Cultivation practices are a common practice in most athletic field maintenance operations. Primarily thatch accumulation and soil compaction are the problems trying to be remedied.
Thatch Management
Thatch is the intermingled layer of living and dead stems, roots, rhizomes, and stolons that develops between the live green vegetation of your turf and the soil surface.
The thatch layer is composed of plant parts at various stages of decay by soil microorganisms. Grass clippings are not considered part of thatch because they are 85% water and easily decomposed by soil microorganisms.
Grasses that spread by rhizomes (underground plant stems that form new plants at nodes) and stolons (above ground horizontally growing plant stems) are prone to forming thatch layers.
The turfgrasses used in the southern athletic fields (bermuda, zoysia, texas bluegrass) are perennial grasses that spread in this manner and, therefore, form thatch.
Having a moderate thatch layer (between ¼ and ½ inch thickness) is beneficial as it makes a turf more resisitant to sports traffic, decreases compaction, protects grass crowns (growing points) from temperature extremes, and acts as a "mulch" to decrease water loss from the soil surface.
Thatch decay is also beneficial as it releases nutrients to the turf environment. Having an excessive thatch layer of greater than ½ inch, though, can lead to serious problems.
Grass species vary in their rate of thatch production and some new grass varieties have especially vigorous growth characteristics that lead to quick thatch accumulation.
Why Does Thatch Accumulate?
Thatch accumulation occurs when the production of plant parts (stems, rhizomes, stolons) is faster than the rate of thatch decomposition by the soil microorganisms. There is a balance between thatch production and decomposition that can be achieved through proper field maintenance techniques.
Thatch production is greatly affected by maintenance practices such as fertilization and mowing. Excessive fertilization (especially using a soluble nitrogen source such as urea or ammonium sulfate) leads to quick production of plant parts that adds to thatch.
Follow the fertility and mowing recommendations of this site for proper management. Thatch decomposition is greatly affected by soil microorganism health and activity and can be aided by cultural practices.
The factors influencing soil microorganism activity are soil pH and adequate, but not excessive soil moisture. Soil pH that is too acidic (below 6) or too basic (above 9) reduces microorganism activity. Soil microbes are aerobic and soil that is too wet from poor drainage or excessive irrigation leads to microorganism death from anaerobic (without oxygen) conditions.
The key is to create a good environment for soil microorganism activity, have high quality and adequate growth from fertility recommendations, and employ some mechanical practices to remove excessive thatch and improve soil drainage if and when they occur.
Thatch Problems
Problems include reduced water, nutrient and air infiltration into the soil (which leads to reduced rooting and decreased drought resistance), and increased disease and insect damage. A field with an excessive thatch layer declines in quality and becomes much harder to manage.
The turf becomes less responsive to fertilizer applications and much more water will be required to wet the soil profile through a thick thatch layer.
How to Check for Thatch?
A field that has too much thatch will feel spongy and tend to scalp even when mowed several times per week. To estimate thatch thickness, use a soil probe, spade, or knife to remove a small section of sod. Measure the thatch layer with a ruler in a couple of places around the field.
If the average thatch thickness is greater than ½ inch, plan to use mechanical dethatching practices in the near future and get on a topdressing schedule.
How to Control Thatch
Thatch control is a preventive and curative process. Thatch prevention can be achieved through proper fertilization, frequent mowing, and frequent light topdressing of about 1/8 inch every two week during the growing season. Avoid applying nitrogen at a rate exceeding 1 pound per thousand square feet especially with a nitrogen source that is soluble.
Maintain adequate, but not excessive soil moisture to encourage thatch decomposition (deep and infrequent watering).
Avoid using compost or other organics for topdressing - your topdressing soil should resemble the rootzone mix or be straight sand. The addition of highly organic materials adds to the already decomposing organic thatch layer and can increase thatch problems.
Curative practices like dethatching and core aerification are designed to remove excess thatch that has built up in the field.
The best time for thatch removal is in the early spring, although many people core aerify several times a year. If using a vertical mowing or power rake unit be sure that the blades penetrate through the thatch layer to the soil surface.
Vertical mowing should be done in two or three directions. Core aerification is the process of pulling soil plugs (2 to 3 inches long and ½ to ¾ inch in diameter) out of the ground.
Ideally you want to get 8 or 9 plugs removed per square foot to significantly enhance water and air movement into the soil.
This may require making several passes over the field. Plugs should be removed, but can be left on the ground.
Various other products on the market that claim to control thatch through addition of bacteria or enzymes to the soil have not been proven through research and are not recommended for use.
Reducing Soil Compaction
Soil compaction occurs in soils high in clay content that receive traffic and sodic soils that have excessive amounts of the sodium ion. Compaction is enhanced in areas of high to excessive soil moisture that receives traffic. When this occurs the soil loses structure and forms an impenetrable layer on the surface to water and air movement.
It is much like the phenomenon of highway rollers wetting a soil immediately prior to rolling over it to get the highest degree of compaction and stability possible.
If soils get compacted and the infiltration of water and air into the soil is impeded, the field will certainly begin to decline. Very hard, empty spots will develop in those area of compaction and "compaction" resistant weeds will probably be the only plants that can survive.
Goosegrass and prostrate Knotweed are the best examples of compaction resistant plants and are good indicators of high soil compaction.
How to Control Soil Compaction
Soil compaction develops mostly in high traffic areas, but is greatly enhanced by excessive soil moisture. That is one reason why we preach a deep and infrequent watering schedule here. In some areas, compaction cannot be avoided and steps must be taken to periodically alleviate the problems.
Core aerification, where you remove soil cores (at least 8 to 9 per square foot), is the best method for alleviating soil compaction.
Sodic Soils
When sodium builds up to high levels in clay soils (this does not occur in sand based soils) - the clay loses its structure and the soil becomes rock hard. Sodium comes mostly from irrigation water and buildup occurs when irrigation water evaporates and leaves sodium and other ions behind on the soil surface.
Sodium can be removed from the soil by the addition of Gypsum and deep, infrequent irrigation. Gypsum replaces sodium ions with calcium ions and the sodium ions are free to leach through the soil profile. The clay regains its structure and the soil loosens up - letting water and air infiltration occur again.
Gypsum application of 40 to 90 pounds per thousand is recommended as needed and this combined with core aerification is the best treatment we now of.
