Soil Sampling Methods, Tools and Providers
Types of Soil Sampling
What is Soil Sampling & Why Does Accuracy Matter?
Fundamentally, soil sampling is taking bags of soil from specific places in the field so they can be analyzed at a soil testing lab for several important properties. Field sampling took off in the 80s after the invention of GPS, and now is integral in site-specific agriculture, including self-driving tractors, drones, air spreaders, and cutting-edge biologicals.
The goal is to manage individual sections of fields where conditions are relatively consistent.
Think of it as a technology investment, rather than as a line item on the fertilizer bill. Unlike fertilizer, soil samples don’t grow crops – but they do increase efficiency, helping you stretch your inputs further.
This article will help you maximize your ROI by understanding the cost/quality tradeoffs in each soil sampling decision.
Soil Sampling Methods: 5 Types of Soil Sampling
The Grids vs Zones Debate
Zones are (typically) divided based on soil type or some other data layer, are less dense than grids, and use distributed samples (see description below). And while there are plenty of exceptions, grids are usually completely uniform and unbiased, more dense than zones, and use point sampling.
Both grids and zones can be extremely accurate – or completely useless. Rather than simply comparing “grids vs zones,” ask yourself (and your agronomist) these 5 questions:
What’s “Normal?”
Dividing fields into uniform grids is still the most common, though zones based on soil type are rising in popularity.
Differences in Quality
A 10 acre zone will probably have more consistent conditions than a 10 acre grid, if that zone is based on solid agronomic data.
BUT grids are usually smaller.
Differences in Cost
Because of the data work involved, it’s more expensive to make zones than grids, but zones are usually cheaper to sample and analyze because they’re often less dense.
Things to Consider
While this is VERY ROUGH, I use a factor of 2 to convert between zones and grids. For example, a 5-ac grid will be roughly as accurate as a 10-ac zone.
Remember that not all zones are equal! The majority of agronomists push for zones from soil types, yield, or topography, though some progressives claim that electrical conductivity, gamma radiation, or satellite imagery layers create even better zones.
What’s “Normal?”
The most common grid size is 2.5 acres. While 5 to 10 acre zones are typical in the midwest, 10 to 20 acre zones are more common further north and in Canada.
Differences in Quality
Grids
Anything below a 2.5 acre grid is considered high end, while anything above 10 acres is considered lower quality.
Zones
Generally speaking, zones smaller than 5 acres are pretty high quality, while anything larger than 20 to 30 acres are lower quality.
Differences in Cost
Increasing density increases costs.
Doubling the density doubles the lab costs, and usually increases extraction costs 20-60%.
Things to Consider
To know what density is right for you, look at soil maps of similar fields in your area. An indicator that the density is right is if you see mostly SMALL differences between adjacent grids. If one grid is at a 7 pH and the next is at 6, that’s a red flag that the field probably should have been sampled more densely.
Final tip, variability largely depends on water movement. If you have hills, lots of rain, or sandy soil, I’d recommend a relatively dense grid.
What’s “Normal?”
“Point sampling,” where you pick a point and pull 6-8 soil cores from across several rows, is typical throughout the corn belt.
“Distributed samples” are where cores are pulled from throughout the grid/zone, sometimes using an “M” pattern. These are more popular in Canada and the Pacific Northwest.
Differences in Quality
Distributed samples can increase the sample’s quality if the grid or zone isn’t very consistent.
If the grid or zone is already pretty consistent, changing the sample type won’t have a significant impact.
Differences in Cost
Distributed samples take more time than point samples, and are more expensive.
Things to Consider
In grids or zones smaller than 10 acres, this won’t have a very significant impact. If your grid or zone is above 10 acres, though, I’d strongly recommend a distributed sample.
What’s “Normal?”
Fertility samples are typically pulled from 6 to 8 inches deep.
Differences in Cost
This usually doesn’t impact costs unless you’re going extremely deep (12+ inches).
Things to Consider
The ideal sampling depth depends on how the fertilizer recommendations will be calculated – so ask your agronomist.
Most fertilizer recommendations come from university studies that simply find correlations between yield and soil test values. You’ll want to use the same sample depth that was used in those studies.
Fun fact, that’s also why many universities are based on similar principles, but have different recs and sampling procedures. If you change one, you have to change the other.
That being said, if any agronomists are reading this, I recommend calibrating your recs on 8+ inch samples, as this reduces the impact of depth variability during sample extraction.
What’s “Normal?”
Most farmers soil sample every 4 years.
Differences in Quality
Frequent sampling is more important when there’s significant leaching because of sandy soil or heavy rain.
Some growers with high-value crops will sample every year.
Differences in Cost
This is pretty simple: sampling twice as often costs twice as much.
Things to Consider
Agronomists can use removal rates to estimate fertility needs between sampling years (higher yields remove more nutrients).
Unfortunately, these estimates will never be perfect because removal is not the only factor that influences nutrient availability. But that can be more accurate if they’re based on the actual results from that field (not an estimated average). Using actual yield from that specific grid or zone would be even better, but this is very uncommon.
Time of Year
Soil sampling in the spring is usually more accurate (because it shows what the soil is like during germination) and more convenient (because you don’t need to worry about getting the results in time to apply fertilizer or till). However, fall sampling is still more popular, and it’s helpful to match what was done in the past so that you can track the field’s fertility changes over time.
Soil Sampling Tools
***Disclaimer: My Company Builds Soil Sampling Robots***
Not all soil cores are the same: The tool being used to sample can significantly change the repeatability and accuracy of that sample by influencing the depth, completeness, and location of the soil cores.
To Test This For Yourself: Simply have a field (10+ samples) sampled 3 times with each method, and see which method was more consistent.
We’ve done similar studies (like this one and this one), and the results back up my claims.
Based on those studies and the countless conversations I’ve had with soil samplers around the country, these are the 8 factors I’d consider when picking a sampling tool:
| Hand Probes | Augers | Hydraulic Probes | ROGO | |
| 1. Relative Cost | 👍👍👍 | 👍 | 👍 | |
| 2. Speed | 👍 | 👍 | ||
| 3. Depth Accuracy | 👍 | 👍 | 👍👍👍 | |
| 4. Soil Core Completness | 👍👍 | 👍 | 👍👍 | |
| 5. Location Accuracy | 👍 | 👍 | 👍 | 👍👍👍 |
| 6. Packaging/Organization | 👍 | 👍 | 👍 | 👍👍👍 |
| 7. Accountability/Verification | 👍👍 | |||
| 8. Overall Field Repeatability | 👍 | 👍👍👍 |
Soil Sampling Companies
These are my priorities when picking a soil sampling provider, in order:
- Find someone who will use the method and tool that I’m looking for. As discussed, there are significant differences between these, and it’s important that you and your provider are on the same page.
- If possible, I recommend getting sampling from the same person or group who writes your fertilizer recs. They’re probably already very familiar with your agronomic goals and can take care of many of the sampling decisions for you. Plus, they’ll already have your maps and past sample points, so you won’t need to worry about sending anyone your field data.
- If you can’t do it through your agronomist, find someone who you like that can get to your fields quickly. Often sampling is just one more piece of the fall/spring rush, so make sure they have a reputation for being reliable.
- Price, of course. Why wasn’t this first on the list? The price of soil sampling varies between $3 and $8 per acre based on the sample density, map type, and other factors. When directly comparing providers (which I do often), most options will be within $1-3/ac of each other. This actually doesn’t have a major impact on my decision, though, and here’s why: If I’m on a 4-year rotation, that’s only a 25 to 75 cents per acre per year difference between the most and least expensive providers. The soil sampling methods and tools will have a far greater impact on price than the service provider you select.
Soil Sampling Management & Data Flow
