How To: Post-Harvest Soil Sampling is Easier Than You Might Think

Sally Krueger

Nov. 09, 2018

After crop removal, agronomists and fertilizer companies are busy collecting soil samples to be sent to a soils laboratory for analysis, but many farmers also collect their own soil samples, send them off for analysis and then review the lab reports to determine any needed change in their fertility plans.

It does not matter who collected the sample, but it is critical that the soil sample be taken correctly to uniformly represent the area sampled.

The best ways to collect soil samples and have them analyzed:

1. Visualize your fields by drawing boundaries in FBN.

Using FBN Maps can help organize your fields for soil sampling as well as to explore multi-year harvest trends, compare your yields to soil types, and examine field elevation and topography to harvest moisture.

By breaking out your data in histograms and pie charts, you have the power to understand your fields like never before. So you not only can see your harvest, planting, seed, soil and elevation data, but can also visualize crop moisture, drydown needs, prescription accuracy, planting speeds and more.

All your maps can be compared and overlaid to one another, and you can add and map an unlimited number of acres.

2. Order a soil test kit from your lab of choice.

You may not have access to drop off or hand deliver your soil samples to a lab—most farmers don’t!

But if you’ve chosen a reputable lab, many offer options for a pickup, or to order a soil test kit that includes bags and shipping labels to ensure your samples arrive carefully and your results are returned to you accurately.

3. Take your soil samples. 

There is no reason that you can’t or shouldn’t take your own soil samples. With guidance from your agronomist, you can manage any needed adjustments to your fertility plan based on what your soil analyses indicate.

The information you receive from a soil analysis is only as good as the soil sample you collected.

Soils laboratories only use a teaspoon of soil after drying and grinding the sample to mix the two pound soil sample you submitted.

If the soil sample you collect represents 40 acres of soil to a depth of 8 inches, that sample represents approximately 90 million pounds of soil—that’s why soil sampling can be such a valuable picture of what’s happening in your fields.

The 5 Best Methods to Take a Reliable Soil Sample

Remember that the purpose of the soil test is to report the nutrient levels present and indicate what nutrients may be deficient to properly produce the next intended crop yield goal.

That’s why soil variability is a concern when collecting soil samples, and there are several methods of soil sampling that can help to avoid this issue:

1. Divide the field into halves or quarters of 20 to 40 acres.

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 you choose.

This method will provide an overall average picture of the area sampled, which includes a specific measurement or value for each soil nutrient, in addition to the average pH, organic matter (OM) and cation exchange capacity (CEC) of the field. These particular measurements help to 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 (which means you wind up applying nutrients where nutrient levels are already adequate, or not applying enough of the needed nutrients in deficient areas.)

2. Collecting by soil types within a field.

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.

3. Collecting samples by recent yield data.

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) differ in yield.

Adjusting pH may be needed 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.

4. Sample the field according to the topography of the field.

This usually means acquiring samples from bottom areas of field 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.

5. Soil sampling in a grid.

With this method, the field is divided into a checkerboard design. Each square of the field is marked by a GPS location.

Typically five soil cores are collected from each grid, and 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 due to the special equipment needed to conduct them.Grid sampling also typically leads to the most efficient method to apply nutrients, but can come with an increased cost due to increased analysis and labor, and because a variable rate fertilizer spreader is also required.

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.

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 yearly soil analyses can also help you to determine if your current fertilizer programs are sufficient enough 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.

What you will need to take a soil sample

Whatever method or combination of methods you choose for soil sampling, you will need the following equipment to collect your soil samples:

  • A common chrome plated soil probe

  • A plastic or stainless steel bucket

  • Soil sample bags that are provided by your soils laboratory

How to take the soil sample

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 of each and every 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

Avoid problem areas and potential sample contamination

Even adding a single core from an unusual location can skew the laboratory results and render them useless. Sample contamination issues can 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, but you’ll likely see the return on that investment for years to come.

Sally Krueger

Nov. 09, 2018