Soil testing takes center stage

Golf Course News, Jul 2005 by Wolff, David

ENVIRONMENTAL ISSUES INCREASE AWARENESS

Soil testing is a tool used to assess the status of soil fertility and the potential for nutrient imbalances in turfgrass. While soil testing has been around for decades, it's becoming more important because of mandatory nutrient management programs.

Mandatory soil-testing programs have been established in some states for certain agriculture production and land application of organic wastes. They also are being applied to golf courses. These programs were prompted by excess levels of phosphorus, nitrogen and pesticides found in soils that have shown to be detrimental to the environment, particularly rivers and lakes. These environmental soil-testing issues will receive even more attention as public awareness and research data increase.

Healthy turf is key

The key to producing healthy turfgrass is to reduce stress on the plant. Physical stress includes soil layering, wet soil, compaction, poor aeration, poor drainage, or soils with high clay or stone content. Chemical stress might include low fertility, excess nutrients (imbalance/toxicity), salts, high or low pH levels and pesticides. Biological stress includes insects, viruses, wilts and blights, nematodes, diseases, susceptible varieties, grass types and animals. Finally, there are weather issues such as temperature extremes, precipitation and light.

Weather and physical factors are always at the top of the plant stress list. Fortunately, superintendents can control nutrient fertility more easily than many of the other stress types. A superintendent's goal should always be to focus on the stresses within his control and be aware of those he can't control. Soil testing alone can be risky when assessing turf nutrient efficiency. Plant tissue analysis used in conjunction with soil testing is one way to verify efficient nutrient management.

Soil tests differ

University researchers have developed various soil-testing methods to provide information about soil nutrient availability. Some soil-testing methods are developed and calibrated to be used on soils with certain properties. For example, there are about six phosphorus soil tests (Mehlich III, Bray I, Olsen, Mehlich I, Morgan and AB-DTPA) performed by laboratories in the United States.

Each of these phosphorus tests has its own unique chemistry, strengths and weaknesses. The strong acid Mehlich I test is used mainly in the Southeast because of the more acid soils in the region. Labs mainly in the Northeast perform the acid Morgan test because that's where it was developed. The Olsen and AB-DTPA tests are recommended in higher pH soils in which their unique chemistry makes them more accurate. The Olsen and AB-DTPA tests also are used extensively for soils in the central and western United States where more alkaline soils are found.

Mark Flock, a laboratory director and agronomist for Brookside Laboratory in New Knoxville, Ohio, is president of the North American Soil and Plant Testing Council and is a member of the USGA Putting Greens Materials Testing Technical Advisory Committee. Flock says the acid Bray I phosphorus test has been a favorite test for many areas.

"It has been shown to be accurate on soils with pH levels below 7-2," he says. "In alkaline soils, this test falls apart and might produce erroneous low phosphorus levels."

The acid Mehlich III test is performed on more soils than any other phosphorus test. Like the Bray I test, it's accurate on all acid soils and on soils with higher pH levels.

"If you perform each of these six tests on one sample, you would get six distinctly different levels of available phosphorus in the plant," Flock says. "Which one is right? Actually, all six could be right. It's important to understand each of these methods has different chemistries. But each has been calibrated, and the calibration scale is different for each test. It's like comparing Fahrenheit and Celsius scales. Both are good, accurate measurements of temperature, but the levels are different for equivalent temperatures. This is why it's critical superintendents work with a laboratory, consultant, field representative or extension agent that understands the testing methods."

Proper interpretation of results can't be made unless the superintendent knows which test was performed and has the calibration data to follow.

"Stick with a good laboratory that is active in proficiency programs and stays active with regional testing meetings," Flock says. "Consistency in sampling, testing methods and reporting of results will make your job much easier. Sending samples to different labs that might be performing different test methods can be confusing. Some laboratories get blamed for poor testing when, in fact, the testing is good, but because the superintendent might be comparing results of different methods, he's comparing apples and oranges."

Most laboratories use tests developed in their region. Universities publish bulletins listing the recommended chemical soil test procedures for different regions of the country. This works fine for agriculture soil testing, but for turf testing, even using local soil testing methods can be challenging because constructed soils on golf courses could be considerably different than natural native soils. Many times golf course tees and greens are constructed with materials such as calcareous sands that might require special testing. Whether a USGA-specified or California green is constructed, if calcareous sand is used, there will be a soil with a pH near 8.0. The local soil test method calibrated for acid soils might not be appropriate for these soils.