Best judgment of the agronomist

Here are direct quotes from books and articles over the past 50 years. Do you notice a trend here? Actually, trend might not be the correct word. I’ll rephrase it like this: do you notice the persistence of an unsolved problem?

Actually, an excellent solution exists. Click to view the spoiler

The MLSN method of soil test interpretation solves many of these problems in one fell swoop. MLSN is based on data from and for turfgrass, MLSN is especially applicable for sand rootzones, and fertilizer recommendations made using MLSN account for the differences between species, among many other advantages.

A 50 year timeline

Few detailed correlation studies have been conducted concerning turfgrass responses to fertilization with various levels of plant nutrients on soils with various inherent fertility levels. Most soil test recommendations for turfgrasses are based on soil test correlation studies conducted with field crops such as corn and certain pasture grasses. Utilizing these correlations as a base, modifications have arbitrarily been made in applying them to turfgrass conditions. It is unfortunate that adequate correlation studies have not been conducted for turfs. Hopefully this problem will be corrected in the future.

— James B. Beard, 1973, in Turfgrass: science and culture, page 442

Unfortunately, turfgrass recommendations appear to be based on research done with other crops, such as forages, results from turfgrass fertility studies not designed to relate to soil testing, and the best judgement [sic]of the agronomist making the recommendations.

— Turner and Waddington, 1978, in Survey of soil testing programs for turfgrasses

Although critical soil K levels must certainly exist for turfgrass establishment, little research has been conducted to determine or document these levels.

— Turner and Hummel, 1992, in Nutritional requirements and fertilization, page 407 of Turfgrass, edited by Waddington, Carrow, and Shearman

Turfgrass sufficiency levels have generally been derived from other crops and adjusted over the years by turfgrass scientists.

— Robert N. Carrow, 1995, in Soil testing for fertilizer recommendations

Unfortunately, very little research has been conducted on turfgrasses to provide the necessary information required for precise interpretation. Thus, interpretation may be based on research with forage or other crops, or in some cases, no research at all … In some cases, turfgrasses have been placed in a “high” P and K requirement category, while pasture grasses were in a “low” category. This decision was based on economics, not agronomics. The cost of fertilization was not considered of primary importance for turf.

— Carrow, Waddington, and Rieke, 2001, in Soil Fertility and Chemical Problems, pages 160–164

Traditionally, ranges for various nutrients are based on the past 60 years of fertility studies, particularly on forages, agronomic and horticultural crops, with adjustments made to fit perennial turfgrasses based on studies and the judgment of experienced university turfgrass scientists.

— Carrow, Stowell, Gelernter, Davis, Duncan, and Skorulski, 2004, in Clarifying soil testing: III. SLAN sufficiency ranges and recommendations.

Relationships between extracted soil potassium, potassium fertilization rates, and turfgrass response needs additional study. Such work is especially missing for the sand-based systems in which many turfgrasses are managed … [and] such research is highly dependent on extractant strength, soil type, and geographic region, making extrapolation of results from region to region less useful. … Soil-testing methods and fertilizer recommendations for phosphorus have long been evaluated for field and pasture crops, but because turf growth and response tends to be a regional issue related to specifics such as crop, soil type, and soil extractant, there are limited studies as related to extractable soil phosphorus, especially in high-sand greens. … Calibration of [soil test phosphorus] with turfgrass growth and subsequent phosphorus fertilizer recommendations is scant, and additional data is needed. Many current recommendations for phosphorus fertilizer for turfgrasses are based on forage- or field-crop calibration data … .

— Frank and Guertal, 2013, in Potassium and phosphorus research in turfgrass, chapter 14 of Turfgrass: Biology, Use, and Management, edited by Stier, Horgan, and Bonos.

In many cases, we are missing key calibration data for turfgrasses. This is true for newer turfgrass species of interest, such as seashore paspalum, or for commonly utilized turfgrass species that may be managed differently by geographic region, such as annual bluegrass.

— Thompson, Guertal, McGroary, Soldat, and Hopkins, 2023, in Considerations with soil testing in turfgrass, a chapter from Achieving Sustainable Turfgrass Management, edited by Michael A. Fidanza

Now for some good news

The MLSN guidelines address almost all these issues. For example, see:

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