Digging in the Dirt: Undue emphasis

This river of green in an otherwise phosphorus-deficient stand of creeping bentgrass was caused by a few drips of a phosphorus solution coming off a leaky spray boom. The subsequent irrigation washed the drips across the plot and the grass greened up, showing not much phosphorus is needed for the grass to respond. While creeping bentgrass tends to turn purple when phosphorus is low, annual bluegrass tends to turn yellow. Photo courtesy Doug Soldat

By Doug Soldat, PhD

Without understanding all the limitations covered in this article, it is easy to see how one could get carried away by attempting to find the ‘ideal’ level of every nutrient in the soil. One common over-interpretation is when soil test reports recommend balancing the soil cations using the base cation saturation ratio (BCSR). These use the same methods as previously described, but recommend the soil cations (i.e. calcium, magnesium, and potassium) are balanced in an ‘ideal’ ratio. Unfortunately, after years of research, there is still no evidence this approach works—the only thing known for certain is someone following this approach ends up spending a lot more money.

To avoid over-interpretation or relying solely on the laboratory’s (or consultant’s) interpretations of the soil-testing results, this author recommends results be compared with Pace Turf’s Minimum Level for Sustainable Nutrition (MLSN) Guidelines. Instead of drawing interpretations from a single study, these minimum levels are based on thousands of soil-testing results where the turf was deemed to be performing average or above average (all soil samples from poor performing turf were thrown out). The ‘minimum level’ was set at the lower one-third of the data set, meaning about 33 per cent of the soil samples with good turf had levels (e.g. for potassium or phosphorus) below that minimum.

Read the full article: Digging in the Dirt: The reliability of soil testing

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