Soil testing is the basis of a sound fertilizer program; however, it is important to understand that the decisions made as a result of soil tests are only as good as the samples themselves. A little extra time spent during collection and submission can pay big dividends later. Following these five steps will help ensure you get the most from your soil samples.
Collect soil cores to the correct depth. The correct depth for soil samples depends on the nutrients being tested, sample purpose, tillage system, intended crop, laboratory request, etc. At the Noble Research Institute, we recommend a 0- to 6-inch core for standard analysis plus a 6- to 12-inch core for nitrogen analysis. The laboratory we use calibrates their analysis based on a 6-inch increment. Many nutrients, such as phosphorus and potassium, will form layers in the soil profile and may be concentrated near the soil surface. If the sample is collected from a depth that is too shallow, results will show that there is a greater concentration of those nutrients and we may not recommend a needed fertilizer. If the sample is collected from a depth that is too deep, results will show a lower concentration of those nutrients and we may recommend unnecessary fertilizer.
Collect enough cores to make the sample representative. There is a great deal of soil variability even over relatively small areas. To get a good average for the area, we need to collect and blend enough representative cores together. The more cores included per sample, the better the sample quality. The absolute minimum is 12 to15 cores per sample.
A sample should not represent too large of an area. We tend to want to make a single sample represent an entire field no matter its size. Due to the variability in soil fertility, it is difficult to get a good average for more than 40 to 50 acres at a time even if the field appears uniform. Divide fields larger than this into smaller areas or zones that can be sampled and analyzed separately. Ideally, these zones are fertilized according to their individual needs, but it is often necessary to average the recommendation across the zones to make a single blend for the entire field.
Collect separate samples to represent different soil or topography types. Often a single field will contain significantly different types of soils or topographies. These should be sampled and analyzed separately much like the zone sampling mentioned above. Additionally, if there are trouble spots in a field that do not produce well, these should be sampled separately to help identify the cause of the problem.
Provide sample information requested on the submission form. The information on the sample submission form is needed to generate an appropriate recommendation. At a minimum, we must know what is being grown, if the crop is established and the field identification. Fertility and lime recommendations depend on the crop and whether or not it is established. The field identification is necessary to tie the sample back to the field where it was collected. Since a sample can be used for up to three years, do not rely on memory to know where it came from. Include a reasonable target yield goal along with the crop variety and percent stand for established crops. The nitrogen recommendation is based on the target yield goal. The variety and percent stand establishment can both affect the nitrogen use efficiency, which will also affect the nitrogen recommendation. Finally, include the crop and yield history along with the amount and types of fertilizers applied. If the fertilizer response for the field is significantly different than normal, that information can be used to refine the fertilizer recommendation.
The above steps require additional time and effort, but, considering the cost of fertilizers today, it is time and effort well spent.