On the other hand, inaccurate soil-nutrient measurements can create financial headaches for both landowner and farmer, says Mitchell. In particular, soil-sample depth and lab error are two potential pitfalls that could hurt people who aren't careful when agreeing to these types of leases, he add.
"When landowners get soil samples, the assumption is that the nutrient value is the same, no matter the depth, and sometimes it is," he says. "However, the reality is that on most fields you can get drastically different results, depending on how deep the soil samples are taken."
Mitchell notes that on some new ground he recently sampled, the fertility levels drop off rapidly beyond the 0-6-in. depth. "On this ground, the topsoil is only several inches deep, but it had three times the nutrients than the soil at 6-12 in. in depth," he says.
The nutrient stratification that occurs on this ground tends to be fairly common in no-till situations, compared to fields where tillage mixes the nutrients more deeply into the soil profile, he says. However, even in situations with more tillage, there's generally a drop-off in fertility below the 12-in. depth, he adds.
"So, if the landowner takes soil samples at a 12-in. depth after the farmer has applied fertilizer based on soil samples from a 6-in. depth, it may look like the farmer has cut the fertility application in half," says Mitchell. "In this case, the farmer might be held liable for what appears to be extreme changes in soil fertility, due to differing nutrient levels at two different depths. For strip-till farmers, there's also a potential liability issue, depending on whether soil samples are taken in the row, where nutrients are applied, or out of the row, where they are not applied."
Soil testing quality, depending on the lab, is the other significant challenge, adds Mitchell. "Landowners want to look at the old soil-nutrient levels and see the changes over time," he says. "However, if the quality of that data is bad, the implications can be huge."
What method a lab employs to test soils can result in highly variable outcomes, cautions Mitchell. "Using field-moist sample preparation methods can be much more consistent and accurate than the air-dried or oven-dried soil methods that many labs have been using for decades, but are now outdated," he says. “Keeping field samples in their naturally moist state preserves the integrity of the sample, and allows us to measure what the plant actually sees."
There is also an emerging class of soil measurements developing which can isolate soil particle-size distribution, providing a new level of insight into a field’s structure and productive capacity, says Mitchell. "These new, high-resolution soil-texture measurements are vastly superior to the two basic methods (the hydrometer and sieving approaches) used to determine soil texture in the past," he says. "The old methods only approximate soil texture based upon percent sand, silt and clay, not particle-size distribution."
Soil texture is critically important for properly managing soil water-holding capacity through irrigation scheduling, tillage considerations and variable-rate adjustments, adds Mitchell. "Soil texture is soil’s single most important physical property," he says. "A better understanding of a field's soil texture alone can provide critical information regarding water flow potential, water holding capacity, nutrient efficiency, compaction risk and (field) traffic load capacity."
Other factors to consider when evaluating land would be to look at maps generated by real-time kinematic (RTK) technology and radioactive, radiometric and electrical conductivity measurements. "These technologies can detect changes in slope and elevation due to erosion," says Mitchell. "Soil erodes from higher elevations to lower areas of the fields. And RTK maps can show where the soil has moved. We can also use RTK maps to compare where we have scraped out waterways and added the soil somewhere else in a field."
A very strong relationship exists between yield and topsoil depth, Mitchell says. "Sadly, the U.S. has lost half of all its topsoil and half of all its organic matter since being used for agricultural purposes," he says. "Yet, now we can measure and manage erosion using these new mapping technologies to see, verify and validate how much soil is moving and benchmark a farm's conservation efforts. Adopting this technology should also help motivate people to act in their own best interest."