FEATURED ARTICLE: Black Layer In Turfgrass Soils
October 12th, 2007
by Dr. Eric A. Brown
Turfgrass Doctors Research and Consulting
Black layer in turfgrass soils seems to be one of those mystifying phenomena that can creep up and cause havoc on an otherwise status-quo agronomic system. The truth is, if you get down to the molecular and biological foundation of the problem, black layer can be mitigated and corrected through an understanding of what the soil and plant system needs.
There are two basic types of cell metabolism: those that use oxygen (aerobic) and those that do not (anaerobic). Aerobic metabolism consumes oxygen for cellular respiration, releasing carbon-dioxide and water. Oxygen is used as a terminal electron acceptor and is converted into water.
Redox (shorthand for reduction/oxidation reaction) reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. There are a number of different electron acceptors, both organic and inorganic. If oxygen is available, it is invariably used as the terminal electron acceptor because it generates the greatest Gibbs free energy change and produces the most energy.
Black Layer In Turfgrass Soils Indicates Oxygen Is In Short Supply To Plants And Microorganisms
In anaerobic environments, the absence of oxygen requires other electron acceptors to be used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules. These alternate electron acceptors are reduced the same way oxygen is reduced to water when oxygen is present.
Probes have been developed to measure the redox potential (Eh) of soils. A restriction on the resupply of oxygen to soils will lower redox potential. Soils that stay wet, such as those in wetlands, will have low redox potential (-300 to 300 mV). Healthy upland soils will have high redox potential (300 to 700 mV) because oxygen should not be in short supply (Ponnamperuma, 1972). Soils with black layer will have reduced redox potentials compared to a soil with good tilth.
When little or no oxygen is available, oxygen consuming roots and microorganisms may die. If cells in a human body are deprived of oxygen they can be damaged and may die, the same is true for oxygen metabolizing microorganisms in the soil. Unlike the human body, soils that have depleted oxygen will become dominated by microorganisms that can survive without oxygen. Microorganisms that can survive without oxygen (called anaerobes) are the cause of black layer. Turf root cells must have oxygen for cellular respiration and so will decline in a black layer environment.
Black layer in soils has been incorrectly blamed on such things as applying too much organic fertilizer, compost, and/or sulfur. Black layer can smell bad because transformation of oxidized molecules to its reduced form (after being an electron acceptor) can result in molecules that have foul odors. When sulfate is used as an electron acceptor the reduced form can ultimately be hydrogen sulfide. When oxygen is the electron acceptor, water is released as the reduced form of oxygen.
Organic matter will build up in anaerobic systems because the breakdown of organic molecules is not as complete or efficient compared to oxygen rich systems. This incomplete breakdown of organic matter in anaerobic conditions further promotes black layer by blocking drainage and gas exchange.
When Turf Suffers From Black Layer, Your Unhealthy Soil System Is Caused By An Oxygen Deficiency
In a healthy soil system, about 25% of the space is air pore space, 25% is water pore space and 50% is soil particles. As plant roots and aerobic microorganisms use up oxygen through respiration, fresh oxygen must diffuse down into the soil from the above ground atmosphere. For this reason, the soil air has less available oxygen than atmospheric air.
If you encounter black layer in turfgrass soils, oxygen is not getting down into the soil air pore space fast enough. If water is occupying most or all of the pore space, available oxygen will decline and black layer conditions become possible. Poor drainage and over watering are causes of black layer because oxygen diffuses through water 10,000 times slower than through air.
What You Can Do To Fix Your Black Layer Problem
Corrective measures for black layer in the soil include aerating the soil to remove barriers for oxygen and carbon-dioxide diffusion. Increasing drainage and monitoring water applied can reduce the amount of water in the soil pore space allowing more space for gaseous oxygen.
In addition to physical aeration, correcting drainage problems, and monitoring water content of the soil profile, there are other tools that can be used to lessen the problems associated with low levels of oxygen in soils. New technology has brought us products that can supply oxygen to oxygen depleted soil systems. Agriox™ is a proprietary slow release soil oxygen product developed and manufactured by Geoponics, LLC.
Black layer in turfgrass soils is a result of the soil-plant system not being able to cycle nutrients and oxygen like a healthy agronomic system is designed. Understanding the fundamental agronomic causes of black layer will help turf managers use the tools and methods necessary to mitigate and even avoid plant and soil conditions that do not promote healthy turfgrass.
Eric A. Brown, Ph.D.
Turfgrass Agronomist
Citation:
Ponnamperuma, F.N. 1972. The chemistry of submerged soils. Advances in Agronomy. 24: 29-96.
Turfgrass Doctors: Pioneers in the Field of Turfgrass Consulting and Research
Offering unbiased, on-site scientific product evaluations, Turfgrass Doctors supports the turfgrass industry by helping golf course superintendents find products and solutions to fit specific situations. Their services range from common consulting issues like weed control and fertilizer types, to more complicated services such as product research and data analysis. For further information, visit their website, email Dr. Brown, or call 352-514-9233.
Before joining Turfgrass Doctors, Dr. Brown worked for Liquid Ag Systems, Inc. which specialized in supplying liquid fertilizers to the golf course market in South Florida. Dr. Brown has also been responsible for the maintenance of collegiate sports turf at the University of Florida and is an Adjunct Professor at Sante Fe Community College in the Department of Natural Sciences. He attended graduate school at the University of Florida receiving a M.S. and Ph.D. in Soil and Water Science. His research focused on phosphorus retention in USGA putting greens, and mass balance of 15N-labeled nitrogen sources to turfgrass. He received a B.S. in Business Administration (Marketing) from Auburn University in 1989.
Entry Filed under: Stabilized Chemical Aeration,Turfgrass Care & Maintenance
Leave a Comment
You must be logged in to post a comment.
Trackback this post | Subscribe to the comments via RSS Feed