The biggest problem encountered when maintaining turfgrass for sports pitches is soil compaction. This compression of the particles together leads to the formation of a denser, poorly draining mass with little capacity for gas exchange that is useful for supplying oxygen to the root system. As a consequence, there is a general decline in the turfgrass, its vigour and its ability to recover from damage caused by wear.
Soil compaction and its consequent negative effects can be reduced by creating a topsoil with a high sand content of appropriate grain size and good physical and chemical characteristics. The consequence of using a substrate with a high sand content is poor water retention capacity, but by using the U.S.G.A. specification this problem is minimised.
In a top-soil with a high sand content nutrients are easily leached out as there is a low cation exchange capacity, particularly in the years immediately following construction. This potential problem can be reduced by the use of slow-release fertilisers and/or foliar fertilisation.
Evaluations are carried out on various top-soil mixtures with different percentages of the various components, until one is found that conforms to standard parameters of each component to be used in the mixture, the correct volume percentages are then given.
Sand based growth medium
The crushed stone used for drainage and the drainage layer in contact with the sand must also be subjected to grain size analysis and compatibility with the top-soil. Along with physical determinations of the sand, chemical determinations such as salt content, pH, P and K levels are also necessary. Sometimes, especially in arid and semi-arid regions, analysis for sodium determination is also necessary. For the physical-chemical determination of the mix, samples must be sent to specialised laboratories equipped to issue a report of compliance with U.S.G.A. regulations.
THE U.S.G.A. System for the creation of intensively used turfgrasses should not be considered the best system in absolute terms, but rather the best system that can be used at the moment. Furthermore, it should not be forgotten that this is a system that has been tried and tested on tens of thousands of greens and sports fields for over 60 years and has always provided excellent results worldwide.
Once the suitable top soil has been created, it must be subjected to laboratory tests. The first test concerns water infiltration, which must be at least 360 mm/h. Laboratory tests are carried out on compacted top-soil without turf.
Topsoil mixtures subjected to percolation tests for 8 hours and then drained at a tension of 40 cm of water, should have a porosity between 35 and 55%. Of the total porosity, the macro-porosity should be between 15 and 30 % and the micro-porosity between 15 and 25 %.
Topsoil mixtures with sand as main component should have a bulk density between 1.1 and 1.6 g/cm³. The water retained by the soil and not drained is the part of the water that is used for grass growth. In the laboratory the topsoil should have a water retention capacity per 40 cm of 12-25 % of the weight of the kiln dried soil at 105/111 °C. The ideal value of water retention capacity is 18%, i.e. 1.8 mm of retained water for every 10 mm of soil. The pH should be between 5.5 and 8, with the optimum value being between 6 and 6.5.
The preferred range of electrical conductivity is between 0 and 1 millimhos/cm, values below 4 are acceptable.
The value of the percentage of exchangeable sodium (ESP) must be less than 15, with preference given to lower values.
The quality of the sand, the main constituent of the mixture, is of great importance. The use of washed, screened sand of a siliceous nature is recommended, limy sand and/or sand with an excessively high pH should be avoided. The shape of the individual particles should not be rounded but angular. From the point of view of the distribution of the particle size fractions see the following table.
|Name||Particle Diameter||Recommendation (by weight)|
|Fine Gravel||2.0-3.4 mm||Not more than 10% of the total particles in this range, including a maximum of 3% fine gravel (preferably none)|
|Very Coarse Sand||1.0-2.0 mm|
|Coarse Sand||0.5-1.0 mm||Minimum of 60% of the particles must fall in this range|
|Medium Sand||0.25-0.50 mm|
|Fine Sand||0.15-0.25 mm||Not more than 20% of hte particles may fall within this range|
|Very Fine Sand||0.05-0.15 mm||Not more than 5%|
|Silt||0.002-0.05mm||Not more than 5%|
|Clay||< 0.002 mm||Not more than 3%|
|Total Fines||Very fine sand + silt + clay||Less than or equal to 10%|
Various types of organic components have been used in the past, providing graduated and different responses in terms of effectiveness. The ideal organic matter should be well decomposed, well fractionated and have 10% ash content, with an acceptable maximum of 15%. It should also be well chopped but not pulverised as this can create problems with water infiltration. Well chopped sphagnum peat and coco peat with an organic matter content of 85/90% is the most commonly used mixing material.
Additional sources of organic matter can be rice husks, ground bark, sawdust or compost, provided these have been tested by an approved laboratory. Compost must be at least one year old; it is also subject to variations depending on the source material and origin, even within the same batch, and therefore requires careful control and testing for possible phytotoxic effects on turfgrass.
Under the 2004 amendments, certain inorganic porous materials may be used as topsoil improvers as an alternative or complement to peat. Porous ceramics, diatomaceous earth and zeolite can be mixed with peat or can even replace it, as long as the particle size ratios and other parameters of the USGA specifications for the topsoil mix are met. However, it must be considered that there are great differences in these products and that there is currently a lack of long-term experience with their use, so it is necessary to rely on experienced suppliers who can testify to positive experiences.
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