COURSE CARE
Interpreting Soil Test Results: Test Your Skills! February 27, 2015

Interpreting Soil Test Results: Test Your Skills!

By Brian Whitlark, Agronomist
January 27, 2009

Of all the tasks superintendents must complete on a daily basis and the projects they must attend to, taking time to read and interpret soil test results is not frequently high on the "to do" list. The mere topic of interpreting soil tests seems to make some superintendents squeamish due to their lack of familiarity with these reports. The skill set required to properly interpret soil reports is simply not used on a routine basis and must be practiced to maintain a certain level of proficiency.

This email update will be the first of several practice sessions that should prove useful when interpreting your own soil tests. The format will be the same for each update with soil and/or water data preceded by a brief description of the area tested and any special challenges to growing healthy turfgrass. This section will be followed by a question and answer session. It will best serve you to read the questions first and formulate your own answers before peeking at the solution. It should be noted that there is never one simple answer to soil and water issues, and if you should develop an alternative solution and would like to share, please contact the Southwest Region office.

For additional guidance when interpreting soil reports, review the following resources:

  • Textbook: Turfgrass Soil fertility and Chemical Problems - Assessment and Management. Carrow, Waddington and Rieke, 2001.
  • Clarifying Soil Testing: III SLAN sufficiency ranges and recommendations. Carrow et al., Golf Course Management , January, 2004: 194-198.
  • Textbook: Salt-Affected Turfgrass Sites - Assessment and Management . Carrow and Duncan, 1998.
  • Soil Fertility and Turfgrass Nutrition 101. James Baird, Green Section Record, Sept-Oct, 2007. /content/dam/usga/pdf/imported/070901.pdf

Section 1: Description of turfgrass area sampled and special challenges.

The data from the soil report shown below was collected from the 12 th fairway on a golf course located 25 miles south of Phoenix, AZ. The soil consists of a clay loam texture that is extremely hard when dry, but can be easily penetrated with a soil probe when wet. Historically, this fairway had been designated as cart path only due to lack of turfgrass coverage. The quality of the overseeded ryegrass in this fairway was considered poor when the samples were collected in April 2005. Although the golf course is more than 20 years old, this fairway still seems to suffer from compaction issues caused by vehicle traffic during construction.

Section 2: Soil Test Data

Section 3: Questions and Answers

Question: What nitrogen (N), phosporus (P), and potassium (K) fertilizer applications might you make for this perennial ryegrass fairway?

Answer: Soil tests for nitrate are typically NOT a good indicator of plant available N, therefore, it is always best to use visual observations of color, turfgrass vigor, and clippings to base N applications. Both P and K are very high in this soil and should not be applied.

Question: What secondary macronutrients (Ca, Mg, S) or micronutrients (Fe, Mn, Zn, B or Cu) might you apply, if any (focus your recommendations on fertility for this question)?

Answer: It is very rare that Ca, Mg and S are deficient in turfgrass systems, especially in the Southwest. None are needed for fertility purposes in this fairway. All micronutrients are high or very high, therefore none need be applied if turf color is acceptable. A spray application of Fe may be useful to enhance ryegrass color if desired. Notice the B levels are listed as "High" at nearly 4ppm. Kentucky bluegrass is more sensitive to B toxicity than other grasses with a tolerance level between 2-4 ppm. However, most grasses will tolerate soil B levels up to 6-10 ppm and therefore is not a major concern in this scenario.

Question: Shifting gears to salinity and sodium issues, what indicators might you use to highlight as the primary problems in this soil?

Answer: Total salt content as expressed by the electrical conductivity (EC) is high at 4.6 mmhos/cm (dS/m) as well as the exchangeable sodium percentage (ESP) at 20.5%, which classifies this soil as saline-sodic. Sodium is high at 1,800 ppm, which equates to an EC of 2.8 dS/m (1,800 ÷ 640). This level of sodium may be toxic to salt-sensitive plants such as Poa annua , but not perennial ryegrass. The threshhold EC, above which perennial ryegrass begins to decline, is 6.5 dS/m, thus the EC level in this soil is not at harmful levels yet, but is high enough to warrant salinity reduction strategies such as aggressive aerification, slicing, and leaching. However, the elevated ESP has resulted in soil dispersion and has made leaching difficult. A soluble source of Ca should be applied to improve soil flocculation and lower Na levels. The best and most economical source of Ca is gypsum for this scenario. In order to replace all the Na on the exchange complex (78 meq/l) approximately seven tons of gypsum per acre is required (if you are interested in the math that led to this figure, please contact the SW Region office). Typical gypsum rates range from 1/2 to 1 ton/acre and can be applied on a semiannual or quarterly basis, therefore it will take several years to apply enough gypsum to offset the Na in this soil.

Question: Are there any other factors that stand out as potential issues on this soil report?

Answer: You may have noticed the sulfate level in this soil is very high at 1,500 ppm. Although sulfates themselves are not harmful , their elevated presence in this soil is a good indicator that water movement below the rootzone is minimal. Why? Because sulfates are extremely soluble and will move with water. This further stresses the need for calcium additions to improve soil flocculation and ultimately drainage.

Please contact me at bwhitlark@usga.org if you should have any questions or simply would like to discuss soil test results.

Brian Whitlark is an agronomist with the Southwest Region of the USGA Green Section located in the Phoenix, Arizona.