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Irrigation is one of the most critical aspects of managing natural grass, especially in high-traffic areas like football fields. The amount of water applied, the timing (morning, noon, or evening), and the frequency of irrigation all depend on factors such as grass species, soil type, and weather conditions. Proper irrigation is essential for maintaining healthy turf that can withstand the stress of athletic activity.
Unlike ornamental lawns in parks, football field turf must endure constant movement—running, jumping, kicking, and sliding—which can cause damage such as matting and tearing. Therefore, it's crucial to irrigate efficiently. Ideally, watering should be done once with a long interval between sessions. It's even more important to manage irrigation during peak usage times, particularly when temperatures are high. However, many school and practice football fields have sandy soils, which drain quickly. This often leads to overwatering due to automatic sprinkler systems, resulting in poor management and issues like early death, stunted growth, disease, and pest infestations.
Soil moisture plays a vital role in plant health. When soil dries to a certain point, plants can no longer absorb the water, leading to wilting. This level is known as the wilting coefficient. On the other hand, when the soil becomes saturated, excess water drains away under gravity, and the remaining moisture is called the field capacity. Effective water content refers to the amount of water available to plant roots and is measured in millimeters per centimeter of soil depth. The range for effective water tension typically falls between pF 2.5–2.7 (field capacity) and pF 3.9–4.2 (wilting point). Different soil types hold varying amounts of effective water, as shown in the table below.
**Effective Water Content in Different Soils**
| Soil Type | Effective Water in 1 cm Soil (mm) |
|------------------|-----------------------------------|
| Sand | 0.33–0.83 |
| Sandy Loam | 0.75–1.08 |
| Loam | 1.08–1.67 |
| Light Sandy Loam | 1.67–1.75 |
| Heavy Loam | 1.50–1.75 |
| Clay | 1.50–1.58 |
For example, in a 20cm deep sandy soil, the effective water would range from 6.6mm to 16.6mm. The deeper the root system, the more water the plant can access, reducing the need for frequent irrigation. Factors such as grass species, temperature, mowing height, and soil compaction also influence this process.
Irrigation is closely tied to evaporation and evapotranspiration. Cold-season grasses typically lose about 0.65–0.8mm of water per day, while warm-season grasses lose 0.55–0.65mm. In temperate regions, daily evaporation can range from 2.5–7.5mm. Based on these rates, irrigation intervals can be estimated. For instance, if the soil holds 15mm of water and evaporates at 5mm per day, irrigation should occur every 3 days.
However, irrigation frequency isn’t solely based on evaporation—it also depends on transpiration, temperature, and irrigation method efficiency. Sandy soils, commonly used on football fields, retain less water. To improve water retention, it’s beneficial to enrich the soil with organic matter, minerals, and soil conditioners. This creates a mixed soil that can extend irrigation intervals by two to three times compared to pure sand.
There are several methods to determine irrigation intervals: theoretical calculations, moisture sensors, and surface temperature monitoring. Moisture sensors placed in the soil provide direct readings, while temperature-based methods can also help identify when irrigation is needed. In some cases, cooling irrigation water may be necessary if temperatures rise too high.
The amount of irrigation required varies depending on grass species, soil quality, and environmental conditions. For example, in sandy soils, the available water at a 15cm depth is approximately 25.4mm. For a standard football field covering 7,000m², this translates to about 210 tons of water. Exceeding this amount can be harmful to the grass. Understanding the effective water content of different soils helps optimize irrigation practices.
**Various Types of Soil and Effective Water**
| Name | Field Capacity (%) | Wilting Coefficient (PF) | Effective Water (mm) |
|--------------|--------------------|---------------------------|------------------------|
| Fine Sand | 0.5 | 3.5 | 3.0 |
| Micro Sandy Soil | 30.2 | 7.7 | 22.5 |
| Cohesive Soil | 30.5 | 17.5 | 13.0 |
In addition to soil type, different grass species have varying water needs. Cold-season grasses, such as fescue and ryegrass, are more sensitive to heat and drought than warm-season grasses like zoysiagrass and Bermuda grass. During hot periods, cold-season grasses may go dormant and benefit from mist irrigation to cool the plants. In northern regions, this approach is particularly effective.
Overall, successful lawn management requires a combination of scientific methods and practical experience. Understanding the specific needs of the grass, soil, and climate ensures that irrigation is both efficient and sustainable.