Subsoil K: "Most Northwest Iowa soils are categorized as having low subsoil K (less than 50 ppm in the 12- to 24-inch depth). This means the topsoil must be managed more carefully because the plants cannot rely on deep reserves." โ Iowa State University.
I've deep-tested over the last 10 years in my root pits and found a lot of available K with standard tests. This graph represents a wide variety of soil types using Haney H3A extraction taken in 2025. How do we know total nutrient availability by only looking at the top 6 inches?