Just Get pH Right? Does getting soil pH “right” lead to more nutrient availability? Yes, but also no. Life in the soil is not one dimensional. When considering pH, we should also be considering Eh (soil redox potential). There are management decisions that can influence both pH and Eh, and these two metrics should be thought of together. Redox Potential (Reduction and Oxidation) Reduction= gain of electrons (H+) Oxidation= loss of electrons or gain of Oxygen To make it easier to understand, I think about a plant oxidizing the atmosphere (adding oxygen) and the roots reducing (adding H+) to the soil. Reference the Pourbaix diagram I’ve attached to see how pH and Eh relate. There are more of these charts you can find to see where different nutrients may become available to the crop. You can find this by looking for Olivier Husson’s work. It makes sense why pH is considered and Eh is not. pH can actually be measured on a soil test. But even pH is not static even though we treat it like it is. Either way, there are management choices that will impact both. Here are some examples of an overly oxidized, reduced, and a more balanced system. Overly Oxidized: -frequent tillage -dry soil conditions, low residue cover -low OM/low WEOC -nitrate based fertility -low biological activity All results in high Eh and slightly alkaline conditions plus nutrient tie-ups Overly Reduced: -heavy clay w/ poor drainage -high OM and/ or manure inputs -compacted, anaerobic subsoil -waterlogged for extended periods of time All results in low Eh and more acidic conditions plus anaerobic microbial activity. Blueish gray looking soil that might smell rotten. A more balanced system: -living roots as much as possible -minimal tillage -mixed nitrogen sources -good biological activity -ground cover -aggregated soil All results in moderate Eh and slightly acidic to neutral pH. Leads to nutrient cycling and resilience.

A Quick History of Grid Sampling Soil sampling and recommendations based on those samples have come a long way since the early 1900s in some ways. But in other ways, they have not. Consider the first recorded field that got recommendations based on soil samples. This was published by Linsley and Bauer in 1929. It was: • A 30.89 acre field that was broken down into ≈1 acre grids. • There were soil samples taken at roughly 6 inch and 12 inch depths. • Based on those soil samples, the recommendation for Lime was about a half ton per acre. The lime would’ve been shoveled onto the field by hand (Franzen & Mulla, 2015). • The areas that didn’t need lime based off of the soil tests saved both labor and money. All of this was a huge advancement in agriculture at the time. But it is hard to believe that we are still using grids (especially larger than the ones used in 1929) in the year 2026. We have learned so much about soil since then, but our soil mapping, testing, and sampling techniques haven’t always kept up with the pace of learning. So What? Everything in agriculture is variable. Fields aren’t always square, soil is dynamic, and weather throws a wrinkle into everything. This makes measuring anything difficult. What we do measure isn’t always black and white. However, we try to treat something as dynamic as the relationship between plants and soils as static. This is what grid sampling and industry standard soil tests do. Grids treat the field as if it works in 2.5 acre (or more) squares. And coupled with standard chemical tests, we shoot for ppm values within those perfect squares. Seems simple right? But soil is not simple. We might know 0.1% about it, actually. Yet we do know that it is alive and that it cycles. Grids and chemical extractions alone do not really account for that. Consider this justification for 2.5 acre grids: “Although most researchers that have sampled whole fields at small spatial scale (less than 0.4 ha (≈1 acre) sampling density) have found that 0.4 ha is a minimum density to sample if nutrient variability is in a range where variable-rate fertilizer application would make a positive difference to the farmer, most grid soil sampling today is conducted in a 1 sample ha grid (2.5 acre grid). The reason that this is acceptable in the central Corn Belt of the United States today is that most fields and areas within those fields have P and K levels in the high range. Although there is considerable variability of P and K levels within the field, most of the field, regardless of soil test level, will result in the same P and K fertilizer recommendation. Therefore, a less dense grid is acceptable, because failure to represent the P and K nutrient level still results in the proper recommendation as provided by university soil fertility experts (Bullock, 2002; Wittry and Mallarino, 2004).” This is from A History of Precision Agriculture (Franzen & Mulla, 2015). That quote works for a system that is static and that has perfect edges. The reason why the 1 acre or less grids were preferred is because they picked up variability that the 2.5+ acre grids could not. Often times, you don’t have the same elevation, water flow accumulation, or topography within that 2.5 acre grid. This leads to differences in soil structure, carbon, biology, and nutrition within that one grid (reference the picture attached that overlays 2.5 acre grids over Baseline Rx zones). And while the quote above references P and K levels to justify 2.5 acre grids, is that enough? This considers chemical P (Bray, Mehlich, Olsen) and chemical K (AA) and that’s it. This is especially concerning if that P and K will have recommendations to spread in the fall which allows for tie-ups of these nutrients. What about Nitrogen, Calcium, or Carbon?