Soil variability is often a concern when collecting soil samples, considering that the purpose of soil testing is to report the nutrient levels present and indicate which nutrients may be deficient in order to to properly achieve the next intended crop yield goal.
However, there are several methods of soil sampling that can help to avoid this soil variability issue. Below, we'll outline five methods for taking a reliable soil sample on your farm and explore how to take a soil sample.
This is the most common soil sampling method if soil variability is not a serious issue.
Take several soil cores in a zigzag pattern across each quadrant, then properly mix and bag so the cores can be analyzed by the soils laboratory.
This method will provide an overall average picture of the area sampled, which includes a specific measurement or value for:
Each soil nutrient
Average pH
Organic matter (OM)
Cation exchange capacity (CEC) of the field
These measurements help you better understand the availability of nutrients to plants.
Keep in mind that if soil variability is an issue in your field, this type of sampling can lead to inefficiencies in your fertility applications. Select a different testing method better suited to soil variability to avoid later applying nutrients where nutrient levels are already adequate, or not applying enough of the needed nutrients in deficient areas.
This method works well as long as there are only two or three soil types within a field, and the field can also then be fertilized in the same way — by soil type.
This type of sampling also provides measurements on pH, OM and CEC and will help you to take note of differences in pH, OM and CEC within the field.
This method can help you to determine why some areas yield more or less than others. You should collect several soil cores from areas of the field that you know (or expect to) differ in yield.
You may need to adjust the pH in order to change the availability of present nutrients, as opposed to adding more nutrients to the low-yielding area, and this method can help you to determine how best to address those areas.
This approach usually means acquiring samples from bottom areas of fields as well as tops of hills and sides of hills.
With this method, you should also sample in the same way you would ultimately apply fertilizer across the field, based on the topography, and what the soil analysis reveals would be needed for each sample area.
This method divides the field into a checkerboard design. Each square of the field is marked by a GPS location, with five soil cores collected from each grid.
Each square or sample should be from a uniform-size area. The grid size can vary from 1 to 4 acres each, but the most common size is 2.5 acres. Grid samples are generally done commercially because of the special equipment needed to conduct them.
While this approach typically leads to the most efficient nutrient application, it requires additional budget because of the increased analysis and labor costs. Later stage fertilizer application also requires a variable rate fertilizer spreader to accommodate the soil variability and associated varying nutritional requirements.
To stretch out this cost, grid samples are normally done every 3 to 4 years, based on cropping and nutrient plans made for that timeframe. In theory, fertilizer is only placed where it is needed by plants, and will bring the entire field to equal nutrient values at the end of the program. The same grid is then resampled to check on the progress of nutrient availability within each grid.
Keep in mind that a combination of any of these methods of soil sampling works, too.
Pick the method and specific timing following harvest that best fits your situation. By collecting soil samples at a similar time of year, you can collect a consistent, reliable history from each field.
Comparing your annual soil analyses can also help you to determine if your current fertilizer programs are sufficient for your yield goals. You don’t want to over-apply nutrients your crops won’t use, but keeping the right amount of nutrients in the soil will help to control pressures from weeds, pests, and diseases.
Maintaining fertile soil also demonstrates good land management and stewardship.
Whatever method or combination of methods you choose for soil sampling, you'll need the following equipment to collect your soil samples:
A common chrome plated soil probe
A plastic or stainless steel bucket
Soil sample bags provided by your soils laboratory
Each soil sample should not represent more than 40 acres, which means you should take a minimum of 20 cores.
Take soil at a depth of either 6 or 8 inches
Maintain a consistent depth for each core you collect
Mix your cores thoroughly before bagging and labeling the sample
Keep your soil samples dry and cool
Deliver your soil samples to the lab as soon as possible, especially if you have included a nitrate (NO3) test
Even adding a single core from an unusual location can skew the laboratory results and render them useless.
Sample contamination issues can also be caused by galvanized buckets or dirty buckets. Be sure to remove any crop residue or foreign material before collecting and bagging your samples.
Other field areas to avoid when soil sampling can include:
Blank areas
Fence rows
Turn rows
Windbreaks
Terrace bottoms
Old farm sites
A field where banded fertilizer was applied (if possible, do not gather a soil core from the band itself)
Soil sampling is a great investment that you can make today for more efficient fertility planning for next year’s crop, and you’ll likely see the return on that investment for years to come.
FBN has a comprehensive selection of crop nutrition products to maximize every crop’s yield and quality potential.
Our solutions span the entire growing season and include starter fertilizers, in-season foliar micronutrients, and soil amendments, including prebiotics and probiotics to support plant health. Mix and match products to meet your crop’s unique nutritional demands.
With 24/7 digital shopping access, direct-to-farm delivery, transparent pricing and savings opportunities, and detailed label information for each product, FBN delivers the information and crop nutrition products you need to maximize your crop’s potential this season.
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