Tag Archives: Doug Soldat

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

Digging in the Dirt: Lack of definite correlation

Phosphorus deficiency of creeping bentgrass on a high-pH sand-based root zone. Deficiency symptoms disappear above 5 parts per million (ppm) Mehlich-3 extractable soil phosphorus. Image courtesy Bill Kreuser


By Doug Soldat, PhD

Soil test reports generally do not offer a user-friendly experience. In fact, most people understandably skip the details and decimal points, heading straight to the section that says either ‘low,’ ‘optimum,’ or ‘excessive.’ The often-overlooked question, however, asks how these assessment categories were developed. Soil test data are specific to a crop and soil type, meaning the ‘optimum’ number for corn on a Batavia silt loam will be different from that on a Miami silt loam. At the same time, the ‘optimum’ level for soybeans on a Batavia silt loam will differ from that of corn on the same soil. This means several studies need to be run for each crop and soil type to have reliable data.

Much of this work has been done in agriculture because of the economic significance of food production. However, soil testing research for turfgrass is hard to find. The little work that has been done is only specific for a particular grass species (or even variety), and the soil type in which it was growing.

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

Digging in the Dirt: Imperfect methods

Tools of the trade: while t-probes (right) are useful for taking samples for nutrient analysis, a profile probe (left) can be useful for identifying layers and other physical problems. Photo courtesy Doug Soldat


By Doug Soldat, PhD

Soil testing is a relatively straightforward practice. Soil is collected and sent to a laboratory. The laboratory dries and grinds the soil, then takes a pinch and adds 15 to 20 mL (0.5 to 0.7 oz) of a chemical extractant and shakes the soil/liquid solution for a few minutes. Next, the solution is poured through a filter and the clear solution is analyzed for the nutrients in the soil. The chemical extractant is usually an acid (pH < 7) combined with salt, or a base (pH > 7) with salt. The acid (or base) is used to extract the plant-available phosphorus, while the salt is used to measure the exchangeable cations (i.e. positively charged ions) such as potassium, calcium, and magnesium.

For acidic soils, commonly used extractants include Bray-1 and Mehlich-3. For high-pH soils, the Olsen extraction is a good choice. Often, soil-testing labs will use several different extractants on the same soil. For example, they may use Bray-1 for phosphorus and ammonium acetate for potassium and other cations. Laboratories usually use the tests most appropriate for the soils in their region, so if sending samples across the country, it is important to ensure the proper extractant is being used on the soils.

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

Digging in the Dirt: The reliability of soil testing

Soil testing can be a useful component of a fertility plan, but many things can go wrong. For example, taking the sample to a consistent depth is more important than one probably thinks. Photo © Bill Kreuser


By Doug Soldat, PhD

Most people have heard the phrase, “Don’t guess, soil test!” or have listened to a speaker tout this procedure’s importance before planning out a fertility program. The truth is a soil test is only as good as the method used and the data behind it. Unfortunately, the methods are not always appropriate and the data may be thin or non-existent. However, soil testing can be an effective part of a fertilizer program when properly used. This article covers the most common soil-testing mistakes and how to avoid them.

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