Soil Test Variability: Why Does It Occur And What To Do About It?

While there is general agreement that soil testing is useful to determine phosphorus (P) and potassium (K) levels, it is not perfect; natural processes and management practices can make it difficult to translate test results into fertilizer recommendations. Following are a resaons why variability occurs and suggestions on how to respond.

Natural Processes

Stover Nutrient Content Study

Nutrient Release from Crop Residue

Unlike P, K is not tied up in organic forms in plants, and thus it can leach out with adequate moisture. For a corn crop of 180 bu/acre, it is not unreasonable to have 200 pounds of K2O per acre tied up in the residue.We observed in a study (Figure 1) that the amount of K leached from residue depends on rain amounts and time elapsed between harvest and sampling. Last fall, lower-than-expected values likely resulted from little rain before most samples were collected. To reduce this problem, it is best to always collect samples in similar moisture conditions and at approximately the same time in the fall.

Seasonal Variability

Soil K levels drop during the summer partly because under warm and wet conditions, some microorganisms can reduce iron oxidation state in some clays, which causes them to collapse and trap K, thus reducing K availability (Figure 2). Slight increase in the fall is probably the result of K release from crop residue. Noticeable increase during winter and spring is related to freeze-thaw or dry-wet cycles, which can release K from certain clays. Some consider spring sampling a better time while others prefer fall. While I am not discussing the pros and cons of either approach, it is best to always sample at the same time of the year and not to try to compare values for fall and spring samplings.

Fig 2. Seasonal variability in soil K levels.

Management and Sampling Strategy

Cropping System Impact Nutrient Removal

Changes in soil P and K levels are often predicted with a budget approach based on amounts of nutrient applied and removed. If more fertilizer is applied than removed, soil test levels are expected to increase; conversely, if less fertilizer is applied than removed, levels are expected to decline.While this approach conceptually makes sense, how quickly or slowly levels change depends on starting test levels, the amount of budgeted surplus or deficit, soil properties, environmental conditions, and cropping system. Sometimes we treat this budget as we would a checking account, when in reality we should view it more like the stock market, which is influenced by many factors. Understanding how the various factors influence the outcome is essential to avoid becoming overly concerned when the final balance is not what was expected.

Localized Nutrient Placement and In-field Variability

Intensive tillage mixes soil and creates uniform P and K concentrations within the 7-inch sampling depth.When soil is not intensively mixed, broadcast P and K applications create a gradient, with the highest concentrations in the soil surface (Figure 3, top graphs). During sampling, a shallow sample in a stratified system results in higher-than-actual test levels, while going deeper or dropping a portion of the surface results in lower-than-actual levels. Also, applying P and K in subsurface bands creates areas of high levels at the location of the band and does not fertilize the rest of the soil (Figure 3, bottom graphs). Sampling these fields correctly can be challenging.

The best way to reduce in-field variability problems is with georeferencing to allow successive sampling in the same location. Also, variability within a sampling zone increases when fewer cores are taken. Thus, it is better to take fewer samples of more cores than more samples of fewer cores.

Variability Between Labs

It is normal to have some variability within the same lab.When samples are sent to different labs the variability normally increases substantially. The best approach is to always send samples to the same certified lab. Finally, remember that the values you get from the lab are not absolute; rather, you should consider them estimates used to track trends in fertility over time. If you are doing things right but your one-year testing results are odd, keep in mind that fertility does not change drastically overnight.

Fig 3.
Fabián Fernández

Fabián Fernández
Assistant Professor
Soil Fertility and Plant Nutrition