Fall is by far the best time to take soil samples. You will have more time to sample properly than in the spring. You will have time to really think about the results and make plans for next year, which might include early ordering of fertilizer, deciding where manure might be most beneficial, etc. Soil and weather conditions are often better for sampling in the fall. You cannot afford to have your crop come up short because of lack of nutrients when you are making a significant investment in the many inputs required to put out the crop. At the same time, you cannot afford to apply extra nutrients that will not give you an economic return. The economics of soil testing are pretty simple. If you sample a 10A field every 3 years as recommended, the cost per acre is around $0.30/A/year. This very small investment in soil testing which results in recommendations that are used to manage typically more than $200 worth of nutrients on a crop that is worth over $1000/A. You can’t risk this magnitude of input costs and potential returns on a guess.
Good soil test begin with good soil samples. Here are some guidelines for getting good soil samples.
- Sample uniform areas
Usually we sample each field individually. However, there may be times when we need to subdivide fields if there is the potential for significant differences across the field. Examples include: significant soil differences, part of the field receives manure but not the whole field, topographic differences such as low areas versus sidehills, etc. Also, there are situations when we can combine fields. For example when we have small strips that are all managed the same we can lump these together into one sample.
- Take lots of cores
At least 15 to 20 cores should be collected to make up a composite sample to send to the lab. More is better.
- Sample to uniform depth
For most routine soil testing samples should be collect to plow depth. Even in no-till or permanent sods. Inconsistent sampling depth is one of the biggest sources of errors in soil sampling. This is especially true in no-till and reduced tillage systems where there is often significant stratification of nutrients in the soil.
Special note in no-till: In no-till fields an acid layer, called an acid roof, can develop right at the surface of the soil. This thin acid layer can have a significant impact on the crop but it can be missed in a normal plow depth sample. If the normal plow depth soil sample in no-till recommends liming, apply the lime as recommended and no further testing is needed. If the normal sample does not recommend lime and the field has been in long-term no-till and has not been limed recently, take a less than 2 in. deep sample and test it for pH. This sample can be submitted to a soil testing lab or you can use a field pH test kit for this purpose. If this sample has a pH less than 6.2, then apply 1 ton of limestone even if the regular sample soil test did not call for any limestone.
- Avoid atypical areas or sample them separately
Sample between the rows and avoid any fertilizer bands as much as possible. Also, many fields have known atypical areas such as dead furrows, old fence rows, lime or manure stacking areas, wet spots, etc. If the areas are too small to manage separately, do not sample them. Taking one or two cores from these odd areas just contaminates the sample for the rest of the field. If the areas are large enough that you are able and willing to manage them separately, then take a separate sample from these areas.
- Handle the sample carefully
Collect the soil cores in a clean bucket so as not to contaminate it, crumble the sample cores and air dry the sample. Mix the cores thoroughly and take a subsample to fill the mailer to send to the lab.
- Fill out the soil test information sheet
Additional information such as the crop, the expected yield, the crop rotation, tillage depth, etc. are used along with the soil test results to make the recommendation. Thus, it is critical that the soil test information sheet be filled out completely and accurately.
Don’t Guess. . . Soil Test!