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The value of the North American lawn |
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Page 5 of 11
Lawns protect, restore, and improve soil
Soil is a basic part of any ecosystem and is practically non-renewable. It takes virtually millions of years to build biologically and chemically active topsoil that can support plant growth. Therefore, protection of the topsoil must be a prime objective for any landscape activity. Turfgrass can protect this invaluable resource by controlling erosion caused by wind and water run-off. The dense leaves, thatch, and roots of turfgrass provide an excellent cover that tremendously reduces soil erosion, even on severe slopes. With well-established and well-maintained lawns, almost no soil will be lost even in heavy rainstorms. To the surrounding environment, this means less mud and dust. On a larger viewpoint, it means conservation of topsoil and less sediment pollution of our rivers and lakes. A study concluded that a 30 minutes storm producing a 76 mm per hour rainfall could cause a soil loss of 223 kg per hectare from bare ground compared to only 10-60 kg from a healthy turfgrass cover (Gross et al., 1991).
How does turfgrass reduces soil erosion? A healthy turfgass lawn absorbs rainfall six times more effectively than a wheat field and four times better than a hay field. Turfgrass stabilizes the soil surface with its numerous roots and shoots. A hectare of turfgrass will typically possess 75 million to >20 billion shoots (Beard, 1973; Lush, 1990). Regular mowing of turfgrass increases shoots compared to an ungrazed grassland. Thus, turfgrass offers a cost-effective means of controlling water and wind erosion of soil and is called the "bandage for the earth".
Turfgrasses substantially contribute to soil quality. Soil scientists agree that the best soils are ones that contain organic matter in various states, from green organic matter to mineralized compounds. Organic matter helps separate clay particles, improves water infiltration rates, helps retain essential nutrients, and helps supports the microbes essential to cycling nutrients and maintaining habitats necessary to support beneficial organisms. Turfgrasses add significant amounts of organic matter to soils over time through root, stolon and rhizome formation and decay. Grass clippings also reach soils through the actions of microbes and invertebrates, especially earthworms. Due to the tremendous amount of organic matter additions from the turnover of roots and other plant tissues, turfgrass improves and restores soils rapidly. In fact a high proportion of the world's most fertile agricultural soils have developed under a vegetative cover of grasses (Gould, 1968). Turfgrass roots may grow from 0.5 to 3 m deep depending upon turfgrass species, extent of mowing, and soil conditions. It has been reported that the root biomass of a Kentucky bluegrass lawn is in the range of 11,000 to 16,000 kg per hectare (Boeker, 1974; Falk, 1976). In the upper 150 mm of the soil there are about 122,000 roots and 6.1 x 107 root hairs per liter of soil, with a combined length of >74 km and a surface area of about 2.6 m2 (Dittmer, 1938). A study has estimated that the annual root system turnover rate is about 42% for a lawn (Falk, 1976). Considering this estimate, 6761 kg of root biomass would be turned over into the soil per hectare each year (Beard and Green, 1994). This estimate is low because it does not account for root secretions, death and decay of fine roots and root hairs, and consumption of roots by soil animals. The amount of root biomass annually produced and turned over into the soil, or root net productivity, for a defoliated grassland is higher than the amount reported for ungrazed prairie ecosystems. This is why accelerated soil restoration of environmentally damaged areas by planting perennial grasses is employed effectively on highly eroded rural landscapes, burned-over lands, garbage dumps, mining operations, and steep timber harvest areas.
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