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Renovating hybrid grass

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The maintenance of a hybrid grass system must include thatch control and annual renewal, alternating every 2-3 years with the removal of natural grass and new seeding. This last operation is known as returfing and aims to remove the organic soil that has been generated on the surface to restore the original benefits of the hybrid grass system. It is understood that only a direct on-site installation of a well designed system and specialised operators can achieve results in very short time.


James Thorp at the “YearBook of Agriculture 1948” in page 56 of his article “HOW SOILS DEVELOP UNDER GRASS” teach us the following: “The formation of grassland soils involves the accumulation of mineral soil materials, the invasion of these materials by grass, and the accumulation of organic matter and development of soil structure.

Mineral soil materials accumulate through the direct chemical and physical weathering of rocks and through the deposition of broken and chemically weathered rock fragments (sand, silt, clay, and gravel) by streams, lakes, glaciers, wind, and down-slope gravitational movement. A very rough estimate is that one-third to one-fourth of the soil materials of the natural grasslands in the United States are the products of direct weathering of rocks of greatly varying composition and hardness. The remaining materials have been deposited by streams and the other agencies I have listed.”

He also explains that: “The extensive loess (wind-blown dust) deposits are ideal for the growth of grass. Loess is a uniform unstratified mixture of silt, very fine sand, and clay. Probably loess is the most extensive single kind of parent material of grassland soils in the world as a whole.”

Moreover, at page 57 explains that: “Where climatic conditions are favorable, grass invades areas of freshly deposited soil materials very soon after they are first exposed. If accumulation of sediments or weathered rock materials is slow, grasses become well established and begin immediately the work of soil building. Roots spread through the soil and sooner or later die, providing organic, humus-forming waste.”

At page 61 we can read that: “After an exhaustive study of the nitrogen and organic matter of hundreds of soils in the United States, Hans Jenny, in Research Bulletin No. 152 of the Missouri Agricultural Experiment Station, drew a number of conclusions regarding the accumulation of soil organic matter in relation to climate. Among them are:

  1. Within regions of equivalent effective moisture, the nitrogen and organic matter contents of soils of medium textures increase 2 to 3 times with every 18° F (10 °C) fall in average annual temperature, from south to north in the United States. The statement holds for both forested and grassland soils.
  2. Within regions of equal average annual temperatures, the nitrogen and organic matter content of grassland soils increases with increasing humidity. The rate of increase is greater in the cool northern regions than in the warmer southern regions. This conclusion does not hold for forested soils.
  3. From data available to Jenny at the time his bulletin was written in 1930, he felt that climate and vegetation were more significant to the accumulation of nitrogen in loamy soils than topography, parent material, or age of the soil.”

Renewing natural and hybrid grass systems on sports fields

Checking root depth and healthChecking root depth and health

When it comes to the maintenance of natural and hybrid grass systems, organic soil should be expected to accumulate on the surface, especially in windy and relatively dry areas where soil erosion is most active. The accumulated organic soil will keep the surface wetter, reduce water infiltration and the ability of roots to penetrate the soil especially where foot traffic is intensive.

When it is hot, many roots will die, even if irrigation is applied, contributing to the formation of organic soil and the grass is less resistant to tearing until the roots grow again, with temperatures in the appropriate range. For this reason, many Groundsmen in the UK, prefer to remove the grass each year along with the organic soil that has formed on the surface; if the accumulation is not excessive, others prefer to renew the existing cover by scarifying to remove weak grass, deep aeration, supplementary seeding and sanding to promote gas exchange in the deeper layers. 
In hybrid grass systems it is imperative to contain the accumulation of organic matter each month with a mechanical rake as otherwise the fibres will soon be buried and the benefit of protecting the crowns of the plants will have worn off.  It is also advisable to renew the existing turf once a year without sanding, and every 2-3 years to completely remove the natural grass and organic soil that has built up on the surface.

The buried synthetic fibres, when associated with shallow roots, form a weak lawn and make it necessary to remove the grass and organic soil, then sow new grass on the original infill with a light addition of sand, to restore the benefits of the hybrid system. This operation (returfing) requires knowledge and experience to detach the old grass, avoiding damage to the synthetic turf which must remain intact. It is also imperative that the hybrid grass system must be designed with certain characteristics, installed with the synthetic fibres protruding on the surface, and must be resistant to mechanical cultivation operations. For this reason, the appropriate hybrid carpet must be selected where the synthetic fibres are strong and resilient and are well attached to the backing, so that they are not damaged or removed during the work of removing the natural grass. It is also essential that the system has been installed directly on site, stitching the rolls of hybrid carpet all together and not with the transplanted sod, to avoid lifting the hybrid carpet and tearing the backing. 

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