What does top dressing mean? Farmers interpret top dressing to mean the surface application of fertiliser (usually nitrogen) to a growing crop. In the turf grass industry the term top dressing is used almost exclusively to describe the surface application to turf of sand, soil, organic material or any combination of these. The key point is that top dressings add to and become part of the rootzone. Thus over a period of years top dressings will alter or sustain the physical and to some extent biological properties of rootzones.

Why are top dressings necessary? Long term maintenance of high quality fine turf on golf and bowling greens is difficult. It is not only a matter of growing good grass, although that is an important aspect. The word "turf" embraces not just "grass" but also the top few centimetres of rootzone which determine several aspects of the playing quality of a green. Trueness, holding character and response to rain or irrigation inputs are controlled or substantially affected by the rootzone surface. It is in order to control properties affecting playing quality as well as turf grass performance that top dressings are used.

Trueness Even moderate usage can cause small holes and undulations to develop which can be corrected by careful top dressing and luting in. It is impossible to create a perfectly smooth surface after turfing but top dressing provides the means to correct the problem. Shallow hollows that may extend over one or more metres on bowling greens can be levelled over several years by selective top dressing.

Thatch and general organic matter control Surface organic matter will accumulate in any fine turf when earthworms, the natural agents incorporating plant residues, are controlled. The rate of accumulation depends upon factors which affect the rates of plant residue addition and decomposition but a build-up cannot be prevented without both active removal of plant residues by scarification and verticutting together with top dressing to "dilute" organic matter at the surface. Top dressing immediately after verticutting and scarification, which prune turf and remove plant material, encourages new root development and tillering. The development of a significant depth of visually evident thatch must be avoided and it is much easier to avoid its development than to remove a thatch layer once developed.

Permeability of the surface Intensive usage of turf usually leads to a degree of scaling of the surface against water infiltration. Spiking and slitting break the seal and improve infiltration temporarily but a top dressing applied afterwards and luted in, helps protect vertical channels and extend the period of benefit.

Composition and physical properties of top dressing Top dressings become part of the rootzone so logically they should have composition and properties similar to specifications for rootzone materials. For most golf and bowling greens this means a particle size composition dominant in medium sand with no more than around 10% in particles less than 125 microns. Organic matter content should not be more than around 2.5% bearing in mind use to control rootzone organic matter content. Despite these general points it is important to use a material consistent with existing (good) rootzone. So, for example, on links courses where the green rootzones are comprised of native dune sand which has a particle size around 180-210 microns top dressings dominant in particles of this size should be used.In all cases the sand in top dressing should be of uniform size, that is with a small gradation index. Cheap top dressings are often comprised of sands with a wide range of particle sizes even when the amount of fine particles is small. When compressed they have a smaller total pore space than uniform sands and are therefore inferior.The pH of top dressings is a poor guide to their likely effect on rootzone pH. The most useful criterion is content of free lime (usually shell) which dissolves over time. A content of about 0.5% is the maximum advisable if you want to maintain an acidic rootzone.Obviously if you are trying to use a consistent top dressing formulation this is not possible if the supplier's material is inconsistent. The supplier used must have in place a quality control testing system to guarantee specifications and tolerances. Lack of quality control may be another reason for cheapness.

Timing and rates of top dressing Typical rates of top dressing per annum are in the range of 2 - 5 kg/m2. Single applications of around 3 kg/m2 or more are difficult to integrate with the rootzone. If the top dressing is left as a distinct layer sandwiching plant material or thatch between it and the underlying mineral rootzone it will cause problems. When this organic sandwich humifies and becomes buried by subsequent top dressings it creates a horizontal discontinuity which will adversely affect rooting depth and water infiltration. It is much easier to integrate small amounts of top dressing and it has become common practice to apply four to six top dressings of 0.5 - 1.0 kg/m2 during the growing season. Such small amounts can be applied quickly and brushed in causing no significant disruption of play. Top dressings should be applied when the grass is growing rather than dormant and scarification (at least) should be carried out immediately beforehand to allow the top dressing to be brushed or luted in, to come into contact with the rootzone surface. Top dressings are an integral part of green maintenance. They serve several functions in sustaining turf quality. Furthermore they have been used successfully by many greenkeepers to build up over the years a depth of high quality rootzone onto inferior material used in the original construction.

Prof. Bill Adams
The University of Wales, Aberystwyth, UK

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