HOW TO CREATE THE MOST EFFECTIVE SOIL SAMPLING PROGRAM Written By Laura Barrera and first published on Agfuse
With harvest either underway or on the horizon for most growers in the U.S., now is a good time to prepare for your soil sampling program. Most universities and the NRCS recommend taking soil samples in the off-season, after the last crop harvest, and before the next cash crop is planted. Mississippi State University Extension advises collecting samples 3-6 months before planting, to allow any lime recommendations to react and change the pH. But before you can grab your soil probe and head out to the field, you need to understand best soil sampling practices and have a plan of action for collecting your samples, so you can accurately capture the soil chemistry and variability of your fields. Poor soil sampling practices can result in incorrect data, which can end up costing you in either inputs or yield. “The foundation of understanding or making an appropriate recommendation comes from having a representative soil sample,” says Jason Ackerson, a Purdue University soil scientist. “That comes down to having a sample that’s demonstrative of the types of soil you see in that field.” In short: The better your soil sampling program is, the more likely you are to see a return on investment from your soil and crop inputs while also producing healthier, higher-yielding crops.
Choose Your Lab In his Soil Sampling Guidelines paper, Ackerson advises farmers to use the same lab for analysis every time they take soil samples because labs can use different methods, which may lead to different results. Using the same lab every time will ensure any changes you see are because of changes in the soil, rather than the testing. If you’re new to soil sampling, it
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can help to pick out a lab before you start collecting samples, primarily for budgeting purposes. While Ackerson would never advise picking a lab solely on cost, knowing the price upfront can be helpful when considering the number of samples you plan to take and how you’re going to collect them. “If a farmer comes to me and has a limited budget for soil sampling, it’s probably not feasible for us to recommend a high-resolution grid sample when they don’t have the resources to pay for that analysis,” he says. In addition to price, consider whether you want just the soil test results or if you’d like additional information, such as fertilizer recommendations, as not labs will provide this. You’ll also want to make sure the lab is accredited, which should be available on the lab’s website. While most states have an accreditation system, there’s also a national accrediting laboratory body. Finally, consider whether the lab’s testing methods are relevant to your soil’s environment. There are a couple of different ways labs can do extractions for nutrient testing, Ackerson says, and there’s usually one method they’re more comfortable with. You want to make sure those methods apply to your soil environment. For example, there are different methods for extracting phosphorus: the Bray-1 method and the Olsen method. The Olsen method is optimized for soils in more arid environments with high levels of carbonates, so it wouldn’t be ideal for a farmer in a region like Indiana to send his samples to a lab that uses it.
Determine Where You’re Going to Collect Samples Whole-Field vs. Sampling
Spatially Explicit
There are two primary strategies for choosing your sampling locations: whole field and spatially explicit. With the whole-field approach, you’ll collect several samples across the field, known as subsamples, which will then be mixed to create one composite sample. As Ackerson explains in “Soil Sampling Guidelines,” composite samples reduce the effects of soil variability and are less sensitive to unusually high or low soil test values that might occur due to concentrated fertilizer applications or natural soil variation.
Jason Ackerson recommends growers who are doing whole-field sampling to follow a zig-zag pattern and collecting subsamples at each dot. This should help capture every soil type equally. Photo from "Soil Sampling Guidelines" published by Purdue University.
Jason Ackerson recommends growers who are doing whole-field sampling to follow a zig-zag pattern and collecting subsamples at each dot. This should help capture every soil type equally. Photo from "Soil Sampling Guidelines" published by Purdue University. Whole-field sampling is most appropriate for farmers who only plan to make blanket applications in their fields. While long-term uniform applications can lead to inconsistencies, Ackerson says growers can find other ways of identifying those areas without moving to spatially explicit sampling. In some cases, they may take soil samples from specific problem areas to confirm any soil issues. “Often a good crop scouter or agronomist can identify nutrient deficiencies based on scouting crops in
ISSUE 12 | JANUARY 2021
