By Harry Brook
Fall is an excellent time to be testing your soil and planning for the 2021 growing season. It saves valuable time in the spring, can allow you to purchase fertilizer for 2021 when nitrogen costs tend to be lower, and, when done when soil temperatures are 5˚ to 7˚ C or lower, can be as accurate as spring sampling.
Soils are rarely uniform in composition in a field. There is often a lot of variability with topsoil depth, organic matter content, pH and nutrient content. With this in mind, the sampling technique used in soil sampling, can affect your results. And ultimately, your fertility decisions. Variability is most noted by topography in a field. Hill tops have thinner topsoil and hill bottoms have greater accumulation. Soil issues include salinity, seasonal flooding, blowout areas and acidic soils. Even flat, featureless fields can have significant soil differences.
Soil sampling can be done in several ways. One of the most common is to get a good, representative sample from the field. This entails taking small samples from many locations (15 to 20/quarter) to get a good, average soil sample. Avoid areas with obvious soil problems, like saline areas. Mix those samples together and take a sub-sample from that for the actual testing. Fertility recommendations are based on that average.
Another technique is called benchmarking. This is where an area is selected in the field that represents most of the field and multiple samples are taken from that site. The GPS co-ordinates are noted and sampling occurs at the same site every year. It doesn’t give a field average, but it does allow for the fertilizer program to adjust as nutrient levels change annually.
There is a third, much more expensive, sampling technique. It involves taking samples from a grid in the field. Many multiple samples can give you an idea of management zones to put in place. A less expensive, but still extensive sampling procedure, would be to benchmark sample the obvious zones in the field, based on topography. Hilltops would have a separate sample from mid-slope or low areas.
Sampling depth is another choice. Most often, a 0" to 6” sample is taken. The science behind this is that the majority of topsoil occurs in the top 6 inches and that contains most of the nutrients available in the soil. A 6” to 12” sample or even a 12” to 24” sample can complete the nutrient picture in conjunction with the 0" to 6” sample. In years with plentiful rain, nitrogen can get washed down into the soil and just testing the top 6 inches of soil gives an incomplete picture.
There are other variations on these three soil sampling techniques but they try to give you an idea of what nutrients are going to be available for the 2021 crop. Samples should be sent as quickly as possible to the lab. You want the soil test to occur as quickly after sampling as possible to ensure the test results are true to the field.
Once you have your sample, it’s time to get a test to see what is happening there and to plan for the future.