Putting Green Topsoil Mixtures that Meet USGA Specifications
In 1960, after years of research, the Green Section of the USGA published its "Specifications for a Method of Putting Green Construction". It was a method that was a departure from what at the time was considered the norm. The use of much more sand in the topsoil mixture was advocated in order to resist compaction and assure good drainage. A principle of soil layering was prescribed that created a "perched water table", which insured a reserve water supply with a soilwater relationship considered ideal for all agricultural crops. It recommended that all soil mixtures be laboratory tested to determine particle size distribution, infiltration rates, and per cent capillary and noncapillary pore space. It stressed the physical as well as the chemical and mechanical tests performed on all top mixtures for greens built to these specifications. Though backed by solid research, this method was slow to be accepted. However, during the 1970's the method began being accepted, and now, every major new golf course being built and almost all greens that are renovated use the USGA Specifications as the criteria for determining what the physical properties of the mix should be.
How USGA Specifications Aid The Golf Course Superintendent
The USGA did not develop these Specifications primarily for the golf course superintendent, but the golf course superintendent probably derives the most benefit from greens built with USGA Specifications. Golf course architects, builders, and contractors by nature of their work often seek materials of the lowest price that are available for building golf greens. If these materials later prove unsatisfactory, it is the golf course superintendent who suffers the most. Poorly constructed greens can cause a much higher degree of maintenance, increased maintenance costs, and poor playing conditions. Greens constructed improperly are usually too hard and compacted or too soft, offer little resistance to compaction, have little resilience, are much harder to control the proper amount of water resulting in greens being too wet or too dry, are harder to fertilize, and are harder to control diseases, nematodes, and weeds. Where specifications meeting USGA requirements are followed, consistent and satisfactory golf greens can be more easily maintained. According to reports, greens built with initial USGA Specifications and properly maintained can expect 20 or more years of satisfactory service. The golf club always suffers the consequences of poorly constructed greens; that is, increased maintenance costs and poor playing conditions.
The Topsoil Mixture
The USGA Specifications for constructing a putting green consist of a topsoil mixture with a minimum thickness of 12 inches, above an optional 1 1/2 to 2 inch coarse sand layer that is sometimes referred to as the "choker layer", which is above a 4 inch layer of washed pea gravel that has a particle size diameter of ¼ to 3/8 inches, which covers the 4 inch tile drain imbedded in the subgrade of native soil or fill material. This discussion will deal only with the 12 inch topsoil mixture.
Native topsoils that meet USGA physical characteristics are almost nonexistent in nature. Therefore, the putting green topsoil mixture must be compounded from locally available sand, soil, and organic amendments. However, because of the extreme variations in these local materials, a high degree of expertise is required in compounding topsoil blends with desired physical properties. Different lots of sand vary considerably in particle size and shape. Native soils also vary greatly in particle size and shape, as well as silt and clay content and in degree of aggregation, organic matter, and fertility including acidity and soluble salt content.
It is unlikely that golf course architects. builders, or superintendents can cope with the variability in construction materials when formulating topsoils and topdressings for greens. Therefore, since successful construction is dependent on the proper combination of physical and hydraulic properties in the topsoil, a physical soil analysis must be made of available construction materials before they are purchased. The abundance of each material to complete the job should also be investigated, and representative samples of each material tested by a competent laboratory.
What Are USGA Specifications?
All of the physical properties considered important by the USGA for a putting green topsoil mixture are listed below.
Particle Size. A sand or soil is composed of gravel, sand, silt, and clay. These classifications are based on the diameter of the particle size. Of course rocks or pebbles should be immediately removed. Gravel is particles larger than 2 mm in diameter. Sand is particles ranging in diameter from 2 mm to 0.05 mm. Silt is between 0.05 to 0.002 mm, and clay, the finest fraction, is less than 0.002 mm in diameter.
Ideally, the topsoil mixture should contain no gravel or particles larger than 2 mm in diameter. Gravel will ride a 2 mm or #10 mesh sieve. The topsoil mixture should contain at least 92% sand, less than 5% silt, and less than 3% clay. Sand is broken down into fractions called very coarse which is between 1 and 2 mm in diameter, coarse which is between 1 and 0.5 mm, medium between 0.5 and 0.25 mm, fine between 0.25 and 0.15 mm, and very fine which is 0.15 to 0.05 mm in diameter. The best sand fraction by far for golf green construction is the medium fraction and good sands for developing golf greens should be high in medium sand, preferably 50% or greater. Of course the coarser the sand, the higher the infiltration rate and the finer the sand the lower the infiltration rate. An ideal sand for golf greens would have from 3-5% very coarse sand, 20-25% coarse sand, 50-55% medium sand, 20-25% fine sand, and 2% very fine sand. It should be kept in mind, however, that excellent greens can be developed from greens without an ideal sand distribution, however, the ideal sand distribution serves as a helpful guideline for classifying sands.
The USGA recommends that sand for topdressing should range from 1.0 to 0.15 mm, which would be sand ranging from coarse to fine. For trap sand, the range is from 1.0 to 0.25 mm, which is sand ranging from coarse to medium. Trap sand should be slightly coarser than topdressing sand as the fine fraction is recommended to be omitted in trap sand.
Water Permeability Rate. This rate is also referred to as infiltration or percolation rate. When the topsoil mixture is compacted at a moisture content equal to field capacity on the green, and maintained under a constant head flow of water for 24 hours at room temperature, as described by USGA procedures, the infiltration rate should be 4 to 8 in/hr. for established bermuda grass greens and 8 to 12 in/hr. for bentgrass greens. Initial rates for bermuda grass greens should be 8 to 10 in/hr. and 12 to 14 in/hr. for bentgrass greens. Since the rates will decrease a little once grass is established, starting out at the upper end of the range will allow the mix to remain longer within the desired range over a greater number of years as the tendency is for the rate to slowly decline with time.
Porosity. Noncapillary pore space is the large pore space and capillary pore space is the small pore space in soils. Compacted topsoil mixtures that have been allowed to percolate water for 24 hrs. and then drained to field capacity (which is a tension of 40 cm of water under laboratory conditions), should not have a % large pore space less than 15%. A good range for % large and % small pore space is from 15 to 25%.
Bulk Density. Topsoil mixtures should have a bulk density between 1.25 and 1.45 grams per cubic centimeter. The more organic matter that is present in a mix, the lower the bulk density. A green with a high bulk density (greater than 1.60) would be a hard, compacted green and a green with a low bulk density (less than 1.20) would be a soft, 'peaty' mix high in organic matter and water holding capacity.
Water Retention. The water that is held by a soil against drainage is the water that supports the growth of turf. In putting greens, the drainage potential from the putting surface to the tile is approximately 40 centimeters or 16 inches. The topsoil should have a laboratory 40 cm tension water retention capacity between 12 and 25%.
Organic Matter. The organic matter content in a putting green topsoil mixture should be between 10 and 20%. If a sand needs more than 20% peat to reduce the infiltration rate of a mix to the desired rates, then the sand is too coarse for use. Peat used in a Greensmix should be more than 80% organic and less than 20% inorganic. Of the peats used in a Greensmix, the most common is sphagnum peat moss, although Michigan and humus peat are also used.
A description of putting green topsoil mixtures that meet USGA Specifications has been presented. Detailed specifications have been presented for all of the physical properties considered important by the USGA for a putting green topsoil mixture. The effect of how a USGA putting green topsoil mixture aids the golf course superintendent was discussed. The Tifton Physical Soil Testing Laboratory specializes in fabricating putting green topsoil mixtures that meet USGA Specifications. For more information contact T. Powell Gaines, 1412 Murray Avenue, Tifton, GA 31794, Phone 229/382-7292.