Soil Organic Matter on Sand Based Rootzones

Tim Butlerin Consultancy
Soil3.jpg High-sand-content athletic fields are being commonly used in modern sports pitch construction. The sand portion of the rootzone mix used for such fields varies greatly with up to 100 percent pure sand being used in some instances. The level of soil organic matter (SOM) to begin with in the rootzone will depend greatly upon the amount of sand that was used in the initial construction, with very low SOM levels found initially in almost pure sand rootzones.

SOM comprises organic materials in all stages of decomposition including roots, clippings, rhizones and stolons. Many plant constituents that get into the root zone contain high levels of materials such as cellulose and lignin which are difficult to breakdown by micro-organisms. Organic matter is composed of relatively stable material, termed humus, that is somewhat resistant to further rapid decomposition.

Organic matter contributes to the chemical and physical properties of a turfgrass rootzone. The build up of organic matter in sand based rootzones can cause a reduction in water infiltration due to an alteration in particle size distribution in the rootzone, and higher water holding capacity, while reducing O2 content within the surface zone (top 2 inches) and O2 diffusion across the surface zone. When rootzone O2 levels are low, root cells loose their ability to take up water and may die back. Other secondary problems could arise with organic matter accumulation such as increased disease activity, softer grass and root decline. Rootzones high in organic matter are often soft and waterlogged after rain. A heavily thatched turf would be an example of a rootzone with excessive organic matter.

SOM serves as a major reserve of plant nutrients, especially nitrogen, phosphorus, potassium and sulphur and, to a degree, may help stabilise rootzones and provide an anchor for turfgrass roots. Without a degree of SOM, the cation exchange capacity of the soil will be very low, and many nutrients, such as K+, may be leached out. Turfgrass rootzones that are low in organic matter are often droughty, hard, and compacted. In some situations, organic matter is added to sand based rootzones at the time of construction. Amendments commonly used include peat, rice hulls and saw dust. Fresh organic residues such as saw dust may tie-up plant nutrients and heat-up the soil for a stage, which is likely to cause management problems. Rootzones with poorly mixed fresh organic residues can lead to areas that get very hot, and turfgrass damage may occur.

USGA specification greens normally contain 1% to 3% (by weight) organic matter throughout the rootzone mix, however research has demonstrated that as organic matter content in a sand mix increases to above 4% to 5% (by weight), then the percentage of macropores of >0.08mm decreases due to plugging by organic matter (2,1,6). On sports pitches, the levels of organic matter are often much higher than on golf greens, however a careful eye should be kept on the organic matter to ensure that levels are not dramatically rising. I have talked to many sports field managers who have reported organic matter levels of around 5% or much higher, even when some cultivation practises were in place. This is not surprising on an athletic field, and many highly managed sand based fields may have higher levels

Management techniques include core cultivation, scarification and top dressing. If a cultivation programme is in use that includes top dressing, then it is important, if possible, to prevent layering and organic matter accumulation through regular top dressing. If top dressing is erratic, then some organic matter will accumulate between top dressing applications, leading to a layered rootzone with poor drainage capacity.
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