Soil sample accuracy determines test accuracy

Stacy Campbell, K-State Extension
Stacy Campbell

Some farmers are accustomed to routinely taking soil samples to determine the next crop nutrient needs. This article may be mundane to those of you that are accustomed to taking soil samples. However, there are always new people entering agriculture and maybe even some that haven’t taken soil samples but are considering it. 

Sample accuracy is extremely important, because it determines test result accuracy. Follow these steps to obtain a good soil sample for crop fields.

You Will Need: Auger, spade or soil sampling tube/probe – many Extension offices have available; Clean pail/bucket(s); Soil Sample Information Sheet; Soil sample shipping containers/bags.

Step 1 - Each area or field you sample should have the same soil texture, color, slope, and previous fertilization and cropping history. For example, if a field is comprised of bottom land, and also upland soils. Ideally they should be sampled separately.

Most growers will pull soil samples to test primarily for nitrogen, phosphorus and potassium, the macro-nutrients. In order to do that two separate samples must be collected in separate buckets. To test for phosphorus (P) and potassium (K) which are immobile nutrients in the soil—take a 0-6” sample. To test for nitrogen (N03) which is a mobile nutrient—take a 0-24” sample.

Step 2 – For the P and K sample and it is standard with this sample to receive the soil pH level as well. From each area/field, take 0-6” samples consisting of a minimum of 12 to 15 cores or slices, mix thoroughly in a clean container, and then fill your soil bag/container. It is optional for Organic Matter and Zinc to be tested for in the 0-6” profile if desired.

Note: For all samples taken always scrape or move away the surface residue before taking your cores or slices.

Another minimum of 12 to 15 cores or slices and mixed together in a separate clean container should be taken for the Profile test; which consists of testing for available nitrogen (NO3). Also you can test for sulfur (S) and chloride (Cl) micro and secondary nutrients as well. It is necessary to take this sample from 0-24 inches. Because NO3, S and Cl are mobile nutrients and move with the soil water. If you cannot obtain a sample to 24” because the soil is to dry, record on the form the average sampling depth you were able to take the samples at. This is important information for the lab to know in making an accurate recommendation.

Optional - on fields that have been reduced-tilled or no-tilled for several years, a split sample from the top 6 inches (i.e. 0 to 3 inches and 3 to 6 inches) is encouraged to assess pH and nutrient stratification near the surface. Caution: For the zinc test collect samples in a plastic container to avoid contamination from galvanized buckets or material made of rubber.

Step 3 - Avoid sampling in old fence rows, dead furrows, low spots, feeding areas, and other areas that might give unusual results. If information is desired on these areas, obtain a separate sample from the area.

Step 4 - Be sure to label the soil container/bag plainly and record the numbers on the soil container and the information sheet and completely fill out the sheet.

Step 5 - Air dry samples as soon as possible for the nitrogen test. Air drying before shipment is desirable, but not essential for all other tests. Do not use heat for drying. If it is going to be a few days before you ship nitrogen samples, it is best to put them in a freezer. This will stop any mineralization of organic nitrogen in the sample bag which can affect the sample accuracy. 

Tip – spraying WD-40 or cheap vegetable oil in the soil sampling tube will make it easier to get the soil cores to come out. The oil will not affect the integrity of the soil sample and results.