agronomy representative.

Jeremy Hogan

Lead Agronomist – Illinois

Last Updated on July 25, 2024