Top Dressings explained
By Peter Jefford, General Manager, Rufford Sports Surface Technology
A frustratingly unpredictable climate, economic pressures and increasing numbers of players all demanding perfect playing conditions seven days a week, 52 weeks of the year. It's all a far cry from the challenges faced by greenkeepers/groundsman 30 years ago and leaves today's successors nursing some major headaches.
Establishing consistency and quality in such a fast changing industry is the key to creating successful sports turf and that very much involves the need for testing.
The rootzone is the layer under the sward in which the grass roots live and breathe and this is really the starting point in the long-term success of any green or pitch. An ideal rootzone for healthy grass is natural, well structured sandy loam and if you were building a lawn at home then this would be the first choice. But then you wouldn't anticipate hundreds of golfers walking over your turf every day of the year (even in saturated winter conditions) or soccer players running and tackling on it. If they did then your new lawn would quickly turn into a mud bath!
Naturally occurring soil simply cannot provide the drainage (or, in the case of football and rugby, the stability) needed for a good commercial playing surface. This is basically because there is insufficient pore space between the particles in a compacted soil. Water drains through soil primarily via voids created by earthworms and particle aggregation. However, excessive foot traffic and play quickly compacts these voids and subsequently slows drainage right down leading to a waterlogged profile. Hence, to create a rootzone for commercial applications an addition is needed and this comes in the form of sand, the physical properties of which provide the drainage and stability usually lacking in natural soils. This is because the space between sand particles (provided you use a correctly graded sand) remains even when it is compacted, hence allowing drainage.
So, today the fundamental ingredient in rootzone materials for golf, soccer and other sports (excluding cricket) is sand. This is also true for the top dressing we apply to the rootzone, which I will discuss later. Considerable expertise is required to grow grass on the sand-dominated rootzones. We only have to visit the seaside to see the problems marram grass has establishing itself on the dunne sands to appreciate the problems greenkeepers face trying to establish fescues and bents on sand-dominated rootzones. The organic amendments used are mainly soil, peat, bark and composts. The USGA have recently revised their recommendations for golf to allow the use of zeolite and others for increased moisture and cation exchange capacity. Furthermore, the stress we put these grass species under is extreme and the expertise required to create and maintain a healthy growing environment is high. With players and the public now seeing the best playing surfaces in the world via TV coverage, their expectations are greater than ever,
What does top dressing mean?
A farmer would interpret top dressing to mean the surface application of fertiliser (usually nitrogen) to the growing crop. This has caused some interesting comments from farmers who have developed golf courses from their farm land and when they have applied top dressing, they have expected grass growth from what they think will be a nitrogen source. So, in sports turf 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. Therefore, 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 turf on golf, winter pitches 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 the surface. Trueness, holding character, stability 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.
Frequent applications of an appropriate top dressing will assist a greenkeeper greatly by providing the following benefits:
1.It helps maintain a smooth, true running surface which in turn improves mowing efficiency
2.In conjunction with scarification and verti-cutting, it will avoid excessive thatch build-up
3.Enables you to control organic matter content, water infiltration and aeration
4.It's essential in maintaining or increasing rootzone depth
5.Improves germination on over-seeded areas
6.Improves or sustains appropriate soil texture
We generally have two options when considering which top dressing to apply to our rootzones. If we are lucky enough to have a new construction design, made with appropriate sands and organic amendments with good drainage characteristics, our task is still not straightforward. We will still need a lot of care in choosing the top dressing to apply to it. Compatibility is the most important factor that should be understood here. If our rootzone is not of new design and is compacted or contains high clay content, we will want to improve this.
There's no way that a top dressing supplier can claim that they have a good product for you without first of all testing your greens. That's because there's no such thing as a universally 'good' top dressing - what might be right for one application could be completely wrong for another because applying a mismatched top dressing can cause major layering problems to the rootzone profile. A major risk is the creation of layers of different pore size systems within shallow depths of the rootzone. If this happens then water retention and air/water balance will be adversely affected and also rooting depth. The frightening fact is that this problem won't become apparent for several years, by which time it may be impossible to rectify through cultivation techniques. Remember, once a top dressing is applied (good or bad) it will stay there, in the profile and it will not wash through like a chemical or fertiliser. If bad, it will haunt you for years to come - So take extra care when selecting your top dressing.
Composition and physical properties of top dressing
The type of sand used in top dressings is vitally important and the user should be aware that most sand sales in the UK are for other uses. The sports turf market is a small user in comparison. So be careful if you are offered cheap materials, as these are often finer, differ in shape, colour, lime content and are more interpacking than the sands specified for sports turf. 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 therefore inferior. For golf, winter pitches and bowling greens the dominant particle range in the sand should be medium sand (0.250 to 0.5mm) size range. For winter pitches, these would include fine sand for stability, which would not normally be necessary for golf and bowls. Increasingly, advisors are now specifying from 0.125 to 0.710mm size range for golf, though the main criteria for the reader is that the majority fall within the medium sand range. The mix should provide the correct balance of pore space and desired infiltration.
Top dressings and rootzones should therefore comprise of a high proportion of sand in the mix, as this will assist with the control of aeration, infiltration, percolation and water retention. The next consideration is the very soil used in the top dressing, a point of major importance. Whilst sand supply in the UK is still reasonably plentiful, suitable top soils are certainly not. With this point in mind we have recently launch a new breed of top dressings using lignite as the organic amendment. The lignite is sourced from our own quarry in Devon and has some unique properties other lignite's and soils don't possess:
2.Excellent Nutrient Retention
4.Consistency of supply
5.Long term availability
If the reader would like more information, the product is called XL top dressing, with details on our web site which is listed at the end of this article.
The best top soils for use in sports turf top dressings are sandy soils, unless the top dressing is for use on cricket squares, when with few exceptions, we need clay contents in the range 27% to 33%. It has been long established that soils with clay contents less than about 24% have insufficient binding strength for cricket squares.
The pH of top dressing is a poor guide to their likely effect on the 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. Therefore the supplier you use must have in place a quality control testing system to guarantee specifications and tolerances. Lack of quality control may be another reason for cheapness.
The 70:30 Myth
People who just ask for a 70:30 sand/soil mix in their top dressing (or any other ratio for that matter) don't realise what lottery punters they are. This specification tells you nothing about the quality of the sand in terms of lime content, grading and conductivity, nor anything about the soil which could range from peat-based to a heavy clay content.
A purchaser should be able to request the precise analysis of the top dressing in terms of: particle size; amount of silt/clay; organic matter content and lime.
Feeling the heat
Another important point to mention is that any top dressing you buy should be heat-treated. If it's too wet or damp then it tends to clog together or simply lies on the surface making an even application impossible. Not to mention the fact that if your top dressing is wet then you're effectively paying your supplier for water!
Thatch and general organic matter control
For golf and bowling greens, the 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 should be applied whenever the surface has been disturbed, especially after verticutting/scarification which prune turf and remove plant material, encourages new root development and tillering. The development of a significant and visually evident thatch must be avoided and it is much easier to avoid its development than to remove a thatch layer once developed.
For soccer and rugby this is much less of a problem because normally they do not have thatch problems due to wear and the reason for top dressing is to dilute fine soils which have been brought to the surface by earth worms. Most Soccer/Rugby pitches have an improved rootzone which has been constructed on top of soil and this finer material is taken to the surface by casting worms.
We would advise 50 tonnes per year; this can be applied in one or two applications.
Golf and Bowling greens
Light frequent applications are always advisable throughout the growing season with annual rates varying from 2-8kg/m².Normally for maintenance the greenkeeper would not use more than 6kg/m², higher rates per annum are being used on clay based greens to increase the build up of surface rootzone. Single applications of around 3kg/m² or more are difficult to integrate with the rootzone and the greenkeeper can lose turf condition if top dressings are over applied. It is much easier to integrate small amounts of top dressing and it has become common practice to apply four to six (sometimes more) top dressings of 0.5-1 kg/m² during the growing season.
The Testing Process Explained
The samples are then sent to our in-house USGA PT accredited laboratory at Oakamoor in Staffordshire for analysis. Tests are carried out to determine particle size distribution, pH and organic matter content. We then take these results and feed them into our computer software. This in turn produces a full analysis that includes, amongst other elements:
Particle size distribution on full and half octave sieve sizes
D values which can be used to calculate the gradation index (D90/ D10) or used to ensure bridging characteristics with suitable gravels
Effective particle size (DK value) = the particle diameter that effectively represents the dominant influence on the hydraulic behaviour of a sand.
It can be used to predict:
Pore-size compatibility with other materials
We can then use this information to identify a top dressing from our range that either matches a good rootzone or improves a poor one.
Top dressings are an integral part of sports turf maintenance. They serve several functions in sustaining turf quality. Furthermore they have been used successfully by many greenkeepers and groundsman to build up over the years a depth of high quality rootzone onto inferior material used in the original construction.
Rufford Sports Surface Technology