When soil sample location or depth change, so can the soil test data. Often when calls come into the lab about inconsistent soil test values overtime, the responding agronomist will look at the impact of sample location and depth first. If the soil sample is collected in a consistent manner including same depth, same time of year, same location/pattern, following the same crop, the values soil test values should be relatively stable over time. There also will be a gradual trend upwards or downwards depending on the nutrient management focus. If the value of the soil organic matter (SOM), phosphorus, or CEC change dramatically, i.e. 30-50% in 2-4 years, further investigation is in order.
Calcium and magnesium are relatively consistent with soil depth. If calcium and magnesium change by more than 20-25% then it is likely that the sample location has changed, or an aggressive application of lime or gypsum has been applied. Since the primary values used in calculating CEC are calcium and magnesium, the CEC should be relatively stable if the sample is collected in the same location.
If the CEC is stable, but the SOM changes by 0.5% or more in soils with SOM levels below 4-5% then most likely the sample depth has changed. Most of the organic matter is near the soil surface and shallow samples concentrate SOM in the sample leading to a higher relative higher SOM. The same is true for phosphorus to a lesser degree. Soil test phosphorus can be more variable naturally in the soil by small changes in locations that may not be indicated by the calcium and magnesium values.
If the samples are sampled using GIS location, be sure to compare individual sample points and not the field averages. Field average can skew what is taking place with individual sample points if all the sampling points are not impacted the same.