Influence of substrate base on sports field covered with bermuda grass ( 1 )

The sports field consists of three layers (sub-base, base (substrate) and grass). The base is responsible for the radicular development of the grass, directly influencing the characteristics that provide quality, water drainage and lawn durability and allows athlete performance. The construction of sports field base generally follows USGA (United States Golf Association) recommendations for golf course greens. Sand is the main component based on its high capacity for drainage. This study aimed to define the best composition of the base for sports fields covered with Bermuda grass. The following treatments were evaluated: T1: sand; T2: sand (80%) + peat (20%); T3: sand (90%) + clay soil (10%); T4: sand (70%) + sandy soil (30%) during 12 months. The experimental design consisted of a randomized block design, with a 4 x 12 factorial design (treatments x months), with 3 replicates and each plot measuring 3 x 4 m. During one full year, the following parameters were evaluated: surface traction of the turf, mechanical resistance of the base to penetration, humidity of the base, concentration of nutrients in grass clippings, and chemical analysis of the base. The tallest grass occurred in substrate of sand mixed with peat. This mixture also promoted the highest N and P levels in the leaves of Bermuda grass. Playability of the sports field, as determined by mechanical strength and surface traction, was unaffected by the type of substrate. The presence of peat, sand and clay soil mixed with sand promoted greater water retention at the base.


INTRODUCTION
Sports field grass differs from ornamental grass owing to its characteristic tolerance to wear and tear, as well as normal field use.Bermuda grass (Cynodon dactylon x C. transvalensis) is the most used grass for sporting purposes in Brazil based on its superior regenerative ability in areas damaged by excessive foot traffic.Bermuda grass is a highly variable, sod forming perennial that spreads by stolons, rhizomes and seed; grows best under extended periods of high temperatures, mild winters and moderate to high rainfall in general, are drought tolerant, but do not tolerate poorly drained sites as compacted sites and heavy clay soils (DUBLE, 2001).
Sports field turf is composed of sub-base, base (substrate), and grass, or vegetation cover.These components, originally adopted for golfing greens in the United States, have been used in other sports fields, among them soccer fields (NIRSA, 2009).The base of sports fields should be composed of sand of different granulometries, allowing for efficient drainage (USGA, 1993).The physical characteristics of the base are extremely important, mainly its density and porosity, which directly influence root development and the availability of air and water to the plant (KÄMPF, 2001;SANTOS et al., 2002;FERRAZ and CENTURION, 2005).These characteristics are directly affected by compaction, which results in compression with correspondingly higher density and mechanical resistance, while, at the same time, reducing soil particle porosity and thus limiting the infiltration and redistribution of water, making gas exchange difficult.These characteristics will, in turn, influence the chemical characteristics of the base and interfere with the movement of nutrients, thus reducing their use efficiency by plants (SANTOS et al., 2002;MORAES et al., 2008).A compacted base will also hinder the practice of sports, leading to increased risk of knee and ankle injury (FIFA, 2010).Agronomic techniques, such as leveling and drainage of the area, irrigation systems, and control of the physical and chemical characteristics of the soil and/or substrate are used to enhance the quality of sports field turf (KUHN and HAYDEN, 2012).
This study aimed to determine the best substrate composition of the base for natural fields covered with Bermuda grass.

MATERIAL AND METHODS
In order to carry out the experiment, an appropriate area at the Faculdade de Ciências Agronômicas, Universidade Estadual Paulista "Julio de Mesquita Filho" (FCA/ UNESP), Botucatu, São Paulo State, Brazil (22º51'S and 48º26'W, average altitude of 786 m, average temperature during experimentation of 20.7 °C and total precipitation of 800 mm), was chosen.
The experimental design was a completely randomized block with 4 treatments with a factorial 4 x 12 (treatments x months), with 3 replicates, each plot having a dimension of 3 x 4 m.The experiment was carried out for 12 months.Four substrate compositions were studied to formulate the base of the sports field: T1 -sand, T2 -80% sand + 20% peat moss (pH -6.0, electrical conductivity -4 dS m -1 , 103% of water retention capacity (WRC), and cation-exchange capacity (CEC) -800 mmol c dm 3 ), T3 -90% sand + 10% clay soil (alic purple latosol) and T4 -70% sand + 30% sandy soil (dystrophic red latosol).The sand was selected from samples taken from companies that mine river sand near the Botucatu region of southeastern Brazil at the confluence of the Piracicaba, Tietê and Paranapanema Rivers.The samples were analyzed in the Soil Physics Laboratory of the Department of Soils and Environmental Resources of FCA/UNESP.A sieve analysis test was used to determine sand sizes.The chosen experimental sample (Porto de Areia Santa Isabel, Igaraçu do Tietê, SP) was close to the USGA standard for sand size and is presented in The experimental area was designed as a regular sports field composed of the three necessary layers: sub-base, base, and turf.The sub-base was composed of soil classified as medium-textured red latosol (EMBRAPA, 2013), and for drainage, two channels 20 cm deep and 20 cm wide were excavated to place tubing measuring 10 cm in diameter in each plot.The entire area was later covered with geotextile blanket and sand.A sprinkler irrigation system, using six retractable sprinklers, was installed between the drainage system and the base (flow from 0.17 to 1.85 m 3 h -1 and reaching 7.5 to 14 m).
The chemical characteristics of samples from each base used as treatment were determined and shown in Table 2. Based on these results and aiming to increase substrate saturation by 65%, 18 g m -2 of P 2 O 5 were applied, except for T2, owing to the already high levels of P 2 O 5 .Experimental grass was an interspecific hybrid of Bermuda grass (Cynodon dactylon x Cynodon transvaalensis) called Tifton 419.Before planting, the grass was washed to remove adherent soil on roots and rhizomes.For better attachment of the sods, a compaction roller was used.Fifteen days after planting, the turf was covered with sand for uniformity and to fill the voids.A border 2 m wide was built at the side of the experiment using the same grass as that of the experimental area.
The experimental area was irrigated daily, and the turf was pruned to a height of 25 mm to encourage regrowth.Sand was applied to maintain a level area, and manual weed control herbicides were applied during the period of establishment and formation of the experiment.
After one year of experimentation, the following evaluations were carried out during the formation and stabilization of the turf. 1) Chemical analysis of the base substrate (pH, organic matter, cation exchange capacity and base saturation) was performed each month from a sample composed of 5 subsamples of each plot (0 to 10 cm deep), following Raij et al. (2001).2) Macronutrient contents of grass (N, P and K) were determined in a composite sample of grass clippings obtained in the total area of each plot, according to the modified methodology of Malavolta et al. (1997).The preparation of the samples was performed according to Plank and Carrow (2003).The cut for removal of the grass clippings for use in all evaluations was performed one week after the previous cut of 25 mm.3) Base moisture was determined monthly at 3 points per plot, using Delta-T equipment (Model ML 2X Theta probe, Delta-T Devices Ltd.).The equipment has 4 metal rods that generate a signal of 100 MHz and extend to the interior of the ground.4) The mechanical resistance of the base to penetration was evaluated monthly with an electronic penetrometer (PLG 5300 Solotrack Falker), having a constant velocity of 10 m s -1 and allowing measurements every centimeter (average of 20 cm), and was realized in 3 points per plot.5) Turf surface traction measurement (Nm) was evaluated monthly (3 points per plot) with a Rotational Resistance Tester (Deltec Metaal®).
The data were submitted to the test of normality, heterogeneity and analysis of variance (ANOVA), and means were compared using the Tukey test at 5% probability with the Minitab 13.0 software (MINITAB, 2000).
Averages followed by uppercase letters in the column and lower case letters in the row do not differ by Tukey's test.
Base T2 also had high values of organic matter (Table 4) that persisted during the entire experimental period.Peat is a chemically complex material composed of four main groups: bitumens (fatty acids, waxes and steroids), humic substances (humic and fulvic acids and humins), carbohydrates (mainly cellulose and proteins) and lignins (polyphenolic substances, e.g., humic acids), with lignin and cellulose as major constituents (FUCHSMAN, 1974).
Averages followed by uppercase letters in the column and lower case letters in the row do not differ by Tukey's test.
Base T1 (sand) showed the lowest CEC (cation exchange capacity) values, and base T2 (peat + sand) the higher CEC values (Figure 1).In addition to its ability to absorb water, peat can increase CEC in soils.The two main properties of peat are its cation-exchange capacity, or CEC, and buffering power since organic colloids have specific exposure to areas two to ten times greater than mineral soil colloids (LAMIM et al., 2001).Base T2 (sand + peat) could retain cations and, thus, reduce the leaching of basic cations, such as calcium, magnesium, and potassium, but retain other cations, such as ammonium, owing to the physicochemical properties of peat.
The desired basal saturation value was reached only on treatment T1 (pure sand) after several months.All other treatments had values lower than 65% (Figure 2).Part of the nitrogen from the fertilizer used for turf maintenance had nitric acid origin, and after absorbing nitrate, plants generate hydroxyls.As a result of sand's low buffering power, even a small supply of hydroxyl was sufficient to maintain base saturation and the higher pH of T1.
The highest dry mass production occurred with peat treatment (T2), and it was significantly higher than that of other treatments (Table 5).The higher water retention capacity and availability of nutrients to the plants by peat were the main reasons for high growth of the grass and, consequently, the dry mass.
After cutting, nitrogen (N) content in the grass clippings over the 12 months of experimentation ranged from 34 to 38 g kg -1 (Table 6).During the experimental period, variations in N concentration in the grass clippings were observed, which could be attributed to fertilization (monthly fertilization and harvesting) and/or precipitation and grass interaction with the base.It should be noted that some N was exported when the turf was cut and grass clippings subsequently collected and removed from the study area.In addition, climatic conditions, such as high temperature and radiation, promote plant transpiration, which favors nitric N uptake via mass flow.The highest N concentrations of grass clippings occurred at month 2 (51 g kg -1 ), corresponding to the beginning of the experiment and formation of the experimental sports field, and the decrease is observed in months 8, 9, 10, 11 and 12 (February to June), when the dry mass of grass clippings was higher (Table 6).The increase of N levels in the grass leaves could be explained by the lower plant growth, resulting in a further increase in nutrient concentration in foliar tissue.Most N contents observed in grass clippings were within the range of 30 to 50 g kg -1 N, defined as appropriate by McCarty et al. (2003), except for month 12, which had lower results.------------------------p ----------------------- T1: Sand (100%); T2: Sand (20%) + Peat (80%); T3: Sand (90%) + Clay soil (10%); T4: Sand (70%) + Sand soil (30%).
Averages followed by uppercase letters in the column and lower case letters in the row do not differ by Tukey's test.
Averages followed by uppercase letters in the column and lower case letters in the row do not differ by Tukey's test.
The highest phosphorus (P) concentration in grass clippings occurred with treatments T1 and T2 (Table 7).This result shows that peat provided the nutrient since T2 did not receive inorganic phosphate fertilization at the planting, as occurred in T1.The range of P concentration sufficiency in Bermuda grass is 1.5 to 5.0 g kg -1 P (WATSON, 1991), indicating that the grass had been adequately fertilized with phosphorus.
Averages followed by uppercase letters in the column and lower case letters in the row do not differ by Tukey's test.
Averages followed by uppercase letters in the column and lower case letters in the row do not differ by Tukey's test.
The measurement of mechanical resistance to penetration of the base reflects the interaction between base and plant.Once established, grass develops a great amount of roots, rhizomes and stolons, which fill the porous spaces existing at the beginning of lawn formation.The mechanical resistance to penetration of the base increased by 2.5 times from the first evaluation up to month 10, observing the average of the treatments in each month (Table 9).This result can be attributed to an accommodation of the particles of the base owing to irrigation and rainfall, as well as heavy foot traffic, especially for cut grass.Holmes and Bell (1986) used a penetrometer to measure the mechanical resistance to penetration in a soccer field and reported at least 1.4 MPa for areas of heavy foot traffic and at least 1.0 MPa for areas of moderate foot traffic.Magni et al. (2004) found great resistance to penetration on turf with sand base (2.5 MPa) when compared to turf with different drainage and non-drainage systems (1.5 MPa).Humidity is the main factor controlling the surface hardness of native soils (BAKER et al., 1991); however, turf with sand base has little variation of moisture content.The values of mechanical resistance to penetration were similar among the treatments, indicating that mixtures composed of sand had no influence on this parameter during the course of the experimental period.In fact, sand, as the major component of the base, is the most important factor affecting mechanical resistance to penetration of the  T1: Sand (100%); T2: Sand (20%) + Peat (80%); T3: Sand (90%) + Clay soil (10%); T4: Sand (70%) + Sand soil (30%).
Averages followed by uppercase letters in the column and lower case letters in the row do not differ by Tukey's test.
Otherwise, the different mixtures in the composition of the base did not affect the playability of this experimental soccer field relative to, for example, mechanical properties and surface traction.

CONCLUSIONS
Although the treatments with peat and clay or sandy soil presented statistically higher values in some chemical characteristics of the base and grass, the quality parameters did not differ between the treatments.Thus, using pure sand is the best option for the composition of the base for natural fields covered with Bermuda grass.

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CONTRIBUTIONS CMAM: conception and design of the research, obtaining data, analyze and interpretation of data, statistical analysis, write and critically analyses of manuscript.MRO: conception and design of the research, obtaining data, analyze and interpretation of data, statistical analysis, write and critically analyses of manuscript.CPRPJ: analyze and interpretation of data, statistical analysis, write and critically analyses of manuscript.ART: analyze and interpretation of data, write and critically analyses of manuscript.MMPS: analyze and interpretation of data and statistical analysis of manuscript.DMF: analyze and interpretation of data, write and critically analyses of manuscript.RLLB: conception and design of the research, obtaining data, analyze and interpretation of data, write and critically analyses of manuscript and financial and obtaining funding.

Table 1 .
Soils were classified according to the system established by The Brazilian Agricultural Research Corporation (EMBRAPA, 2013).

Table 1 .
Granulometric distribution of sand to base composition as recommended by the United States Golf Association (USGA).

Table 2 .
Chemical composition of the substrates composing the base.

Table 4 .
Organic matter of the base during experimentation (n=3).

Table 9 .
Mechanical resistance of the base to penetration during experimentation (n=3).