Matt Greenspan

Soybean roots nearly disappear just above the claypan. That's not a figure of speech. On Centralia claypan ground, depth to that clay ran about 12 inches on the hilltops, 4 inches on the eroded hillsides, and up to 40 inches at the bottom of the slope. Roots were thick near the surface, then almost vanished right above the claypan - then showed up again 8 to 16 inches below the top of that clay, about 20 to 30% thinner. And the water story is even simpler: 6 inches of topsoil holds about half the water of 39 inches. Corn struggled where topsoil was under 6 inches. That same field had lost about 5 inches of topsoil over roughly 150 years - as much as 12 inches on the north end. So when a yield map stays stuck on those shoulders, it isn't always begging for more P and K. You can't fertilize a 4-inch topsoil shoulder into a 40-inch toeslope. Not every low zone needs more P and K. USDA-ARS / MU

That spring K test can look fine - until July shows up. Here's the part a lot of people miss: a regular soil-test K reading only sees what's in solution and hanging on exchange sites. It doesn't see the K stuck between 2:1 clay layers. On high-smectite ground, those layers can collapse when the soil dries and trap that K for a while. NDSU split their corn trials at a smectite-to-illite ratio of 3.5. Below that, the critical mark was 150 ppm. At 3.5 or higher, it jumped to 200 ppm. On those high-smectite sites, a dry July or August still threw K-deficiency symptoms and more than 20 bu/ac of response - even when spring tests were already above that old 150 ppm bar. Our Mexico and Adco claypans are Fine, smectitic, and that clay layer is usually more than 50% clay. So a grid that "looked fine" in a wet fall can still get cranky on those eroded shoulders when July dries out. Fertilizer as a whole is a gut punch right now. DTN's latest national retail survey had anhydrous at $1,007 a ton and MAP at $987. Potash is still the reasonable one in that lineup, at $510. If a dry July is leaving K bushels on the table, this isn't the year I'd skip it just to save a little money. Not a new Missouri critical. MU and Tri-State don't split K recs by clay type. Just a reason to keep an eye on those shoulders. NDSU / Franzen

Saturation Risks: why it matters. Warm soil + standing water = N gone fast. Above 60°F, full of water 2–5 days, 50%+ of N can be lost. At 55–60°F: ~1–2% of nitrate a day. At 65–70°F: ~4–5% a day. Only nitrate is at risk. Ammonium holds.

Fall burndown sets up a clean start in spring. Henbit comes up mostly in fall and is a strong SCN host. Fall control of emerged marestail is more effective and consistent than spring burndown. Spray on a 50°F+ day. Still scout before planting.

Your N bill comes with a lime bill. MU Extension: every 100 lb of N from NH3, urea or UAN takes 72 lb ENM of lime to neutralize. Ammonium sulfate? 220. So 180 lb N/ac as NH3 ≈ 130 lb ENM/ac. Compare lime quotes by ENM, not by the ton. extension.missouri.edu/publi…

Your K soil test can change with the weather. Iowa State's 2023 update: on moderately poor to poorly drained soils, the moist K test was much more reliable than the dry test. Prime suspect: wet/dry swings. Drying can collapse clay sheets and trap K. agronext.iastate.edu/soilfer…

Claypan K isn't one number. In MU plots near Columbia, soil-test K ran 4.4 kg/ha lower per extra cm of topsoil, and raising soil-test K took 4x more fertilizer K at 44 cm topsoil than at 0 cm. Map depth to claypan before you build K. doi.org/10.2134/agronj2017.0…

A full 1,200-bu grain cart can put 35–40 tons on one axle. On wet soil that packs 2–3 ft down, below any tillage, and it can still cost yield in a drought or wet year a decade later. Unload on headlands, stay in combine tracks, and drop tire pressure.